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Low Voltage, Low Power, Factory-Calibrated 16-/24-Bit Dual - ADC AD7719
Low-Side Power Switches Reference Detect Circuit APPLICATIONS Sensor Measurement Temperature Measurement Pressure Measurements Weigh Scales Portable Instrumentation 4-20 mA Transmitters GENERAL DESCRIPTION
FEATURES HIGH RESOLUTION - ADCs Two Independent ADCs (16- and 24-Bit Resolution) Factory-Calibrated (Field Calibration Not Required) Output Settles in One Conversion Cycle (Single Conversion Mode) Programmable Gain Front End Simultaneous Sampling and Conversion of Two Signal Sources Separate Reference Inputs for Each Channel Simultaneous 50 Hz and 60 Hz Rejection at 20 Hz Update Rate
ISOURCE Select TM 24-Bit No Missing Codes--Main ADC 13-Bit p-p Resolution @ 20 Hz, 20 mV Range 18-Bit p-p Resolution @ 20 Hz, 2.56 V Range
INTERFACE 3-Wire Serial SPITM, QSPITM, MICROWIRETM and DSP-Compatible Schmitt Trigger on SCLK POWER Specified for Single 3 V and 5 V Operation Normal: 1.5 mA Typ @ 3 V Power-Down: 10 A (32 kHz Crystal Running) ON-CHIP FUNCTIONS Rail-Rail Input Buffer and PGA 4-Bit Digital I/O Port On-Chip Temperature Sensor Dual Switchable Excitation Current Sources
The AD7719 is a complete analog front end for low frequency measurement applications. It contains two high resolution sigmadelta ADCs, switchable matched excitation current sources, low-side power switches, digital I/O port, and temperature sensor. The 24-bit main channel with PGA accepts fully differential, unipolar, and bipolar input signal ranges from 1.024 x REFIN1/128 to 1.024 x REFIN1. Signals can be converted directly from a transducer without the need for signal conditioning. The 16-bit auxiliary channel has an input signal range of REFIN2 or REFIN2/2. The device operates from a 32 kHz crystal with an on-chip PLL generating the required internal operating frequency. The output data rate from the part is software programmable. The peak-to-peak resolution from the part varies with the programmed gain and output data rate. The part operates from a single 3 V or 5 V supply. When operating from 3 V supplies, the power dissipation for the part is 4.5 mW with both ADCs enabled and 2.85 mW with only the main ADC enabled in unbuffered mode. The AD7719 is housed in 28-lead SOIC and TSSOP packages.
FUNCTIONAL BLOCK DIAGRAM
DVDD AVDD IOUT1 IEXC1 200 A IEXC2 200 A DGND REFIN1(+) REFIN1(-) XTAL1 REFERENCE DETECT XTAL2
AD7719
OSC. AND PLL
IOUT2 AIN1 AIN2 AIN3 AIN4 AVDD MUX1 AGND AIN5 AIN6 MUX2 MUX TEMP SENSOR AVDD AGND REFIN2 PWRGND AUXILIARY CHANNEL 16-BIT - ADC BUF PGA AVDD I/O PORT MAIN CHANNEL 24-BIT - ADC
SERIAL INTERFACE AND CONTROL LOGIC
DOUT DIN SCLK CS RDY RESET
P1/SW1 P2/SW2 P3
P4
ISOURCE Select is a trademark of Analog Devices. SPI and QSPI are trademarks of Motorola Inc. MICROWIRE is a trademark of National Semiconductor Corporation.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2001
AD7719
TABLE OF CONTENTS
FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . 1 SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . . 6 ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 TIMING CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . 7 DIGITAL INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 PIN CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 PIN FUNCTION DESCRIPTIONS . . . . . . . . . . . . . . . . . . 9 TYPICAL PERFORMANCE CHARACTERISTICS . . . . 11 DUAL-CHANNEL ADC CIRCUIT INFORMATION . . . 12 Main Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Auxiliary Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Both Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 MAIN AND AUXILIARY ADC NOISE PERFORMANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 ON-CHIP REGISTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Communications Register (A3, A2, A1, A0 = 0, 0, 0, 0) . . . . . . . . . . . . . . . . . . . . 19 Status Register (A3, A2, A1, A0 = 0, 0, 0, 0; Power-On Reset = 00 Hex) . . . . . . . . . . . . . . . . . . . . . 20 Mode Register (A3, A2, A1, A0 = 0, 0, 0, 1; Power-On-Reset = 00 Hex) . . . . . . . . . . . . . . . . . . . . . 21 Operating Characteristics when Addressing the Mode and Control Registers . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Main ADC Control Register (AD0CON): (A3, A2, A1, A0 = 0, 0, 1, 0; Power-On Reset = 07 Hex) . . . . . . . . . . . . . . . . . . . . . 22 Aux ADC Control Registers (AD1CON): (A3, A2, A1, A0 = 0, 0, 1, 1; Power-On Reset = 01 Hex) . . . . . . . . . . . . . . . . . . . . . 23 Filter Register (A3, A2, A1, A0 = 0, 1, 0, 0; Power-On Reset = 45 Hex) . . . . . . . . . . . . . . . . . . . . . 24 I/O and Current Source Control Register (IOCON): (A3, A2, A1, A0 = 0, 1, 1, 1; Power-On Reset = 0000 Hex) . . . . . . . . . . . . . . . . . . . 24 Main ADC Data Result Registers (DATA0): (A3, A2, A1, A0 = 0, 1, 0, 1; Power-On Reset = 000000 Hex) . . . . . . . . . . . . . . . . . 26 Aux ADC Data Result Registers (DATA1): (A3, A2, A1, A0 = 0, 1, 1, 0; Power-On Reset = 0000 Hex) . . . . . . . . . . . . . . . . . . . 26
Main ADC Offset Calibration Coefficient Registers (OF0): (A3, A2, A1, A0 = 1, 0, 0, 0; Power-On Reset = 800000 Hex) . . . . . . . . . . . . . . . . . 26 Aux ADC Offset Calibration Coefficient Registers (OF1): (A3, A2, A1, A0 = 1, 0, 0, 1; Power-On Reset = 8000 Hex) . . . . . . . . . . . . . . . . . . . 26 Main ADC Gain Calibration Coefficient Registers (GNO): (A3, A2, A1, A0 = 1, 0, 1, 0; Power-On Reset = 5XXXX5 Hex) . . . . . . . . . . . . . . . . 25 Aux ADC Gain Calibration Coefficient Registers (GN1): (A3, A2, A1, A0 = 1, 0, 1, 1; Power-On Reset = 59XX Hex) . . . . . . . . . . . . . . . . . . . 26 ID Register (ID): (A3, A2, A1, A0 = 1, 1, 1, 1; Power-On Reset = 0X Hex) . . . . . . . . . . . . . . . . . . . . . 26 User Nonprogrammable Test Registers . . . . . . . . . . . . . . 26 CONFIGURING THE AD7719 . . . . . . . . . . . . . . . . . . . . . 27 MICROCOMPUTER/MICROPROCESSOR INTERFACING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 AD7719-to-68HC11 Interface . . . . . . . . . . . . . . . . . . . . . 28 AD7719-to-8051 Interface . . . . . . . . . . . . . . . . . . . . . . . . 28 AD7719-to-ADSP-2103/ADSP-2105 Interface . . . . . . . . 30 CIRCUIT DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . 30 Analog Input Channels . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Programmable Gain Amplifier . . . . . . . . . . . . . . . . . . . . . 32 Bipolar/Unipolar Configuration . . . . . . . . . . . . . . . . . . . . 33 Data Output Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Burnout Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Excitation Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Reference Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Reference Detect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Reset Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Power-Down Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 ADC Disable Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Grounding and Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Pressure Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . . 37 3-Wire RTD Configurations . . . . . . . . . . . . . . . . . . . . . . 38 Smart Transmitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . 40
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DD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, DVDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, REFIN(+) = 2.5 V; REFIN(-) = AGND; AGND = DGND = 0 V; XTAL1/XTAL2 = 32.768 kHz Crystal; all specifications TMIN to TMAX unless otherwise noted.)
AD7719-SPECIFICATIONS1 (AV
Parameter ADC CHANNEL SPECIFICATION Output Update Rate MAIN CHANNEL No Missing Codes 2 Resolution Output Noise and Update Rates Integral Nonlinearity Offset Error3 Offset Error Drift vs. Temperature 4 Full-Scale Error 5, 8, 9 Gain Drift vs. Temperature 4 Power Supply Rejection (PSR) ANALOG INPUTS Differential Input Voltage Ranges ADC Range Matching Absolute AIN Voltage Limits Analog Input Current2 DC Input Current DC Input Current Drift Absolute AIN Voltage Limits Analog Input Current DC Input Current DC Input Current Drift Normal-Mode Rejection2, 6 @ 50 Hz @ 60 Hz Common-Mode Rejection @ DC @ 50 Hz2 @ 60 Hz2 REFERENCE INPUT (REFIN1) REFIN1 Voltage REFIN1 Voltage Range 2 REFIN1 Common-Mode Range Reference DC Input Current Reference DC Input Current Drift Normal-Mode Rejection2, 6 @ 50 Hz @ 60 Hz Common-Mode Rejection @ DC @ 50 Hz @ 60 Hz Reference Detect Levels AUXILIARY CHANNEL No Missing Codes 2 Resolution Output Noise and Update Rates Integral Nonlinearity AD7719B 5.4 105
Unit Hz min Hz max Bits min Bits p-p Bits p-p ppm of FSR max V typ nV/C typ V typ ppm/C typ dB min
Test Conditions Both Channels Synchronized 0.732 ms Increments 20 Hz Update Rate 20 mV Range, 20 Hz Update Rate 2.56 V Range, 20 Hz Update Rate Typically 2 ppm. FSR =
2 x 1.024 REFIN 1 Gain
24 13 18 See Tables II to V 10 3 10 10 0.5 80
At the Calibrated Conditions Input Range = 2.56 V, 100 dB typ. 110 dB typ on 20 mV Range REFIN1 = REFIN1(+) - REFIN1(-) GAIN = 1 to 128. Input Voltage = 19 mV on All Ranges BUF = 0; Buffered Mode of Operation BUF = 0
1.024 x REFIN1/GAIN 2 AGND + 100 mV AVDD - 100 mV 1 5 AGND - 30 mV AVDD + 30 mV 125 2 100 100 90 100 100 2.5 1 AVDD AGND - 30 mV AVDD + 30 mV 0.5 0.01 100 100 110 110 110 0.3 0.65 16 16 See Tables VI and VIII 15
V nom V typ V min V max nA max pA /C typ V min V max nA/V typ pA/V/C typ dB min dB min dB min dB min dB min V nom V min V max V min V max A/V typ nA/V/C typ dB min dB min dB typ dB typ dB typ V min V max Bits min Bits p-p ppm of FSR max
BUF = 1; Unbuffered mode of operation. BUF = 1. Unbuffered Mode of Operation. Input Current Varies with Input Voltage 50 Hz 1 Hz, 16.65 Hz Update Rate, SF = 82 60 Hz 1 Hz, 20 Hz Update Rate, SF = 68 Input Range = 2.56 V, AIN = 1 V. 100 dB typ. 110 dB typ on 20 mV Range 50 Hz 1 Hz, Range = 2.56 V, AIN = 1 V 60 Hz 1 Hz, Range = 2.56 V, AIN = 1 V REFIN1 = REFIN1(+) - REFIN1(-)
50 Hz 1 Hz, SF = 82 60 Hz 1 Hz, SF = 68 Input Range = 2.56 V, AIN = 1 V 50 Hz 1 Hz, Range = 2.56 V, AIN = 1 V 60 Hz 1 Hz, Range = 2.56 V, AIN = 1 V NOXREF Bit Active if VREF < 0.3 V NOXREF Bit Inactive if VREF > 0.65 V
2.5 V Range, 20 Hz Update Rate
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AD7719
Parameter AUXILIARY CHANNEL (continued) Offset Error3 Offset Error Drift vs. Temperature 4 Full-Scale Error8, 9 Gain Drift vs. Temperature4 Negative Full-Scale Error Power Supply Rejection (PSR) ANALOG INPUTS Differential Input Voltage Ranges Absolute AIN Voltage Limits Analog Input Current DC Input Current DC Input Current Drift Normal-Mode Rejection2, 6 @ 50 Hz @ 60 Hz Common-Mode Rejection @ DC @ 50 Hz2 @ 60 Hz2 REFERENCE INPUT (REFIN2) REFIN2 Voltage REFIN2 Range2 Reference DC Input Current 2 Reference DC Input Current Drift EXCITATION CURRENT SOURCES (IEXC1 and IEXC2) Output Current Initial Tolerance at 25C Drift Initial Current Matching at 25C Drift Matching Line Regulation (AVDD) Load Regulation Output Compliance LOW-SIDE POWER SWITCHES (SW1 AND SW2) RON Allowable Current2 TEMPERATURE SENSOR Accuracy TRANSDUCER BURNOUT AIN(+) Current AIN(-) Current Initial Tolerance @ 25C Drift SYSTEM CALIBRATION 10, 2 Full-Scale Calibration Limit Zero-Scale Calibration Limit Input Span AD7719B 3 10 0.75 0.5 1 70 REFIN2 REFIN2/2 AGND - 30 mV AVDD + 30 mV 125 2 100 100 85 90 90 2.5 1 AVDD 0.2 0.003 Unit V typ nV/C typ LSB typ ppm/C typ LSB typ dB min V nom V nom V min V max nA/V typ pA/V/C typ dB min dB min dB min dB min dB min V nom V min V max A/V typ nA/V/C typ Test Conditions Selected Channel = AIN5/AIN6
AIN = 1 V Input Range = 2.5 V, Typically 80 dB ARN = 1 ARN = 0 Unbuffered Input Input Current Varies with Input Voltage 50 Hz 1 Hz, SF = 82 60 Hz 1 Hz, SF = 68 Input Range = 2.5 V, AIN = 1 V 50 Hz 1 Hz, Range = 2.5 V, AIN = 1 V 60 Hz 1 Hz, Range = 2.5 V, AIN = 1 V With Respect to AGND
200 10 20 1 1 2.1 300 AVDD - 0.6 AGND -30 mV
A nom % typ ppm/C typ % typ ppm/C typ A/V max nA/V typ V max V min
Matching between IEXC1 and IEXC2 No Load AVDD = 5 V 5%. Typically 1.25 A/V
5 7 20 See TPC 5 -100 +100 15 0.03 1.05 x FS11 -1.05 x FS 0.8 x FS 2.1 x FS
max max mA max C typ nA typ nA typ % typ %/C typ V max V min V min V max
AVDD = 5 V. Typically 3 AVDD = 3 V. Typically 4.5 Continuous Current per Switch
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AD7719-SPECIFICATIONS1
Parameter LOGIC INPUTS All Inputs Except SCLK and XTAL1 2 VINL, Input Low Voltage VINH, Input High Voltage SCLK Only (Schmitt-Triggered Input) 2 VT(+) VT(-) VT(+) - VT(-) VT(+) VT(-) VT(+) - VT(-) XTAL1 Only2 VINL, Input Low Voltage VINH, Input High Voltage VINL, Input Low Voltage VINH, Input High Voltage Input Currents Input Capacitance2 LOGIC OUTPUTS (Excluding XTAL2) VOH, Output High Voltage 2 VOL, Output Low Voltage2 VOH, Output High Voltage 2 VOL, Output Low Voltage 2 Floating-State Leakage Current Floating-State Output Capacitance Data Output Coding I/O PORT7 VINL, Input Low Voltage2 VINH, Input High Voltage2 Input Currents 0.8 0.4 2.0 10 -70 10 AVDD - 0.6 0.4 4 0.4 10 10 300 1 1 300 V max V max V min A max A max pF typ V min V max V min V max A max pF typ ms typ ms typ ms typ ms typ AD7719B Unit Test Conditions 0.8 0.4 2.0 1.4/2 0.8/1.4 0.3/0.85 0.95/2 0.4/1.1 0.3/0.85 0.8 3.5 0.4 2.5 10 -70 10 DVDD - 0.6 0.4 4 0.4 10 10 Binary Offset Binary V max V max V min V min/V max V min/V max V min/V max V min/V max V min/V max V min/V max V max V min V max V min A max A max pF typ V min V max V min V max A max pF typ DVDD = 5 V DVDD = 3 V DVDD = 3 V or 5 V DVDD = 5 V DVDD = 5 V DVDD = 5 V DVDD = 3 V DVDD = 3 V DVDD = 3 V DVDD = 5 V DVDD = 5 V DVDD = 3 V DVDD = 3 V VIN = DVDD VIN = DGND, Typically -40 A at 5 V and -20 A at 3 V All Digital Inputs DVDD = 3 V, ISOURCE = 100 A DVDD = 3 V, ISINK = 100 A DVDD = 5 V, ISOURCE = 200 A DVDD = 5 V, ISINK = 1.6 mA
Unipolar Mode Bipolar Mode I/O Port Voltages Are with Respect to AVDD and AGND AVDD = 5 V AVDD = 3 V AVDD = 3 V or 5 V VIN = AVDD VIN = AGND, Typically -40 A at AVDD = 5 V and -20 A at AVDD = 3 V All Digital Inputs AVDD = 3 V, ISOURCE = 100 A AVDD = 3 V, ISINK = 100 A AVDD = 5 V, ISOURCE = 200 A AVDD = 5 V, ISINK = 1.6 mA
Input Capacitance VOH, Output High Voltage 2 VOL, Output Low Voltage2 VOH, Output High Voltage 2 VOL, Output Low Voltage2 Floating-State Output Leakage Current Floating-State Output Capacitance START-UP TIME From Power-On From Idle Mode From Power-Down Mode POWER REQUIREMENTS Power Supply Voltages AVDD - AGND DVDD - DGND Power Supply Currents DIDD Current (Normal Mode) 12
Osc. Active in Power-Down Osc. Powered Down
2.7/3.6 4.75/5.25 2.7/3.6 4.75/5.25 0.6 0.75
V min/max V min/max V min/max V min mA max mA max
AVDD = 3 V nom AVDD = 5 V nom DVDD = 3 V nom DVDD = 5 V nom DVDD = 3 V, 0.5 mA typ DVDD = 5 V, 0.6 mA typ
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AD7719
Parameter
Power Supply Currents (Continued) AIDD Current (Main ADC)
AD7719B
1.1 0.55
Unit
mA max mA max mA max mA max mA max mA max mA max A max A max A max A max A max
Test Conditions
AVDD = 3 V or 5 V, Buffered Mode, 0.85 mA typ AVDD = 3 V or 5 V, Unbuffered Mode, 0.45 mA typ AVDD = 3 V or 5 V, 0.25 mA typ AVDD = 3 V or 5 V, Main ADC Buffered, 1 mA typ DVDD = 3 V, 0.25 mA typ DVDD = 5 V, 0.3 mA typ AVDD = 3 V or 5 V DVDD = 3 V, 32.768 kHz Osc. Running DVDD = 3 V, Oscillator Powered Down DVDD = 5 V, 32.768 kHz Osc. Running DVDD = 5 V, Oscillator Powered Down AVDD = 3 V or 5 V
AIDD Current (Aux ADC) AIDD Current (Main and Aux ADC) DIDD (ADC Disable Mode)13 AIDD (ADC Disable Mode) DIDD (Power-Down Mode)
0.3 1.25 0.35 0.4 0.15 10 2 30 8 1
AIDD (Power-Down Mode)
NOTES 1 Temperature Range -40C to +85C. 2 Guaranteed by design and/or characterization data on production release. 3 System zero calibration will remove this error. 4 A calibration at any temperature will remove this drift error. 5 The Main ADC is factory-calibrated with AV DD = DVDD = 4 V, TA = 25C, REFIN1(+) - REFIN1(-) = 2.5 V. If the user power supplies or temperature conditions are significantly different from these, internal full-scale calibration will restore this error to the published specification. System calibration can be used to reduce this error to the order of the noise. Full scale error applies to both positive and negative full scale. 6 Simultaneous 50 Hz and 60 Hz rejection is achieved using 19.8 Hz (SF = 69) update rate. Normal mode rejection in this case is 60 dB min. 7 Input and Output levels on the I/O Port are with respect to AV DD and AGND. 8 A system full-scale calibration will remove this error. 9 A typical gain error of 10V results following a user self calibration. 10 After a calibration if the analog input exceeds positive full scale, the converter will output all 1s. If the analog input is less than negative full scale then the device will output all 0s. 11 FS = Full-Scale Input. FS = 1.024 x REFIN1/Gain on the Main ADC, where REFIN1 = REFIN1(+) - REFIN1(-). FS = REFIN2 on the aux ADC when ARN = 1 in the aux ADC control register (AD1CON) and REFIN2/2 on the aux ADC when ARN = 0. 12 Normal Mode refers to the case where both main and aux ADCs are running. 13 ADC disable is entered by setting both the AD0EN and AD1EN bits in the Main and Aux ADC control registers to a 0 and setting the mode bits (MD2, MD1, MD0) in the Mode register to non-0. Specifications subject to change without notice.
ABSOLUTE MAXIMUM RATINGS 1
(TA = 25C unless otherwise noted)
AVDD to AGND . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V AVDD to DGND . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V DVDD to AGND . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V DVDD to DGND . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V AGND to DGND2. . . . . . . . . . . . . . . . . . . -20 mV to +20 mV PWRGND to AGND . . . . . . . . . . . . . . . . -20 mV to +20 mV AVDD to DVDD . . . . . . . . . . . . . . . . . . . . . . . . . . -5 V to +5 V Analog Input Voltage to AGND . . . . -0.3 V to AVDD +0.3 V Reference Input Voltage to AGND . . -0.3 V to AVDD +0.3 V Total AIN/REFIN Current (Indefinite) . . . . . . . . . . . . 30 mA Digital Input Voltage to DGND . . . . -0.3 V to DVDD +0.3 V Digital Output Voltage to DGND . . . -0.3 V to DVDD +0.3 V Operating Temperature Range . . . . . . . . . . . -40C to +85C Storage Temperature Range . . . . . . . . . . . . -65C to +150C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150C SOIC Package JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 71.4C/W JC Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 23C/W TSSOP Package JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 97.9C/W JC Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 14C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . . . 215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220C
NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 AGND and DGND and connected internally within the AD7719.
ORDERING GUIDE
Model AD7719BR AD7719BRU EVAL-AD7719EB
Temperature Range -40C to +85C -40C to +85C
Package Description SOIC TSSOP Evaluation Board
Package Option R-28 RU-28
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AD7719 TIMING CHARACTERISTICS1, 2 (AV
Parameter t1 t2 Read Operation t3 t4 t5 4 t5A4, 5 t6 t7 t8 t9 6 t10 Write Operation t11 t12 t13 t14 t15 t16
DD = 2.7 V to 3.6 V or AVDD = 4.75 V to 5.25 V; DVDD = 2.7 V to 3.6 V or DVDD = 4.75 V to 5.25 V; AGND = DGND = 0 V; XTAL = 32.768 kHz; Input Logic 0 = 0 V, Logic 1 = DVDD unless otherwise noted.)
Limit at TMIN, TMAX (B Version) 32.768 50 0 0 0 60 80 0 60 80 100 100 0 10 80 100 0 30 25 100 100 0
Unit kHz typ ns min ns min ns min ns min ns max ns max ns min ns max ns max ns min ns min ns min ns min ns max ns max ns min ns min ns min ns min ns min ns min
Conditions/Comments Crystal Oscillator Frequency. RESET Pulsewidth RDY to CS Setup Time CS Falling Edge to SCLK Active Edge Setup Time3 SCLK Active Edge to Data Valid Delay3 DVDD = 4.75 V to 5.25 V DVDD = 2.7 V to 3.6 V CS Falling Edge to Data Valid Delay3 DVDD = 4.75 V to 5.25 V DVDD = 2.7 V to 3.6 V SCLK High Pulsewidth SCLK Low Pulsewidth CS Rising Edge to SCLK Inactive Edge Hold Time3 Bus Relinquish Time after SCLK Inactive Edge3 SCLK Active Edge to RDY High3, 7 CS Falling Edge to SCLK Active Edge Setup Time3 Data Valid to SCLK Edge Setup Time Data Valid to SCLK Edge Hold Time SCLK High Pulsewidth SCLK Low Pulsewidth CS Rising Edge to SCLK Edge Hold Time
NOTES 1 Sample tested during initial release to ensure compliance. All input signals are specified with t R = tF = 5 ns (10% to 90% of DVDD) and timed from a voltage level of 1.6 V. 2 See Figures 2 and 3. 3 SCLK active edge is falling edge of SCLK. 4 These numbers are measured with the load circuit of Figure 1 and defined as the time required for the output to cross the V OL or VOH limits. 5 This specification only comes into play if CS goes low while SCLK is low. It is required primarily for interfacing to DSP machines. 6 These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then extrapolated back to remove effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus relinquish times of the part and as such are independent of external bus loading capacitances. 7 RDY returns high after a read of both ADCs. The same data can be read again, if required, while RDY is high, although care should be taken that subsequent reads do not occur close to the next output update.
ISINK (1.6mA WITH DVDD = 5V 100 A WITH DVDD = 3V) TO OUTPUT PIN 50pF ISOURCE (200 A WITH DVDD = 5V 100 A WITH DVDD = 3V)
1.6V
Figure 1. Load Circuit for Timing Characterization
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD7719 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
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AD7719
DIGITAL INTERFACE
As previously outlined, the AD7719's programmable functions are controlled using a set of on-chip registers. Data is written to these registers via the part's serial interface and read access to the on-chip registers is also provided by this interface. All communications to the part must start with a write operation to the Communications Register. After power-on or RESET, the device expects a write to its Communications Register. The data written to this register determines whether the next operation to the part is a read or a write operation and also determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the part starts with a write operation to the Communications Register followed by a write to the selected register. A read operation from any other register on the part (including the output data register) starts with a write operation to the Communications Register followed by a read operation from the selected register. The AD7719's serial interface consists of five signals, CS, SCLK, DIN, DOUT, and RDY. The DIN line is used for transferring data into the on-chip registers while the DOUT line is used for accessing data from the on-chip registers. SCLK is the serial clock input for the device and all data transfers (either on DIN or DOUT) take place with respect to this SCLK signal. The RDY line is used as a status signal to indicate when data is ready to be read from the AD7719's data register. RDY goes low when a new data word is available in the output register of either the main or Aux ADCs. It is reset high when a read operation from the data register is complete. It also goes high prior to the updating of the output register to indicate when not to read from the device
CS
to ensure that a data read is not attempted while the register is being updated. CS is used to select the device. It can be used to decode the AD7719 in systems where a number of parts are connected to the serial bus. Figures 2 and 3 show timing diagrams for interfacing to the AD7719 with CS used to decode the part. Figure 3 is for a read operation from the AD7719's output shift register while Figure 2 shows a write operation to the input shift register. It is possible to read the same data twice from the output register even though the RDY line returns high after the first read operation. Care must be taken, however, to ensure that the read operations have been completed before the next output update is about to take place. The AD7719 serial interface can operate in 3-wire mode by tying the CS input low. In this case, the SCLK, DIN, and DOUT lines are used to communicate with the AD7719 and the status of RDY bits (RDY0 and RDY1) can be obtained by interrogating the STATUS Register. This scheme is suitable for interfacing to microcontrollers. If CS is required as a decoding signal, it can be generated from a port bit. For microcontroller interfaces, it is recommended that the SCLK idles high between data transfers. The AD7719 can also be operated with CS used as a frame synchronization signal. This scheme is suitable for DSP interfaces. In this case, the first bit (MSB) is effectively clocked out by CS since CS would normally occur after the falling edge of SCLK in DSPs. The SCLK can continue to run between data transfers provided the timing numbers are obeyed.
t11
SCLK
t14
t16
t12 t13
DIN MSB
t15
LSB
Figure 2. Write Cycle Timing Diagram
RDY
t3
t10
CS
t4
SCLK
t6
t8
t5 t5A
DOUT MSB
t7
t6 t9
LSB
Figure 3. Read Cycle Timing Diagram
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AD7719
The serial interface can be reset by exercising the RESET input on the part. It can also be reset by writing a series of 1s on the DIN input. If a Logic 1 is written to the AD7719 DIN line for at least 32 serial clock cycles the serial interface is reset. This ensures that in 3-wire systems, if the interface gets lost either via a software error or by some glitch in the system, it can be reset back to a known state. This state returns the interface to where the AD7719 is expecting a write operation to its Communications Register. This operation resets the contents of all registers to their power-on reset values. Some microprocessor or microcontroller serial interfaces have a single serial data line. In this case, it is possible to connect the AD7719's DATA OUT and DATA IN lines together and connect them to the single data line of the processor. A 10 k pull-up resistor should be used on this single data line. In this case, if the interface gets lost, because the read and write operations share the same line the procedure to reset it back to a known state is somewhat different than previously described. It requires a read operation of 24 serial clocks followed by a write operation where a Logic 1 is written for at least 32 serial clock cycles to ensure that the serial interface is back into a known state.
PIN CONFIGURATION
IOUT1 IOUT2 AVDD AGND REFIN1(-) REFIN1(+) AIN1 AIN2 AIN3
1 2 3 4 5 6
28 27 26 25
XTAL1 XTAL2 DVDD DGND DIN
AD7719
24
TOP VIEW 23 DOUT 7 (Not to Scale) 22 RDY
8 9 21 20 19 18 17 16 15
CS SCLK RESET P1/SW1 PWRGND P2/SW2 P3
AIN4 10 AIN5 11 AIN6 12 REFIN2 13 P4 14
PIN FUNCTION DESCRIPTIONS
Pin No. 1 2 3 4 5 6
Mnemonic IOUT1 IOUT2 AVDD AGND REFIN1(-) REFIN1(+)
Function Output for Internal 200 A Excitation Current Source. Either current source IEXC1 or IEXC2 or both can be switched to this output. Output for Internal 200 A Excitation Current Source. Either IEXC1 or IEXC2 current source or both can be switched to this output. Analog Supply Voltage. Analog Ground. Negative Reference Input for Main ADC Channel. This reference input can lie anywhere between AGND and AVDD - 1 V. Positive Reference Input for Main ADC Channel. REFIN1(+) can lie anywhere between AVDD and AGND + 1 V. The nominal reference voltage (REFIN1(+) - REFIN1(-)) is 2.5 V, but the part is functional with a reference range from 1 V to AVDD. Analog Input. AIN1 is dedicated to the main channel. Analog Input. AIN2 is dedicated to the main channel. Analog Input. AIN3 can be multiplexed to either the main or auxiliary channel Analog Input. AIN4 can be multiplexed to either the main or auxiliary channel. Analog Input. AIN5 is dedicated to the auxiliary channel and is referenced to AIN6 or AGND Analog Input. AIN6 is dedicated to the auxiliary channel. It forms a differential input pair with AIN5 in fully differential input mode or is referenced to AGND in pseudo-differential mode. Single-Ended Reference Input for Auxiliary Channel. The nominal input reference is 2.5 V. The auxiliary channel will function with an input reference range from 1 V to AVDD. General-Purpose I/O Bit. The input and output voltage levels are referenced to AVDD and AGND. General-Purpose I/O Bit. The input and output voltage levels are referenced to AVDD and AGND. -9-
7 8 9 10 11 12 13 14 15 REV. 0
AIN1 AIN2 AIN3 AIN4 AIN5 AIN6 REFIN2 P4 P3
AD7719
PIN FUNCTION DESCRIPTIONS (continued)
Pin No. 16 17 18 19 20
Mnemonic P2/SW2 PWRGND P1/SW1 RESET SCLK
Function Dual-Purpose Pin. It can act as a general purpose output (P2) bit referenced between AVDD and AGND or as a low-side power switch (SW2) to PWRGND. Ground Point for the Low-Side Power Switches SW2 and SW1. PWRGND must be tied to AGND. Dual-Purpose Pin. It can act as a general-purpose output (P1) bit referenced between AVDD and AGND or as a low-side power switch (SW1) to PWRGND. Digital Input Used to Reset the ADC to Its Power-On-Reset Status. This pin has a weak pull-up internally to DVDD. Serial Clock Input for Data Transfers to and from the ADC. The SCLK has a Schmitt triggered input making the interface suitable for opto-isolated applications. The serial clock can be continuous with all data transmitted in a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information being transmitted to or from the AD7719 in smaller batches of data. A weak pull-up to DVDD is provided on the SCLK input. Chip Select Input. This is an active low logic input used to select the AD7719. CS can be used to select the AD7719 in systems with more than one device on the serial bus or as a frame synchronization signal in communicating with the device. CS can be hardwired low allowing the AD7719 to be operated in 3-wire mode with SCLK, DIN and DOUT used to interface with the device. A weak pull-up to DVDD is provided on the CS input. RDY is a Logic Low Status Output from the AD7719. RDY is low if either the Main ADC or auxiliary ADC channel has valid data in their data register. This output returns high on completion of a read operation from the data register. If data is not read, RDY will return high prior to the next update indicating to the user that a read operation should not be initiated. The RDY pin also returns low following the completion of a calibration cycle. The RDY pin is effectively the digital NOR function of the RDY0 and RDY1 bits in the Status register. If one of the ADCs is disabled the RDY pin reflects the active ADC. RDY does not return high after a calibration until the mode bits are written to enabling a new conversion or calibration. Since the RDY pin provides information on both the main and aux ADCs, therefore, when either the main or aux ADC is disabled it is recommended to immediately read its data register to ensure that its RDY bit goes inactive and releases the RDY pin to indicate output data updates on the remaining active ADC. Serial Data Output Accessing the Output Shift Register of the AD7719. The output shift register can contain data from any of the on-chip data, calibration or control registers. Serial Data Input Accessing the Input Shift Register on the AD7719. Data in this shift register is transferred to the calibration or control registers within the ADC depending on the selection bits of the Communications register. A weak pull-up to DVDD is provided on the DIN input. Ground Reference Point for the Digital Circuitry. Digital Supply Voltage, 3 V or 5 V Nominal. Output from the 32 kHz Crystal Oscillator Inverter. Input to the 32 kHz Crystal Oscillator Inverter.
21
CS
22
RDY
23 24
DOUT DIN
25 26 27 28
DGND DVDD XTAL2 XTAL1
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Typical Performance Characteristics- AD7719
8389600 8389400
26
24
NO MISSING CODES - Min
0 100 200 300 400 500 600 READING NO. 700 800 900 1000 8389200 8389000 8388800 8388600 8388400 8388200 8388000
CODE READ
22
20
18
16 0 10 20 30 40 50 60 70 80 90 100 110 UPDATE RATE - Hz
AVDD = DVDD = 5V INPUT RANGE = 20mV REFIN1(+)-REFIN1(-)=2.5V UPDATE RATE=19.79Hz
MAIN ADC IN BUFFERED MODE RMS NOISE = 0.58 Vrms TA = 25 C VREF = 2.5V
TPC 1. Typical Noise Plot on 20 mV Input Range with 19.79 Hz Update Rate
TPC 4. No-Missing-Codes Performance
9 8 7
1400 THE AMBIENT TEMPERATURE VARIES FROM 25 C TO 30 C WHILE RECORDING THE DATA FROM THE DEVICES.
1200
1000
6
4 3
HITS
5
800
600
400
2
200
1
8388382 8388449 8388499 8388547 8388579 8388615
8388779 8388805 8388841 8388874 8388906 8388941
8388687 8388721
8388039
8388985 8389033
8388657
8388754
8389110
0
0
10
20 30 40 TEMPERATURE SENSOR - C
50
TPC 2. Noise Distribution Histogram
TPC 5. Temperature Sensor Accuracy
3.0 1200 2.5 2.56V RANGE 1000 RMS NOISE - V 2.0 800 1.5 AVDD = DVDD = 5V VREF = 2.5V INPUT RANGE = 2.56V UPDATE RATE = 19.79Hz TA = 25 C
HITS
20mV RANGE 3.0 3.5 4.0 4.5 5.0
600
1.0
400
0.5
200
0 1.0
1.5
2.0
2.5
0 -30
-20
VREF - Volts
-10 0 MAIN CAL ACC. @ 4V - V
10
20
TPC 3. RMS Noise vs. Reference Input
TPC 6. Full-Scale Error Distribution
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AD7719
AVDD = DVDD = 5V TA = 25 C
VDD
OSCILLATOR
TIME BASE = 100ms/DIV TRACE 1 = TRACE 2 = 2V/DIV
TPC 7. Typical Oscillator Power-Up
DUAL-CHANNEL ADC CIRCUIT INFORMATION
Overview The AD7719 incorporates two independent - ADC channels (main and auxiliary) with on-chip digital filtering intended for the measurement of wide dynamic range, low frequency signals such as those in weigh-scale, strain-gauge, pressure transducer, or temperature measurement applications.
Main Channel
train whose duty cycle contains the digital information. A Sinc3 programmable low-pass filter is then employed to decimate the modulator output data stream to give a valid data conversion result at programmable output rates from 5.35 Hz (186.77 ms) to 105.0 3 Hz (9.52 ms). A chopping scheme is also employed to minimize ADC channel offset errors. A block diagram of the Main ADC input channel is shown in Figure 4. The sampling frequency of the modulator loop is many times higher than the bandwidth of the input signal. The integrator in the modulator shapes the quantization noise (which results from the analog-todigital conversion) so that the noise is pushed toward one-half of the modulator frequency. The output of the sigma-delta modulator feeds directly into the digital filter. The digital filter then band-limits the response to a frequency significantly lower than one-half of the modulator frequency. In this manner, the 1-bit output of the comparator is translated into a bandlimited, low noise output from the AD7719 ADC. The AD7719 filter is a low-pass, Sinc3 or (SIN(x)/x)3 filter whose primary function is to remove the quantization noise introduced at the modulator. The cutoff frequency and decimated output data rate of the filter are programmable via the SF word loaded to the filter register. A chopping scheme is employed where the complete signal chain is chopped, resulting in excellent dc offset and offset drift specifications, and is extremely beneficial in applications where drift, noise rejection, and optimum EMI rejection are important factors. With chopping the ADC repeatedly reverses its inputs. The decimated digital output words from the Sinc3 filters therefore have a positive offset and negative offset term included. As a result, a final summing stage is included so that each output word from the filter is summed and averaged with the previous filter output to produce a new valid output result to be written to the ADC data register.
Auxiliary Channel
This channel is intended to convert the primary sensor input. This channel can be operated in buffered or unbuffered mode and can be programmed to have one of eight input voltage ranges from 20 mV to 2.56 V. This channel can be configured as either two fully differential inputs (AIN1/AIN2 and AIN3/AIN4) or three pseudo-differential input channels (AIN1/AIN4, AIN2/ AIN4, and AIN3/AIN4). Buffering the input channel means that the part can accommodate significant source impedances on the analog input and that R, C filtering (for noise rejection or RFI reduction) can be placed on the analog inputs if required. Operating in unbuffered mode leads to lower power consumption in low power applications, but care must be exercised in unbuffered mode as source impedances can introduce gain errors. The main ADC also features sensor burnout currents that can be switched on and off. These currents can be used to check that a transducer is still operational before attempting to take measurements. The ADC employs a sigma-delta conversion technique to realize up to 24 bits of no-missing-codes performance. The sigma-delta modulator converts the sampled input signal into a digital pulse
fCHOP fIN fMOD fCHOP
The Auxiliary (Aux) channel is intended to convert supplementary inputs such as from a cold junction diode or thermistor. This channel is unbuffered and has an input range of REFIN2 or REFIN2/2 determined by the ARN bit in the auxiliary ADC control register (AD1CON). AIN3 and AIN4 can be multiplexed into the auxiliary channel as single ended inputs with respect to AGND while AIN5 and AIN6 can operate as a differential input pair or with AIN6 tied to AGND, AIN5 can be operated as an additional single-ended input. A block diagram of the Auxiliary ADC channel is shown in Figure 5.
fADC
ANALOG INPUT
S-D MUX BUF PGA MOD0 XOR
(8 1 SF )
3
3
(8
SF )
1 2
DIGITAL OUTPUT
SINC3 FILTER AIN + V OS AIN - V OS
Figure 4. Main ADC Channel Block Diagram
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AD7719
fCHOP fMOD fCHOP fADC
S-D MUX MOD1 XOR
ANALOG INPUT
(8 1 SF )
3
3
(8
SF )
1 2
DIGITAL OUTPUT
SINC3 FILTER AIN + VOS AIN - VOS
Figure 5. Auxiliary ADC Channel Block Diagram
Both Channels
The operation of the AUX channel is identical to the Main channel with the exception that there is no PGA on the AUX channel. The input chopping is incorporated into the input multiplexer while the output chopping is accomplished by an XOR gate at the output of the modulator. The chopped modulator bit stream is applied to a Sinc3 filter. The programming of the Sinc3 decimation factor is restricted to an 8-bit register SF, the actual decimation factor is the register value times 8. The decimated output rate from the Sinc3 filter (and the ADC conversion rate) will therefore be: f ADC = where fADC is the ADC update rate. SF is the decimal equivalent of the word loaded to the filter register. fMOD is the modulator sampling rate of 32.768 kHz. Programming the filter register determines the update rate for both the main and aux ADC. Both ADCs operate with the same update rate. The chop rate of the channel is half the output data rate. The frequency response of the filter H (f) is as follows:
1 sin (2 x x f / fOUT ) 1 sin (SF x 8 x x f / f MOD ) x x x sin ( x f / f MOD ) sin ( x f / fOUT ) 2 SF x 8
3
at odd integer multiples of fADC/2. The 3 dB frequency for all values of SF obeys the following equation: f (3 dB) = 0.24 x fADC The signal chain is chopped as shown in Figures 4 and 5. The chop frequency is:
f fCHOP = ADC 2
As shown in the block diagram, the Sinc3 filter outputs alternately contain +VOS and -VOS, where VOS is the respective channel offset. This offset is removed by performing a running average of 2. This average by 2 means that the settling time to any change in programming of the ADC will be twice the normal conversion time, while an asynchronous step change on the analog input will not be fully reflected until the third subsequent output.
2 tSETTLE = = 2 x t ADC f ADC
11 x x f MOD 3 8 x SF
The allowable range for SF is 13 to 255, with a default of 69 (45H). The corresponding conversion rates, conversion times and settling times are tabulated in Table I. Note that the conversion time increases by 0.732 ms for each increment in SF.
Table I. ADC Conversion and Settling Times for Various SF Words
where fMOD = 32,768 Hz SF = value programmed into SF SFR. fOUT = fMOD/(SF x 8 x 3) The following shows plots of the filter frequency response for the SF words shown in Table I. The overall frequency response is the product of a Sinc3 and a sinc response. There are Sinc3 notches at integer multiples of 3 x fADC and there are sinc notches
SF Word 13 69 (Default) 255
Data Update Rate fADC (Hz) 105.3 19.79 5.35
Settling Time tSETTLE (ms) 19.04 101.07 373.54
Normal-mode rejection is the major function of the digital filter on the AD7719. The normal mode 50 1 Hz rejection with an SF word of 82 is typically -100 dB. The 60 1 Hz rejection with SF = 68 is typically -100 dB. Simultaneous 50 Hz and 60 Hz rejection of better than 60 dB is achieved with an SF of 69. Choosing an SF word of 69 places notches at both 50 Hz and 60 Hz. Figures 6 to 9 show the filter rejection for a selection of SF words.
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AD7719
0 -20 -40
0 -20 -40
-60
ATTENUATION - dB
ATTENUATION - dB
-80 -100 -120 -140 -160 -180 -200 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 FREQUENCY - Hz SF = 13 OUTPUT DATA RATE = 105Hz INPUT BANDWIDTH = 25.2Hz FIRST NOTCH = 52.5Hz 50Hz REJECTION = -23.6dB, 50Hz 1Hz REJECTION = -20.5dB 60Hz REJECTION = -14.6dB, 60Hz 1Hz REJECTION = -13.6dB
-60 -80 -100 -120 -140 -160 0 10 20 30 40 50 60 70 80 90 100 FREQUENCY - Hz SF = 69 OUTPUT DATA RATE = 19.8Hz INPUT BANDWIDTH = 4.74Hz FIRST NOTCH = 9.9Hz 50Hz REJECTION = -66dB, 50Hz 1Hz REJECTION = -60dB 60Hz REJECTION = -117dB, 60Hz 1Hz REJECTION = -94dB
Figure 6. Filter Profile with SF = 13
0 -20 -40
Figure 8. Filter Profile with Default SF = 69 Giving Filter Notches at Both 50 Hz and 60 Hz
0 -20 -40
ATTENUATION - dB
ATTENUATION - dB
-60 -80 -100 -120 -140 -160 0 10 20 30 40 50 60 70 80 90 100 FREQUENCY - Hz SF = 82 OUTPUT DATA RATE = 16.65Hz INPUT BANDWIDTH = 4Hz 50Hz REJECTION = -171dB, 50Hz 1Hz REJECTION = -100dB 60Hz REJECTION = -58dB, 60Hz 1Hz REJECTION = -53dB
-60 -80 -100 -120 -140 -160 0 10 20 30 40 50 60 70 80 90 100 FREQUENCY - Hz SF = 255 OUTPUT DATA RATE = 5.35Hz INPUT BANDWIDTH = 1.28Hz 50Hz REJECTION = -93dB, 50Hz 1Hz REJECTION = -93dB 60Hz REJECTION = -74dB, 60Hz 1Hz REJECTION = -68dB
Figure 7. Filter Profile with SF = 82
Figure 9. Filter Profile with SF = 255
MAIN AND AUXILIARY ADC NOISE PERFORMANCE
Tables II to VII show the output rms noise and output peak-topeak resolution in bits (rounded to the nearest 0.5 LSB) for a selection of output update rates on both the main and auxiliary ADCs. The numbers are typical and generated at a differential input voltage of 0 V. The output update rate is selected via the SF7-SF0 bits in the Filter Register. It is important to note that the peak-to-peak resolution figures represent the resolution for which there will be no code flicker within a six-sigma limit. The output noise comes from two sources. The first is the electrical
noise in the semiconductor devices (device noise) used in the implementation of the modulator. Secondly, when the analog input is converted into the digital domain, quantization noise is added. The device noise is at a low level and is independent of frequency. The quantization noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. The numbers in the tables are given for the bipolar input ranges. For the unipolar ranges the rms noise numbers will be the same as the bipolar range, but the peak-topeak resolution is now based on half the signal range, which effectively means losing 1 bit of resolution.
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AD7719
Table II. Typical Output RMS Noise vs. Input Range and Update Rate for Main ADC (Buffered Mode) Output RMS Noise in V
SF Word 13 69 255
Data Update Rate (Hz) 105.3 19.79 5.35
20 mV 1.50 0.60 0.35
40 mV 1.50 0.65 0.35
80 mV 1.60 0.65 0.37
Input Range 160 mV 320 mV 1.75 0.65 0.37 3.50 0.65 0.37
640 mV 4.50 0.95 0.51
1.28 V 6.70 1.40 0.82
2.56 V 11.75 2.30 1.25
Table III. Peak-to-Peak Resolution vs. Input Range and Update Rate for Main ADC (Buffered Mode) Peak-to-Peak Resolution in Bits
SF Word 13 69 255
Data Update Rate (Hz) 105.3 19.79 5.35
20 mV 12 13 14
40 mV 13 14 15
80 mV 14 15 16
Input Range 160 mV 320 mV 15 16 17 15 17 18
640 mV 15.5 17.5 18.5
1.28 V 16 18 18.8
2.56 V 16 18.5 19.2
Table IV. Typical Output RMS Noise vs. Input Range and Update Rate for Main ADC (Unbuffered Mode) Output RMS Noise in V
SF Word 13 69 255
Data Update Rate (Hz) 105.3 19.79 5.35
20 mV 1.27 0.52 0.30
40 mV 1.27 0.56 0.30
80 mV 1.35 0.56 0.32
Input Range 160 mV 320 mV 1.48 0.56 0.32 2.95 0.56 0.32
640 mV 3.82 0.82 0.44
1.28 V 5.69 1.21 0.71
2.56 V 10.2 2.00 1.10
Table V. Peak-to-Peak Resolution vs. Input Range and Update Rate for Main ADC (Unbuffered Mode) Peak-to-Peak Resolution in Bits
SF Word 13 69 255
Data Update Rate (Hz) 105.3 19.79 5.35
20 mV 12 13 14
40 mV 13 14 15
80 mV 14 15 16
Input Range 160 mV 320 mV 15 16 17 15 17 18
640 mV 15.5 17.5 18.5
1.28 V 16 18 19
2.56 V 16 18.5 19.5
Table VI. Typical Output RMS Noise vs. Update Rate for Auxiliary ADC Unbuffered)
Table VII. Peak-to-Peak Resolution vs. Update Rate for Auxiliary ADC (Unbuffered Mode)
SF Word 13 69 255
Data Update Rate (Hz) 105.3 19.79 5.35
Input Range 2.5 V 10.75 V 2.00 V 1.15 V
SF Word 13 69 255
Data Update Rate (Hz) 105.3 19.79 5.35
Input Range 2.5 V 16 Bits 16 Bits 16 Bits
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AD7719
ON-CHIP REGISTERS
Both the main and auxiliary ADC channels are controlled and configured via a number of on-chip registers as shown in Figure 10 and described in more detail in the following pages. In the following descriptions, SET implies a Logic 1 state and CLEARED implies a Logic 0 state, unless otherwise stated.
COMMUNICATIONS REGISTER DIN WEN R/W 0 0 A3 A2 A1 A0
DOUT
DOUT
ADC STATUS REGISTER
DIN DOUT MODE REGISTER
DIN DOUT MAIN ADC CONTROL REGISTER
DIN DOUT AUX ADC CONTROL REGISTER
DIN DOUT I/O CONTROL REGISTER
DIN DOUT FILTER REGISTER REGISTER SELECT DECODER
DOUT
MAIN ADC DATA REGISTER
DOUT
AUX ADC DATA REGISTER DIN
DOUT
MAIN ADC OFFSET REGISTER DIN
DOUT
AUX ADC OFFSET REGISTER DIN
DOUT
MAIN ADC GAIN REGISTER DIN
DOUT
AUX ADC GAIN REGISTER
DOUT
ID REGISTER
DIN DOUT TEST REGISTER
Figure 10. On-Chip Registers
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AD7719
Table VIII. Registers--Quick Reference Guide
Register Name Communications
MSB
WEN W R/W
Type Write Only
Size 8 Bits
Power-On/Reset Default Value Not Applicable
LSB
Function All operations to other registers are initiated through the Communications Register. This controls whether subsequent operations are read or write operations and also selects the register for that subsequent operation. Provides status information on conversions, calibrations, error conditions, and the validity of the reference voltage.
0
0
A3
A2
A1
A0
Status Register
MSB
RDY0 RDY1
Read Only
8 Bits
00 Hex
LSB
CAL
NOREF
ERR0
ERR1
0
LOCK
Mode Register
MSB
0 BUF 0
Read/Write
8 Bits
00 Hex
LSB
Controls functions such as mode of operation, channel configuration, oscillator operation in power-down.
CHCON
OSCPD
MD2
MD1
MD0
Main ADC (AD0CON) Control Register
MSB
AD0EN WL CH1 CH0 U/B RN2 RN1
Read/Write
8 Bits
07 Hex
LSB
RN0
This register is used to enable the Main ADC and configure the Main ADC for range, channel selection, 16-/24-bit operation, and unipolar or bipolar operation.
Aux ADC (AD1CON) Control Register
MSB
AD1EN ACH2 ACH1 ACH0 U/B 0 0
Read/Write
8 Bits
01 Hex
LSB
ARN
This register is used to enable the aux ADC and configure the Aux ADC for range, channel selection, unipolar or bipolar operation, and input range.
I/O (IOCON) Control Register
MSB
PSW2 PSW1 0 BO I2PIN I1PIN I2EN I1EN
Read/Write
16 Bits
0000 Hex
This register is used to control and configure the various excitation and burnout current source options available on-chip along with controlling the I/O port.
LSB
P4DIR P3DIR P2EN P1EN P4DAT P3DAT P2DAT P1DAT
Filter Register
MSB SF7 SF6
Read/Write
8 Bits
45 Hex
LSB
SF5
SF4
SF3
SF2
SF1
SF0
This register determines the amount of averaging performed by the sinc filter and consequently determines the data update rate of the AD7719. The filter register determines the update rate for both the main and aux ADCs.
REV. 0
-17-
AD7719
Register Name Main ADC (DATA0) Data Register Read Only 16 Bits or 24 Bits 000000 Hex Provides the most up-to-date conversion result from the Main ADC. Main ADC data register length can be programmed to be 16-bit or 24-bit. Type Size Power-On/Reset Default Value Function
Aux ADC (DATA1) Data Register Read Only 16 Bits 0000 Hex Provides the most up-to-date conversion result from the auxiliary ADC. Aux ADC data register length is 16 bits.
Main ADC Offset Register Read/Write 24 Bits 800000 Hex Contains a 24-bit word that is the offset calibration coefficient for the part. The contents of this register are used to provide offset correction on the output from the digital filter. There are three Offset Registers on the part and these are associated with input channel pairs as outlined in the AD0CON register. Contains a 24-bit word that is the gain calibration coefficient for the part. The contents of this register are used to provide gain correction on the output from the digital filter. There are three Gain Registers on the part and these are associated with input channel pairs as outlined in the AD0CON register. Contains a 16-bit word that is the offset calibration coefficient for the part. The contents of this register are used to provide offset correction on the output from the digital filter. Contains a 16-bit word that is the gain calibration coefficient for the part. The contents of this register are used to provide gain correction on the output from the digital filter. Contains an 8-bit byte that is the identifier for the part. Controls the test modes of the part which are used when testing the part. The user is advised not to change the contents of these registers.
Main ADC Gain Register Read/Write 24 Bits 5XXXX5 Hex
Aux ADC Offset Register Read/Write 16 Bits 8000 Hex
Aux ADC Gain Register Read/Write 24 Bits 59XX Hex
ID Register
Read
8 Bits
0X Hex
Test Registers
Read/Write
16 Bits
0000 Hex
-18-
REV. 0
AD7719
Communications Register (A3, A2, A1, A0 = 0, 0, 0, 0)
The Communications Register is an 8-bit write-only register. All communications to the part must start with a write operation to the Communications Register. The data written to the Communications Register determines whether the next operation is a read or write operation, and to which register this operation takes place. For read or write operations, once the subsequent read or write operation to the selected register is complete, the interface returns to where it expects a write operation to the Communications Register. This is the default state of the interface, and on power-up or after a RESET, the AD7719 is in this default state waiting for a write operation to the Communications Register. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high, returns the AD7719 to this default state by resetting the part. Table IX outlines the bit designations for the Communications Register. CR0 through CR7 indicate the bit location, CR denoting the bits are in the Communications Register. CR7 denotes the first bit of the data stream.
CR7 WEN (0) CR6 R/W (0) CR5 0 (0) CR4 0 (0) CR3 A3 (0) CR2 A2 (0) CR1 A1 (0) CR0 A0 (0)
Table IX. Communications Register Bit Designations
Bit Location CR7
Bit Name WEN
Description Write Enable Bit. A 0 must be written to this bit so the write operation to the Communications Register actually takes place. If a 1 is written to this bit, the part will not clock on to subsequent bits in the register. It will stay at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits will be loaded to the Communications Register. A zero in this bit location indicates that the next operation will be a write to a specified register. A one in this position indicates that the next operation will be a read from the designated register. A zero must be written to this bit position to ensure correct operation of the AD7719. A zero must be written to this bit position to ensure correct operation of the AD7719. Register Address Bits. These address bits are used to select which of the AD7719's registers are being accessed during this serial interface communication. A3 is the MSB of the three selection bits.
Table X. Register Selection Table
CR6 CR5 CR4 CR3-CR0
R/W Zero Zero A3-A0
A3 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
A2 0 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1
A1 0 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1
A0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
Register Communications Register during a Write Operation Status Register during a Read Operation Mode Register Main ADC Control Register (AD0CON) Aux ADC Control Register (AD1CON) Filter Register Main ADC Data Register Aux ADC Data Register I/O Control Register Main ADC Offset Calibration Register Aux ADC Offset Calibration Register Main ADC Gain Calibration Register Aux ADC Gain Calibration Register Test 1 Register Test 2 Register Undefined ID Register
REV. 0
-19-
AD7719
Status Register (A3, A2, A1, A0 = 0, 0, 0, 0; Power-On Reset = 00 Hex)
The ADC Status Register is an 8-bit read-only register. To access the ADC Status Register, the user must write to the Communications Register selecting the next operation to be a read and load bits A3-A0 with 0, 0, 0, 0. Table XI outlines the bit designations for the Status Register. SR0 through SR7 indicate the bit location, SR denoting the bits are in the Status Register. SR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
SR7 RDY0 (0) SR6 RDY1 (0) SR5 CAL (0) SR4 NOXREF (0) SR3 ERR0 (0) SR2 ERR1 (0) SR1 (0) SR0 LOCK (0)
Table XI. Status Register Bit Designations
Bit Location SR7
Bit Name RDY0
Description Ready Bit for Main ADC. Set when data is written to Main ADC data registers or on completion of calibration cycle. The RDY0 bit is cleared automatically after the Main ADC data register has been read or a period of time before the data register is updated with a new conversion result. This bit is also cleared by a write to the mode bits to indicate a conversion or calibration. Ready Bit for Aux ADC. Set when data is written to Aux ADC data registers or on completion of calibration cycle. The RDY1 bit is cleared automatically after the Aux ADC data register has been read or a period of time before the data register is updated with a new conversion result. This bit is also cleared by a write to the mode bits to indicate a conversion or calibration. Calibration Status Bit. Set to indicate completion of calibration. It is set at the same time that the RDY0 and/or RDY1 bits are set high. Cleared by a write to the mode bits to start another ADC conversion or calibration. No External Reference Bit. (Only active if Main ADC is active and applies to REFIN1 only.) Set to indicate that one or both of the REFIN1 pins is floating or the applied voltage is below a specified threshold. When Set, conversion results are clamped to all ones. Cleared to indicate valid Reference applied between REFIN1(+) and REFIN1(-). Main ADC Error Bit. Set to indicate that the result written to the Main ADC data registers has been clamped to all zeros or all ones. After a calibration this bit also flags error conditions that caused the calibration registers not to be written. Error sources include Overrange, Underrange and NOXREF. Cleared by a write to the mode bits to initiate a conversion or calibration. Aux ADC Error Bit. Set to indicate that the result written to the Aux ADC data registers has been clamped to all zeros or all ones. After a calibration this bit also flags error conditions that caused the calibration registers not to be written. Error sources include Overrange, Underrange, and NOXREF. Cleared by a write to the mode bits to initiate a conversion or calibration. Reserved for Future Use. PLL Lock Status Bit. Set if the PLL has locked onto the 32 kHz crystal oscillator clock. If the user is worried about exact sampling frequencies etc., the LOCK bit should be interrogated and the result discarded if the LOCK bit is zero.
SR6
RDY1
SR5
CAL
SR4
NOXREF
SR3
ERR0
SR2
ERR1
SR1 SR0
0 LOCK
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REV. 0
AD7719
Mode Register (A3, A2, A1, A0 = 0, 0, 0, 1; Power-On-Reset = 00 Hex)
The Mode Register is an 8-bit register from which data can be read or to which data can be written. This register configures the operating modes of the AD7719. Table XII outlines the bit designations for the Mode Register. MR7 through MR0 indicate the bit location, MR denoting the bits are in the Mode Register. MR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
MR7 0 (0) MR6 BUF (0) MR5 0 (0) MR4 MR3 MR2 MD2 (0) MR1 MD1 (0) MR0 MD0 (0)
CHCON (0) OSCPD (0)
Table XII. MODE Register Bit Designations
Bit Location MR7 MR6
Bit Name 0 BUF
Description Reserved for Future Use. Configures the Main ADC for buffered or unbuffered mode of operation. If set, the Main ADC operates in unbuffered mode, lowering the power consumption of the AD7719. If cleared, the Main ADC operates in buffered mode allowing the user to place source impedances on the front end without contributing gain errors to the system. Reserved for Future Use. Channel Configure Bit. If this bit is set, the Main ADC operates with three pseudo-differential input channels and the AUX ADC does not have AIN3/AIN4 as an input option. If cleared, the Main ADC operates with two fully differential input channels and the aux channel operates as one fully differential input and two single-ended inputs or three single-ended inputs. Oscillator Power-Down Bit. If this bits is set, placing the AD7719 in standby mode will stop the crystal oscillator, reducing the power drawn by the AD7719 to a minimum. The oscillator will require 300 ms to begin oscillating when the ADC is taken out of standby mode. If this bit is cleared, the oscillator is not shut off when the ADC is put into standby mode and will not require the 300 ms start-up time when the ADC is taken out of standby. Main and Aux ADC Mode Bits. These bits select the operational mode of the enabled ADC as follows: Power-Down Mode (Power-On Default). The current sources, power switches, and PLL are shut off in power-down mode. Idle Mode. In Idle Mode the ADC filter and modulator are held in a reset state although the modulator clocks are still provided. Single Conversion Mode. In Single Conversion Mode, a single conversion is performed on the enabled channels. On completion of the conversion the ADC data registers are updated, the relevant flags in the STATUS register are written and idle mode is entered with the MD2-MD0 being written accordingly to 001. Continuous Conversion. In continuous conversion mode the ADC data registers are regularly updated at the selected update rate (see Filter register) Internal Zero-Scale Calibration. Internal short automatically connected to the enabled channel(s). Returns to Idle mode (001) when complete. Internal Full-Scale Calibration. External VREF is connected automatically to the ADC input for this calibration. Returns to idle mode when complete. System Zero-Scale Calibration. User should connect system zero-scale input to the channel input pins as selected by CH1/CH0 and ACH1/ACH0 bits in the control registers. System Full-Scale Calibration. User should connect system full-scale input to the channel input pins as selected by CH1/CH0 and ACH1/ACH0 bits in the control registers. -21-
MR5 MR4
0 CHCON
MR3
OSCPD
MR2-MR0 MD2 0 0
MD2-MD0 MD0 0 1
MD1 0 0
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
REV. 0
AD7719
Operating Characteristics when Addressing the Mode and Control Registers
1. Any change to the MD bits will immediately reset both ADCs. A write to the MD2-0 bits with no change is also treated as a reset. (See exception to this in Note 3.) 2. If AD0CON is written when AD0EN = 1, or if AD0EN is changed from 0 to 1, both ADCs are also immediately reset. In other words, the Main ADC is given priority over the Aux ADC and any change requested on main is immediately responded to. 3. On the other hand, if AD1CON is written to only the Aux ADC is reset. For example, if the Main ADC is continuously converting when the Aux ADC change or enable occurs, the Main ADC continues undisturbed. Rather than allow the Aux ADC to operate with a phase difference from the Main ADC, the Aux ADC will fall into step with the outputs of the Main ADC. The result is that the first conversion time for the Aux channel will be delayed up to three outputs while the Aux ADC update rate is synchronized to the Main ADC. 4. Once the MODE has been written with a calibration mode, the RDY0/1 bits (STATUS) are immediately reset and the calibration commences. On completion the appropriate calibration registers are written, the relevant bits in STATUS are written, and the MD2-0 bits are reset to 001 to indicate the ADC is back in Idle Mode. 5. Any calibration request of the Aux ADC while the temperature sensor is selected will fail to complete. 6. Calibrations are performed with the maximum allowable SF value. SF register is reset to user configuration after calibration.
Main ADC Control Register (AD0CON): (A3, A2, A1, A0 = 0, 0, 1, 0; Power-On Reset = 07 Hex)
The Main ADC control Register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the Main ADC for range, channel selection, 16-/24-bit operation and unipolar or bipolar coding. Table XIII outlines the bit designations for the Main ADC control register. AD0CON7 through AD0CON0 indicate the bit location, AD0CON denoting the bits are in the Main ADC Control Register. AD0CON7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
AD0CON7 AD0EN (0) AD0CON6 WL (0) AD0CON5 CH1 (0) AD0CON4 CH0 (0) AD0CON3 U/B (0) AD0CON2 RN2 (1) AD0CON1 RN1 (1) AD0CON0 RN0 (1)
Table XIII. Main ADC Control Register (AD0CON) Bit Designations
Bit Location AD0CON7
Bit Name AD0EN
Description Main ADC Enable Bit. Set by user to enable the Main ADC. When set the Main ADC operates according to the MD bits in the mode register. Cleared by the user to power down the Main ADC. 16-/24-Bit Operating Mode. Set by user to enable 16-bit mode. The conversion results from the Main ADC will be rounded to 16 bits and the Main ADC data register will be 16 bits wide. Cleared by user to enable 24-bit mode. The conversion results from the Main ADC will be rounded to 24 bits and the Main ADC data register will be 24 bits wide. Main ADC Channel Selection Bits. Written by the user to select the differential input pairs used by the Main ADC as follows: Note: The CHCON bit resides in the MODE register. CH0 0 1 0 1 0 1 0 1 Positive Input AIN1 AIN3 AIN2 AIN3 AIN1 AIN3 AIN4 AIN2 Negative Input AIN2 AIN4 AIN2 AIN2 AIN4 AIN4 AIN4 AIN4 Calibration Register Pair 0 1 0 1 0 1 0 2
AD0CON6
WL
AD0CON5 AD0CON4 CHCON 0 0 0 0 1 1 1 1
CH1 CH0 CH1 0 0 1 1 0 0 1 1
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AD7719
Table XIII (continued)
AD0CON3
U/B
Main ADC Unipolar/Bipolar Bit. Set by user to enable unipolar coding i.e., Zero differential input will result in 000000 Hex output and a full-scale differential input will result in FFFFFF Hex output when operated in 24-bit mode. Cleared by user to enable bipolar coding, Negative full-scale differential input will result in an output code of 000000 Hex, Zero differential input will result in an output code of 800000 Hex and a Positive full-scale differential input will result in an output code of FFFFFF Hex.
AD0CON2 AD0CON1 AD0CON0
RN2 RN1 RN0
Main ADC Range Bits. Written by the user to select the Main ADC input range as follows. RN2 0 0 0 0 1 1 1 1 RN1 0 0 1 1 0 0 1 1 RN0 0 1 0 1 0 1 0 1 Selected Main ADC Input Range (VREF = 2.5 V) 20 mV 40 mV 80 mV 160 mV 320 mV 640 mV 1.28 V 2.56 V
Aux ADC Control Registers (AD1CON ): (A3, A2, A1, A0 = 0, 0, 1, 1; Power-On Reset = 01 Hex)
The Aux ADC control Register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the Aux ADC for range, channel selection and unipolar or bipolar coding. Table XIV outlines the bit designations for the Aux ADC control register register. AD1CON7 through AD1CON0 indicate the bit location, AD1CON denoting the bits are in the Aux ADC Control Register. AD1CON7 denotes the first bit of the data stream. The number in brackets indicates the power-on/ reset default status of that bit.
AD1CON7 AD1EN (0) AD1CON6 ACH2 (0) AD1CON5 ACH1 (0) AD1CON4 ACH0 (0) AD1CON3 U/B (0) AD1CON2 0 (0) AD1CON1 0 (0) AD1CON0 ARN (1)
Table XIV. Aux ADC Control Register (AD1CON) Bit Designations
Bit Location AD1CON7
Bit Name AD1EN
Description Aux ADC Enable Bit. Set by user to enable the Aux ADC. When set, the aux ADC operates according to the MD bits in the mode register. Cleared by the user to power-down the aux ADC. Aux ADC Channel Selection Bits. Written by the user to select the active input channels used by the Aux ADC as follows: ACH1 0 0 1 1 0 0 0 1 1 0 0 1 1 ACH0 0 1 0 1 0 0 1 0 1 0 1 0 1 Positive Input AIN3 AIN4 AIN5 Temp Sensor AGND AIN5 AIN6 AIN5 Temp Sensor AGND Not Defined Not Defined Not Defined Negative Input AGND AGND AIN6 (Temp Sensor Routed to the ADC Inputs) AGND (Internal Short) AGND AGND AIN6 (Temp Sensor Routed to the ADC Inputs) AGND (Internal Short)
AD1CON6 AD1CON5 AD1CON4 CHCON 0 0 0 0 0 1 1 1 1 1 X X X
ACH2 ACH1 ACH0 ACH2 0 0 0 0 1 0 0 0 0 1 1 1 1
REV. 0
-23-
AD7719
Table XIV (continued)
AD1CON3
U/B
Aux ADC Unipolar/Bipolar Selection Bit. Set by user to enable unipolar coding i.e., Zero differential input will result in 0000 Hex output. Cleared by user to enable bipolar coding, Zero differential input will result in 8000 Hex output. Must be zero for specified operation. Must be zero for specified operation. Auxiliary Channel Input Range Bit. When set by the user the input range is REFIN2. When cleared by the user the input range is REFIN2/2.
AD1CON2 AD1CON1 AD1CON0
0 0 ARN
NOTES 1. When the temperature sensor is selected, the AD7719 automatically selects its internal reference. The temperature sensor is not factory calibrated. Temp sensor is suitable for relative temperature measurements. The temperature sensor yields conversion results where a conversion result of 8000H equates to typically 0C. 2. A 1C change in temperature will normally result in a 256 LSB change in the AD1 data register (ADC conversion result).
Filter Register (A3, A2, A1, A0 = 0, 1, 0, 0; Power-On Reset = 45 Hex)
The Filter Register is an 8-bit register from which data can be read or to which data can be written. This register determines the amount of averaging performed by the sinc filter. Table XV outlines the bit designations for the Filter Register. FR7 through FR0 indicate the bit location, FR denoting the bits are in the Filter Register. FR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. The number in this register is used to set the decimation factor and thus the output update rate for the Main and Aux ADCs. The filter register cannot be written to by the user while either ADC is active. The update rate is used for both Main and Aux ADCs and is calculated as follows: f ADC = where fADC = ADC Output Update Rate. fMOD = Modulator Clock Frequency = 32.768 kHz (Main and Aux ADC) SF = Decimal Value Written to SF Register.
FR7 SF7 (0) FR6 SF6 (1) FR5 SF5 (0) FR4 SF4 (0) FR3 SF3 (0) FR2 SF2 (1) FR1 SF1 (0) FR0 SF0 (1)
1 1 x x f MOD 3 8 x SF
The allowable range for SF is 13dec to 255dec. Examples of SF values and corresponding conversion rate (fADC) and time (tADC) are shown in Table XV. It should also be noted that both ADC input channels are chopped to minimize offset errors. This means that the time for a single conversion or the time to the first conversion result is 2 x tADC.
Table XV. Update Rate vs. SF WORD
SF (dec)
13 69 255
SF (Hex)
0D 45 FF
fADC (Hz)
105.3 19.79 5.35
tADC (ms)
9.52 50.34 186.77
I/O and Current Source Control Register (IOCON): (A3, A2, A1, A0 = 0, 1, 1, 1; Power-On Reset = 0000 Hex)
The IOCON Register is a 16-bit register from which data can be read or to which data can be written. This register is used to control and configure the various excitation and burnout current source options available on-chip along with controlling the I/O port. Table XVI outlines the bit designations for this register. IOCON15 through IOCON0 indicate the bit location, IOCON denoting the bits are in the I/O and Current Source Control Register. IOCON15 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. A write to the IOCON register has immediate effect and does not reset the ADCs. Thus, if a current source is switched while the ADC is converting the user will have to wait for the full settling time of the filter before getting a fully-settled output. Since the ADC is chopped this equates to three outputs.
IOCON15 PSW2 (0) IOCON7 P4DIR (0) IOCON14 PSW1 (0) IOCON6 P3DIR (0) IOCON13 0 (0) IOCON5 P2EN (0) IOCON12 BO (0) IOCON4 P1EN (0) IOCON11 I2PIN (0) IOCON3 P4DAT (0) IOCON10 I1PIN (1) IOCON2 P3DAT (0) IOCON9 I2EN (0) IOCON1 P2DAT (0) IOCON8 I1EN (0) IOCON0 P1DAT (0)
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AD7719
Table XVI. IOCON (I/O and Current Source Control Register) Bit Designations
Bit Location IOCON15
Bit Name PSW2
Description Power Switch 2 Control Bit. Set by user to enable Power switch P2 to PWRGND. Cleared by user to enable use as a standard I/O pin. When the ADC is in standby mode the power switches are open. Power Switch 1 Control Bit. Set by user to enable Power switch P1 to PWRGND. Cleared by user to enable use as a standard I/O pin. When ADC is in standby mode the power switches are open. This bit must be zero for correct operation. Burnout Current Enable Bit. Set by user to enable the 100 nA current sources in the Main ADC signal path. A 100 nA current source is applied to the positive input leg while a 100 nA sink is applied to the negative input. Cleared by user to disable both transducer burnout current sources. IEXE2, 200 A Current Source Direction Bit. Set by user to enable IEXC2 current source to IOUT1. Cleared by user to enable IEXC2 current source to IOUT2. IEXE1, 200 A Current Source Direction Bit. Set by user to enable IEXC1 current source to IOUT2. Cleared by user to enable IEXC1 current source to IOUT1. IEXC2 Current Source Enable Bit. Set by user to turn on the IEXC2 excitation current source. Cleared by user to turn off the IEXC2 excitation current source. IEXC1 Current Source Enable Bit. Set by user to turn on the IEXC1 excitation current source. Cleared by user to turn off the IEXC1 excitation current source. P4, I/O Direction Control Bit. Set by user to enable P4 as an output. Cleared by user to enable P4 as an input. There are weak active pull-ups internally when enabled as an input. P3, I/O Direction Control Bit. Set by user to enable P3 as an output. Cleared by user to enable P3 as an input. There are weak active pull-ups internally when enabled as an input. P2 Digital Output Enable Bit. Set by user to enable P2 as a regular digital output pin. Cleared by user to three-state P2 output. PSW2 takes precedence over P2EN. P1 Digital Output Enable Bit. Set by user to enable P1 as a regular digital output pin. Cleared by user to three-state P1 output. PSW1 takes precedence over P1EN. Digital I/O Port Data Bits.
IOCON14
PSW1
IOCON13 IOCON12
0 BO
IOCON11
I2PIN
IOCON10
I1PIN
IOCON9
I2EN
IOCON8
I1EN
IOCON7
P4DIR
IOCON6
P3DIR
IOCON5
P2EN
IOCON4
P1EN
IOCON3 IOCON2 IOCON1 IOCON0
P4DAT P3DAT P2DAT P1DAT
The readback value of these bits indicate the status of their respective pin when the I/O port is active as an input. The values written to these data bits appear at the output port when the I/O bits are enabled as outputs. P2 and P1 are outputs only, so reading P2DAT and P1DAT will return what was last written to these bits.
REV. 0
-25-
AD7719
Main ADC Data Result Registers (DATA0): (A3, A2, A1, A0 = 0, 1, 0, 1; Power-On Reset = 000000 Hex)
The conversion results for the Main ADC channel is stored in the Main ADC data register (DATA0). This register is either 16 or 24 bits wide, depending on the status of the WL bit in the Main ADC control register (AD0CON). This is a read only register. On completion of a read from this register the RDY0 bit in the status register is cleared.
Aux ADC Data Result Registers (DATA1): (A3, A2, A1, A0 = 0, 1, 1, 0; Power-On Reset = 0000 Hex)
The conversion results for the Aux ADC channel is stored in the aux ADC data register (DATA1). This register is 16 bits wide and is a read only register. On completion of a read from this register the RDY1 bit in the status register is cleared.
Main ADC Offset Calibration Coefficient Registers (OF0): (A3, A2, A1, A0 = 1, 0, 0, 0; Power-On Reset = 800000 Hex)
The offset calibration registers hold the 24-bit data offset calibration coefficient for the Main ADC. There are three registers associated with the Main ADC channel. In fully-differential operating mode there are two input channels and a register is dedicated to each input. When operating in pseudo-differential mode the Main ADC can be configured for 3 input channels and there is a dedicated register for each pseudo-differential input. These registers have a power-on reset value of 800000H. The channel bits in association with the communication register address for the OF0 register allow access to these registers. These registers are read/write registers. The calibration registers can only be written to if the ADC is inactive (MD bits in the mode register = 000 or 001 or both AD0EN and AD1EN bits in the control registers are cleared). Reading of the calibration registers does not clear the RDY0 bit.
Aux ADC Offset Calibration Coefficient Registers (OF1): (A3, A2, A1, A0 = 1, 0, 0, 1; Power-On Reset = 8000 Hex)
The offset calibration register OF1 holds the 16-bit data offset calibration coefficient for the Aux ADC. This register has a poweron-reset value of 8000 Hex. The channel bits in association with the communication register address for the OF1 register allow access to these registers. These registers are read/write registers. The calibration registers can only be written to if the ADC is inactive (MD bits in the mode register = 000 or 001 or both AD0EN and AD1EN bits in the control registers are cleared). Reading of the calibration registers does not clear the RDY1 bit.
Main ADC Gain Calibration Coefficient Registers (GNO): (A3, A2, A1, A0 = 1, 0, 1, 0; Power-On Reset = 5XXXX5 Hex)
The gain calibration registers hold the 24-bit data gain calibration coefficient for the Main ADC. These registers are configured at power-on with factory calculated internal full-scale calibration coefficients. Every device will have different coefficients. However, these bytes will be automatically overwritten if an internal or system full-scale calibration is initiated by the user via MD2-0 bits in the Mode register. There are three gain calibration registers associated with the Main ADC channel. In fully-differential operating mode there are two input channels and a register is dedicated to each input. When operating in pseudo-differential mode the Main ADC can be configured for three input channels and there is a dedicated register for each pseudo-differential input. These registers are read/write registers. The calibration registers can only be written to if the ADC is inactive (MD bits in the mode register =000 or 001 or both AD0EN and AD1EN bits in the control registers are cleared). Reading of the calibration registers does not clear the RDY1 bit.
Aux ADC Gain Calibration Coefficient Registers (GN1): (A3, A2, A1, A0 = 1, 0, 1, 1; Power-On Reset = 59XX Hex)
The gain calibration register GN1 holds the 16-bit data gain calibration coefficient for the Aux ADC. This register is configured at power-on with factory calculated internal zero-scale calibration coefficients. Every device will have different coefficients. However, these coefficients will be automatically overwritten if an internal or system zero-scale calibration is initiated by the user via the MD2-0 bits in the MODE register. These registers are read/write registers. The calibration registers can only be written to if the ADC is inactive (MD bits in the mode register = 000 or 001 or both AD0EN and AD1EN bits in the control registers are cleared). Reading of the calibration registers does not clear the RDY1 bit.
ID Register (ID): (A3, A2, A1, A0 = 1, 1, 1, 1; Power-On Reset = 0X Hex)
This register is a read-only 8-bit register. The contents are used to determine the die revision of the AD7719. Table XVII indicates the bit locations.
Table XVII. ID Register Bit Designations
ID7 0
ID6 0
ID5 0
ID4 0
ID3 X
ID2 X
ID1 X
ID0 X
User Nonprogrammable Test Registers
The AD7719 contains two test registers. The bits in this test register control the test modes of the AD7719 which are used for the testing of the device. The user is advised not to change the contents of these registers.
-26-
REV. 0
AD7719
CONFIGURING THE AD7719 MICROCOMPUTER/MICROPROCESSOR INTERFACING
All user-accessible registers on the AD7719 are accessed via the serial interface. Communication with any of these registers is initiated by first writing to the Communications Register. Figure 11 outlines a flow diagram of the sequence used to configure all registers after a power-up or reset on the AD7719. The flowchart shows two methods of determining when it is valid to read the data register or determine when a calibration cycle is complete. The first method is hardware polling of the RDY pin and the second method involves software interrogation of bits in the status and mode registers. The flowchart details all the necessary programming steps required to initialize the ADC and read data from the main and aux channel following a power-on or reset. The steps can be broken down as follows: 1. Configure and initialize the microcontroller or microprocessor serial port. 2. Initialize the AD7719 by configuring the following registers: a) IOCON to configure the current sources and digital I/O port. b) FILTER to configure the update rate for both channels. c) AD1CON to enable the aux channel, select the analog input, select unipolar or bipolar operation and input range. d) AD0CON to enable the Main ADC channel and select 16-/24-bit mode, analog input range and either unipolar or bipolar operation. e) MODE to configure the operating mode. Operating mode consists of calibration or conversion. All of these operations consist of a write to the communications register to specify the next operation as a write to a specified register. Data is then written to this register. When each sequence is complete, the ADC defaults to waiting for another write to the communications register to specify the next operation. 3. When the operating mode is selected, the user needs to determine when its valid to read the data in conversion mode or when the calibration is complete in calibration mode. This is accomplished either by polling the RDY pin (hardware polling) or by interrogating the bits in either the STATUS or MODE registers (software polling). Both are shown in Figure 11. It is assumed that both the main and aux ADCs are being used and calibration required. If the AD7719 is operated at the factory-calibrated conditions, a field calibration will not be required and these steps can be bypassed.
The AD7719's flexible serial interface allows for easy interface to most microcomputers and microprocessors. The flowchart of Figure 11 outlines the sequence that should be followed when interfacing a microcontroller or microprocessor to the AD7719. Figures 12, 13, and 14 show some typical interface circuits. The serial interface on the AD7719 is capable of operating from just three wires and is compatible with SPI interface protocols. The 3-wire operation makes the part ideal for isolated systems where minimizing the number of interface lines minimizes the number of opto-isolators required in the system. The serial clock input is a Schmitt-triggered input to accommodate slow edges from optocouplers. The rise and fall times of other digital inputs to the AD7719 should be no longer than 1 s. Most of the registers on the AD7719 are 8-bit registers, which facilitates easy interfacing to the 8-bit serial ports of microcontrollers. The Main Channel Data Register (AD0) on the AD7719 can be either 16 or 24 bits, the aux ADC data register (AD1) is 16 bits wide and the offset and gain registers are 24-bit registers, but data transfers to these registers can consist of multiple 8-bit transfers to the serial port of the microcontroller. DSP processors and microprocessors generally transfer 16 bits of data in a serial data operation. Some of these processors, such as the ADSP-2105, have the facility to program the amount of cycles in a serial transfer. This allows the user to tailor the number of bits in any transfer to match the register length of the required register in the AD7719. Even though some of the registers on the AD7719 are only eight bits in length, communicating with two of these registers in successive write operations can be handled as a single 16-bit data transfer if required. For example, if the Filter Register is to be updated, the processor must first write to the Communications Register (saying that the next operation is a write to the Filter Register) and then write eight bits to the Setup Register. If required, this can all be done in a single 16-bit transfer because once the eight serial clocks of the write operation to the Communications Register have been completed, the part immediately sets itself up for a write operation to the Setup Register.
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AD7719
START POLL RDY PIN WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ FROM THE MODE REGISTER
POWER-ON/RESET FOR AD7719 NO CONFIGURE AND INITIALIZE C/ P SERIAL PORT RDY LOW? READ FROM MODE REGISTER
YES WRITE TO THE COMMUNICATIONS REGISTER SELECTING NEXT OPERATION TO BE A WRITE TO THE IOCON REGISTER WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER NO MD BITS = 001
WRITE TO THE IOCON REGISTER TO CONFIGURE THE CURRENT SOURCES, DIGITAL I/O PORT, AND POWER SWITCHES
YES WRITE TO MODE REGISTER SELECTING CONTINUOUS CONVERSION MODE WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER
WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE FILTER REGISTER
WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ FROM THE MAIN ADC DATA REGISTER (AD0)
WRITE TO MODE REGISTER SELECTING CONTINUOUS CONVERSION MODE
WRITE TO FILTER REGISTER CONFIRMING THE REQUIRED UPDATE RATE
POLL RDY PIN WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER NO RDY LOW?
WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE AUX CHANNEL ADC CONTROL REGISTER (AD1CON)
WRITE TO MODE REGISTER SELECTING CONTINUOUS CONVERSION MODE
WRITE TO AD1CON REGISTER ENABLING THE AUX ADC, SELECT THE INPUT CHANNEL BIPOLAR/ UNIPOLAR OPERATION AND INPUT RANGE
YES READ FROM DATA REGISTER (AD0) WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ OF STATUS REGISTER
WRITE TO COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MAIN CHANNEL ADC CONTROL REGISTER (AD0CON)
POLL RDY PIN
READ STATUS REGISTER
WRITE TO AD1CON REGISTER ENABLING THE MAIN ADC, SELECT THE INPUT CHANNEL, WORD LENGTH, BIPOLAR/UNIPOLAR OPERATION, AND INPUT RANGE
NO
RDY LOW?
NO
RDY0 = 1
WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER
YES WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ FROM THE AUX ADC DATA REGISTER (AD1)
YES WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ OF THE MAIN ADC DATA REGISTER (AD0)
WRITE TO MODE REGISTER SELECTING ZERO-SCALE CALIBRATION
READ FROM DATA REGISTER (AD0) READ AD0
HARDWARE POLLING
SOFTWARE POLLING
SOFTWARE POLLING
WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ OF STATUS REGISTER
HARDWARE POLLING
READ STATUS REGISTER WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ FROM THE MODE REGISTER NO
POLL RDY PIN
READ FROM MODE REGISTER
RDY1 = 1
YES NO RDY LOW? NO MD BITS = 001 WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A READ OF THE AUX ADC DATA REGISTER (AD1)
YES WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER
YES READ AD1 WRITE TO THE COMMUNICATIONS REGISTER SETTING UP NEXT OPERATION TO BE A WRITE TO THE MODE REGISTER
WRITE TO MODE REGISTER SELECTING FULL-SCALE CALIBRATION
WRITE TO MODE REGISTER SELECTING FULL-SCALE CALIBRATION
Figure 11. Flowchart for Initializing, Calibrating, and Reading Data from the AD7719 Main and Aux Channels
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AD7719
AD7719-to-68HC11 Interface AD7719-to-8051 Interface
Figure 12 shows an interface between the AD7719 and the 68HC11 microcontroller. The diagram shows the minimum (3-wire) interface with CS on the AD7719 hard-wired low. In this scheme, the RDY bits of the Status Register are monitored to determine when the Data Register is updated. RDY0 indicates the status of the Main ADC channel while RDY1 indicates the status of the Aux channel. An alternative scheme, which increases the number of interface lines to four, is to monitor the RDY output line from the AD7719. The monitoring of the RDY line can be done in two ways. First, RDY can be connected to one of the 68HC11's port bits (such as PC0), which is configured as an input. This port bit is then polled to determine the status of RDY. The second scheme is to use an interrupt driven system, in which case the RDY output is connected to the IRQ input of the 68HC11. For interfaces that require control of the CS input on the AD7719, one of the port bits of the 68HC11 (such as PC1), which is configured as an output, can be used to drive the CS input. The 68HC11 is configured in the master mode with its CPOL bit set to a Logic 1 and its CPHA bit set to a Logic 1. When the 68HC11 is configured like this, its SCLK line idles high between data transfers. The AD7719 is not capable of full duplex operation. If the AD7719 is configured for a write operation, no data appears on the DOUT lines even when the SCLK input is active. Similarly, if the AD7719 is configured for a read operation, data presented to the part on the DIN line is ignored even when SCLK is active.
VDD VDD SS RESET
An interface circuit between the AD7719 and the 8XC51 microcontroller is shown in Figure 13. The diagram shows the minimum number of interface connections with CS on the AD7719 hardwired low. In the case of the 8XC51 interface, the minimum number of interconnects is just two. In this scheme, the RDY bits of the Status Register are monitored to determine when the Data Register is updated. The alternative scheme, which increases the number of interface lines to three, is to monitor the RDY output line from the AD7719. The monitoring of the RDY line can be done in two ways. First, RDY can be connected to one of the 8XC51's port bits (such as P1.0) which is configured as an input. This port bit is then polled to determine the status of RDY.
8XC51
DVDD RESET 10k P3.0 DOUT DVDD
AD7719
DIN
P3.1
SCLK
CS
68HC11
AD7719
Figure 13. AD7719-to-8XC51 Interface
SCK
SCLK
MISO
DOUT
MOSI
DIN
CS
Figure 12. AD7719-to-68HC11 Interface
The second scheme is to use an interrupt-driven system, in which case the RDY output is connected to the INT1 input of the 8XC51. For interfaces that require control of the CS input on the AD7719, one of the port bits of the 8XC51 (such as P1.1), which is configured as an output, can be used to drive the CS input. The 8XC51 is configured in its Mode 0 serial interface mode. Its serial interface contains a single data line. As a result, the DOUT and DIN pins of the AD7719 should be connected together with a 10 k pull-up resistor. The serial clock on the 8XC51 idles high between data transfers. The 8XC51 outputs the LSB first in a write operation, while the AD7719 expects the MSB first so the data to be transmitted has to be rearranged before being written to the output serial register. Similarly, the AD7719 outputs the MSB first during a read operation while the 8XC51 expects the LSB first. Therefore, the data read into the serial buffer needs to be rearranged before the correct data word from the AD7719 is available in the accumulator.
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AD7719
AD7719-to-ADSP-2103/ADSP-2105 Interface
Figure 14 shows an interface between the AD7719 and the ADSP-2103/ADSP-2105 DSP processor. In the interface shown, the RDY bits of the Status Register are again monitored to determine when the Data Register is updated. The alternative scheme is to use an interrupt-driven system, in which case the RDY output is connected to the IRQ2 input of the ADSP-2103/ ADSP-2105. The serial interface of the ADSP-2103/ADSP-2105 is set up for alternate framing mode. The RFS and TFS pins of the ADSP-2103/ADSP-2105 are configured as active low outputs and the ADSP-2103/ADSP-2105 serial clock line, SCLK, is also configured as an output. The CS for the AD7719 is active when either the RFS or TFS outputs from the ADSP-2103/ ADSP-2105 are active. The serial clock rate on the ADSP-2103/ ADSP-2105 should be limited to 3 MHz to ensure correct operation with the AD7719.
DVDD
end source impedances so as not to introduce gain errors. Onchip burnout currents are available and can be used to check that a transducer on the selected channel is still operational before attempting to take measurements. The second or auxiliary ADC is intended to convert secondary inputs such as those from a cold junction diode or thermistor. This ADC is unbuffered and has a fixed input range of 0 V to REFIN2 (ARN bit = 1) or 0 to REFIN2/2 (ARN bit = 0). Again, this ADC can be configured for differential or pseudo differential inputs via the ACH2, ACH1, and ACH0 bits in the auxiliary ADC control register (AD1CON). The auxiliary ADC is specified for 16-bit performance and, since its analog inputs are unbuffered, care must be exercised when placing filtering on the front end to avoid introducing gain errors into the measurement system. The basic connection diagram for the AD7719 is shown in Figure 15. This shows both the AVDD and DVDD pins of the AD7719 being driven from the analog 5 V supply. Some applications will have AVDD and DVDD driven from separate supplies. AVDD and DVDD can be operated independently of each other, allowing the device to be operated with 5 V analog supply and 3 V digital supply or vice versa. An AD780/REF195, precision 2.5 V reference, provides the reference source for the part. A quartz crystal or ceramic resonator provides the 32 kHz master clock source for the part. In some cases, it will be necessary to connect capacitors on the crystal or resonator to ensure that it does not oscillate at overtones of its fundamental operating frequency. The values of capacitors will vary depending on the manufacturer's specifications.
ANALOG 5V SUPPLY 10 F 0.1 F 0.1 F
ADSP-2103 / ADSP-2105
AD7719
RESET
RFS TFS
CS
DR
DOUT
DT
DIN
SCLK
SCLK
Figure 14. AD7719-to-ADSP-2103/ADSP-2105 Interface
CIRCUIT DESCRIPTION
AVDD IOUT1 IOUT2
DVDD RESET CS
5V
The AD7719 is a sigma-delta A/D converter incorporating two independent sigma-delta A/D converters with on-chip digital filtering, intended for the measurement of wide dynamic range, low frequency signals such as those in weigh scale, pressure, temperature, industrial control or process control applications. The main ADC is intended to convert the primary sensor input. The main ADC employs a sigma-delta conversion technique to realize up to 24 bits of no-missing-codes performance. The sigma-delta modulator converts the sampled input signal into a digital pulse train whose duty cycle contains the digital information. A Sinc3 programmable low-pass filter is then employed to decimate the modulator output data stream to give a valid data conversion result at programmable output rates from 5.35 Hz (186.77 ms) to 105.03 Hz (9.52 ms). A Chopping scheme is also employed to minimize ADC offset and offset and gain drift errors. The analog input to the main ADC can be operated in buffered or unbuffered mode and can be programmed for one of eight input ranges from 20 mV to 2.56 V. The input channels can be configured for either fully differential inputs or pseudodifferential input channels via the CH1 and CH0 bits in the main ADC control register (AD0CON) and the CHCON bit in the mode register. When configured for buffered mode (BUF = 0) the input channels are internally buffered allowing the part to handle significant source impedances on the analog input, allowing R/C filtering (for noise rejection or RFI reduction) to be placed on the analog inputs if required. When operating in unbuffered mode, care has to be exercised when selecting front -30-
AD7719
AIN1 AIN2 DIN AIN3 AIN4 AIN5 P2/SW2 ANALOG 5V SUPPLY 10 F REFIN1(+) 0.1 F VIN VOUT REFIN1(-) AIN6 REFIN2 P3 P4 XTAL1 XTAL2 SCLK P1/SW1 DOUT
CHIP SELECT RECEIVE (READ) SERIAL DATA (WRITE) SERIAL CLOCK
32kHz CRYSTAL
AD780/ REF195
GND
PWRGND
AGND
DGND
Figure 15. Basic Connection Diagram
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AD7719
Analog Input Channels
The main ADC has four associated analog input pins (labeled AIN1 to AIN4) which can be configured as two fully differential input channels or three pseudo-differential input channels. Channel selection bits CHI and CH0 in the ADC0CON register, along with the CHCON bit of the mode register, detail the different configurations. The auxiliary ADC has four external input pins (labeled AIN3 to AIN6) as well as an internal connection to the internal on-chip temperature sensor. Channel selection bits ACH2, ACH1, and ACH0 in the ADC1CON register, along with the CHCON bit in the mode register, detail the various configurations on these input channels. Two input multiplexers (MUX1 and MUX2) switch the selected input channel to the on-chip buffer amplifier in the case of the main ADC when operated in buffered mode, and directly to the sigma-delta modulator input in the case of the auxiliary ADC and when the main ADC is operated in unbuffered mode. When the analog input channel is switched, the settling time of the part must elapse before a new valid word is available from the ADC. Figure 16 shows the analog input channel configurations available to the user when the CHCON bit in the mode register is set to a zero. In this case the main ADC can be configured as one or two fully differential input channels (AIN1/AIN2 and AIN3/ AIN4) and the aux can be configured as two single-ended inputs with respect to AGND (AIN3/AGND and AIN4/AGND) and one fully differential input AIN5/AIN6). The aux can also be configured as three single-ended inputs with respect to AGND (AIN3/AGND, AIN4/AGND and AIN5/AGND) by tying AIN6 externally to AGND. The temp sensor is available as an internal connection.
MUX1 AIN1 FULLY DIFFERENTIAL AIN2 AIN2 MAIN CHANNEL AIN3 FULLY DIFFERENTIAL AIN4 AIN4 (MAIN ADC) AIN3 AIN(-) AIN1 AIN(+)
Figure 17 shows the analog input channel configurations available to the user when the CHCON bit in the mode register is set to a one. In this case the main ADC is configured as three pseudo-differential input channels (AIN1/AIN4, AIN2/AIN4, and AIN3/AIN4) and the aux can be configured as two singleended inputs with respect to AGND (AIN5/AGND and AIN6/ AGND) and one fully differential input AIN5/AIN6). The temp sensor is available as an internal connection.
PSEUDODIFFERENTIAL INPUT AIN1/AIN4 AIN2 AIN2/AIN4 AIN3 AIN3/AIN4 AIN4 PSEUDODIFFERENTIAL INPUT AIN4 (MAIN ADC) AIN3 AIN(-) AIN2 MAIN CHANNEL MUX1 AIN1 AIN(+)
AIN1
MUX2 AIN3
AIN5 FULLY DIFFERENTIAL AIN6
SINGLEENDED INPUT
AIN4
AIN(+)
AIN5
AUX CHANNEL AIN(-)
AIN6 AGND SINGLEENDED INPUT (AUX ADC)
Figure 17. Input Channel Configurations with CHCON = 1
In buffered mode (BUF = 0), the output of the main ADC multiplexer feeds into a high-impedance input stage of the buffer amplifier. As a result, the main ADC inputs can handle significant source impedances and are tailored for direct connection to external resistive-type sensors like strain gages or Resistance Temperature Detectors (RTDs). The auxiliary ADC and the main ADC when operated with BUF = 1, however, are unbuffered resulting in higher analog input current. It should be noted that these unbuffered input paths provide a dynamic load to the driving source. Therefore, resistor/capacitor combinations on the input pins can cause dc gain errors depending on the output impedance of the source that is driving the ADC inputs. Table XVIII and XIX show the allowable external resistance/capacitance values for unbuffered mode such that no gain error at the 16- and 20-bit level respectively is introduced. The absolute input voltage range on the main ADC when operated in buffered mode is restricted to a range between AGND + 100 mV and AVDD - 100 mV. Care must be taken in setting up the common-mode voltage and input voltage range so that these limits are not exceeded; otherwise there will be a degradation in linearity and noise performance.
SINGLEENDED INPUT SINGLEENDED INPUT
MUX2 AIN3
AIN4
AIN(+)
AIN5 FULLY DIFFERENTIAL AIN6
AIN5
AUX CHANNEL AIN(-)
AIN6 AGND (AUX ADC)
Figure 16. Input Channel Configurations with CHCON = 0
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AD7719
Table XVIII. Max Resistance for No 16-Bit Gain Error (Unbuffered Mode)
External Capacitance Gain 1 2 4 8-128 0 pF 111.3K 53.7K 25.4K 10.7K 50 pF 27.8K 13.5K 6.4K 2.9K 100 pF 16.7K 8.1K 3.9K 1.7K 500 pF 4.5K 2.2K 1.0K 480 1000 pF 2.58K 1.26K 600 270 5000 pF 700 360 170 75
Table XIX. Max Resistance for No 20-Bit Gain Error (Unbuffered Mode)
External Capacitance Gain 1 2 4 8-128 0 pF 84.9K 42.0K 20.5K 8.8K 50 pF 21.1K 10.4K 5.0K 2.3K 100 pF 12.5K 6.1K 2.9K 1.3K 500 pF 3.2K 1.6K 790K 370
19.372 19.371
ADC INPUT VOLTAGE - mV
1000 pF 1.77K 880 430 195
5000 pF 440 220 110 50
The absolute input voltage range on the auxiliary ADC and the main ADC in unbuffered mode includes the range between AGND - 30 mV to AVDD + 30 mV as a result of being unbuffered. The negative absolute input voltage limit does allow the possibility of monitoring small true bipolar signals with respect to AGND.
Programmable Gain Amplifier
19.370 19.369 19.368 19.367 19.366 19.365
The output from the buffer on the main ADC is applied to the input of the on-chip programmable gain amplifier (PGA). The PGA can be programmed through eight different unipolar and bipolar ranges. The PGA gain range is programmed via the range bits in the ADC0CON register. With an external 2.5 V reference applied the unipolar ranges are 0 mV to 20 mV, 0 to 40 mV, 0 mV to 80 mV, 0 mV to 160 mV, 0 mV to 320 mV, 0 mV to 640 mV, 0 V to 1.28 V and 0 to 2.56 V while bipolar ranges are 20 mV, 40 mV, 80 mV, 160 mV, 320 mV, 640 mV, 1.28 V and 2.56 V. These are the ranges that should appear at the input to the on-chip PGA. The ADC range matching specification of 2 V (typ) across all ranges means that calibration need only be carried out on a single range and does not have to be repeated when the PGA range is changed. This is a significant advantage when compared with similar ADCs available on the market. Typical matching across ranges is shown in Figure 18. Here, the primary ADC is configured in fully-differential, bipolar mode with an external 2.5 V reference, while an analog input voltage of just greater than 19 mV is forced on its analog inputs. The ADC continuously converts the DC voltage at an update rate of 5.35 Hz, i.e., SF = FF Hex. 800 conversion results in total are gathered. The first 100 results gathered with the primary ADC operating in the 20 mV. The ADC range is then switched to 40 mV and 100 more results are gathered, and so on until the last 100 samples are gathered with the ADC configured in the 2.5 V range. From Figure 18, the variation in the sample mean through each range, i.e., the range matching, is seen to be on the order of 2 V. The auxiliary ADC does not incorporate an eight range PGA. The aux ADC operates at a gain of 1 or a gain of 2 as determined by the ARN bit in the AD1CON register.
19.364 SAMPLE COUNT 0
20mV
100
40mV
200
80mV
300
400
160mV 320mV
500
640mV
600
1.28V
700
2.56V
800
ADC RANGE
Figure 18. Main ADC Range Matching
Bipolar/Unipolar Configuration
The analog inputs on the AD7719 can accept either unipolar or bipolar input voltage ranges. Bipolar input ranges does not imply that the part can handle negative voltages with respect to system AGND. Unipolar and bipolar signals on the AIN(+) input on the main ADC are referenced to the voltage on the respective AIN(-) input. AIN(+) and AIN(-) refer to the signals seen by the modulator that come from the output of the multiplexer as shown in Figures 16 and 17. For example, if AIN(-) is 2.5 V and the main ADC is configured for an analog input range of 0 mV to 20 mV, the input voltage range on the AIN(+) input is 2.5 V to 2.52 V. If AIN(-) is 2.5 V and the AD7719 is configured for an analog input range of 1.28 V, the analog input range on the AIN(+) input is 1.22 V to 3.78 V (i.e., 2.5 V 1.28 V). Bipolar or unipolar options are chosen by programming the main and Auxiliary U/B bit in the ADC0CON and ADC1CON registers respectively. This programs the relevant ADC for either unipolar or bipolar operation. Programming for either unipolar or bipolar operation does not change any of the input signal conditioning; it simply changes the data output coding and the points on the transfer function where calibrations occur.
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AD7719
Data Output Coding Excitation Currents
When the ADC is configured for unipolar operation, the output coding is natural (straight) binary with a zero differential input voltage resulting in a code of 000 . . . 000, a midscale voltage resulting in a code of 100 . . . 000, and a full-scale input voltage resulting in a code of 111 . . . 111. The output code for any analog input voltage on the main ADC can be represented as follows: Code = (AIN x GAIN x 2N)/(1.024 x VREF) Where AIN is the analog input voltage, GAIN is the PGA gain, i.e., 1 on the 2.56 V range and 128 on the 20 mV range, and N = 16 in 16-bit mode and N = 24 in 24-bit mode of operation. The output code for any analog input voltage on the aux ADC can be represented as follows: Code = (AIN x GAIN x 2N)/VREF Where AIN is the analog input voltage, GAIN is one or two determined by the ARN bit in the aux ADC control register, i.e., 1 on the VREF range and 2 on the VREF/2 range and N = 16. When an ADC is configured for bipolar operation, the coding is offset binary with a negative full-scale voltage resulting in a code of 000 . . . 000, a zero differential voltage resulting in a code of 100 . . . 000, and a positive full-scale voltage resulting in a code of 111 . . . 111. The output code from the main ADC for any analog input voltage can be represented as follows: Code = 2N-1 x [(AIN x GAIN/(1.024 x VREF)) + 1] Where AIN is the analog input voltage, GAIN in the PGA gain, i.e., 1 on the 2.56 V range and 128 on the 20 mV range and N = 16 in 16 bit mode and N = 24 in 24-bit mode of operation. The output code from the aux ADC for any analog input voltage can be represented as follows: Code = 2N-1 x [(AIN x GAIN/VREF) + 1] Where AIN is the analog input voltage, GAIN is one or two determined by the ARN bit in the aux ADC control register, i.e., 1 on the VREF range, 2 on the VREF/2 range, and N = 16.
Burnout Currents
The AD7719 also contains two matched, software configurable 200 A constant current-sources. Both source current from AVDD that is directed to either the IOUT1 or IOUT2 pins of the device. These current sources are controlled via bits in the IOCON register. The configuration bits enable the current sources and they can be configured to source 200 A individually to both pins or a combination of both currents, i.e., 400 A to either of the selected output pins. These current sources can be used to excite external resistive bridge or RTD sensors.
Crystal Oscillator
The AD7719 is intended for use with a 32.768 kHz watch crystal. A PLL internally locks onto a multiple of this frequency to provide a stable 4.194304 MHz clock for the ADC. The modulator sample rate is the same as the crystal oscillator frequency. The start-up time associated with 32 kHz crystals is typically 300 ms. The OSPD bit in the mode register can be used to prevent the oscillator from powering down when the AD7719 is placed in power-down mode. This avoids having to wait 300 ms after exiting power-down to start a conversion at the expense of raising the power-down current.
Reference Input
The main ADC on the AD7719 contains two 100 nA constant current generators, one sourcing current from AVDD to AIN(+), and one sinking current from AIN(-) to AGND. The currents are switched to the selected analog input pair. Both currents are either on or off, depending on the Burnout Current Enable (BO) bit in the IOCON register. These currents can be used to verify that an external transducer is still operational before attempting to take measurements on that channel. Once the burnout currents are turned on, they will flow in the external transducer circuit, and a measurement of the input voltage on the analog input channel can be taken. If the resultant voltage measured is full-scale the user needs to verify why this is the case. A full-scale reading could mean that the front end sensor is open circuit, it could also mean that the front end sensor is overloaded and is justified in outputting full-scale or the reference may be absent and the NOXREF bit is set, thus clamping the data to all ones. When reading all ones from the output the user needs to check these three cases before making a judgement. If the voltage measured is 0 V, it may indicate that the transducer has short circuited. For normal operation, these burnout currents are turned off by writing a 0 to the BO bit in the IOCON register. The current sources work over the normal absolute input voltage range specifications with buffers on. REV. 0
The AD7719 has a fully-differential reference input capability for the main channel while the auxiliary channel accepts only a single-ended reference. On the main channel, the reference inputs REFIN1(+) and REFIN1(-) provide a differential reference input capability. The common-mode range for these differential inputs is from AGND to AVDD. The reference input is unbuffered and therefore excessive R-C source impedances will introduce gain errors. The nominal reference voltage, VREF, (REFIN1(+) - REFIN1(-), for specified operation is 2.5 V, but the AD7719 is functional with reference voltages from 1 V to AVDD. In applications where the excitation (voltage or current) for the transducer on the analog input also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source will be removed as the application is ratiometric. If the AD7719 is used in a nonratiometric application, a low noise reference should be used. Recommended reference voltage sources for the AD7719 include the AD780, REF43, and REF192. It should also be noted that the reference inputs provide a high impedance, dynamic load. Because the input impedance of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the output impedance of the source that is driving the reference inputs. Reference voltage sources like those recommended above (e.g., AD780), will typically have low output impedances and are therefore tolerant to having decoupling capacitors on the REFIN1(+) without introducing gain errors in the system. Deriving the reference input voltage across an external resistor, as shown in Figure 19, will mean that the reference input sees a significant external source impedance. External decoupling on the REFIN1(+) and REFIN1(-) pins would not be recommended in this type of circuit configuration. The auxiliary channel conversion results are based on the voltage applied to REFIN2. This is a single-ended reference input specified for 2.5 V operation but functional with input voltages from 1 V to AVDD.
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AD7719
Reference Detect
The AD7719 includes on-chip circuitry to detect if the part has a valid reference on the main ADC for conversions or calibrations. If the voltage between the external REFIN1(+) and REFIN1(-) pins goes below 0.3 V or either the REFIN1(+) or REFIN1(-) inputs are open circuit, the AD7719 detects that it no longer has a valid reference. In this case, the NOXREF bit of the STATUS register is set to a 1. If the AD7719 is performing normal conversions and the NOXREF bit becomes active, the conversion results revert to all 1s. Therefore, it is not necessary to continuously monitor the status of the NOXREF bit when performing conversions. It is only necessary to verify its status if the conversion result read from the ADC Data Register is all 1s. If the AD7719 is performing either an offset or gain calibration and the NOXREF bit becomes active, the updating of the respective calibration registers is inhibited to avoid loading incorrect coefficients to these registers, and the ERR0 bit in the STATUS register is set. If the user is concerned about verifying that a valid reference is in place every time a calibration is performed, the status of the ERR0 bit should be checked at the end of the calibration cycle.
Reset Input
non-0. In this Mode the internal PLL is enabled and the user can activate the current sources and power switches but the power consumption of the ADC is reduced as both ADCs are disabled. In this mode the AIDD is reduced to 0.15 mA and the DIDD is reduced to 0.35 mA max at 3 V and to 0.4 mA max with DVDD = 5 V.
Calibration
The AD7719 provides four calibration modes that can be programmed via the mode bits in the mode register. One of the major benefits of the AD7719 is that it is factory- calibrated as part of the final test process with the generated coefficients stored within the ADC. At power-on, the factory gain calibration coefficients are automatically loaded to the gain calibration registers on the AD7719. Each ADC (primary and auxiliary) has dedicated calibration register pairs as outlined in the AD0CON and AD1CON register descriptions. Given that the ADC is factory-calibrated and a chopping scheme is employed that gives excellent offset and drift performance, it is envisaged that in the majority of applications the user will not need to perform any field calibrations. However, the factory calibration values in the ADC calibration registers will be overwritten if any one of the four calibration options are initiated. Even though an internal offset calibration mode is described below, it should be recognized that both ADCs are chopped. This chopping scheme inherently minimizes offset and means that an internal offset calibration should never be required. Also, because factory 25C gain calibration coefficients are automatically present at power-on, an internal full-scale calibration will only be required if the part is being operated at temperatures significantly different from 25C or away from the calibration conditions. The AD7719 offers "internal" or "system" calibration facilities. For full calibration to occur on the selected ADC, the calibration logic must record the modulator output for two different input conditions. These are "zero-scale" and "full-scale" points derived by performing a conversion on the different input voltages provided to the input of the modulator during calibration. The result of the "zero-scale" calibration conversion is stored in the Offset Calibration Registers for the appropriate ADC. The result of the "full-scale" calibration conversion is stored in the Gain Calibration Registers for the appropriate ADC. With these readings, the calibration logic can calculate the offset and the gain slope for the input-to-output transfer function of the converter. During an "internal" zero-scale or full-scale calibration, the respective "zero" input and "full-scale" input are automatically connected to the ADC input pins internally to the device. A "system" calibration, however, expects the system zero-scale and system full-scale voltages to be applied to the external ADC pins before the calibration mode is initiated. In this way external ADC errors are taken into account and minimized as a result of system calibration. It should also be noted that to optimize calibration accuracy, all AD7719 ADC calibrations are automatically carried out at the slowest update rate. Internally in the AD7719, the coefficients are normalized before being used to scale the words coming out of the digital filter. The offset calibration coefficient is subtracted from the result prior to the multiplication by the gain coefficient. From an operational point of view, a calibration should be treated like another ADC conversion. A zero-scale calibration (if required)
The RESET input on the AD7719 resets all the logic, the digital filter, and the analog modulator while all on-chip registers are reset to their default state. RDY is driven high and the AD7719 ignores all communications to any of its registers while the RESET input is low. When the RESET input returns high, the AD7719 operates with its default setup conditions and it is necessary to set up all registers and carry out a system calibration if required after a RESET command.
Power-Down Mode
Loading 0, 0, 0 to the MD2, MD1, MD0 bits in the ADC mode register places the AD7719 in device power-down mode. Device power-down mode is the default condition for the AD7719 on power-up. Individual ADCs (main or auxiliary) can be put in power-down mode using the AD0EN in the main ADC control register (AD0CON) to power off the main ADC or the AD1EN in the auxiliary ADC control register (AD1CON) to power off the auxiliary ADC. The AD7719 retains the contents of all its on-chip registers (including the data register) while in powerdown or ADC disable mode. The device power-down mode does not affect the digital interface, and it does affect the status of the RDY pin. Putting the AD7719 into power-down or idle mode will reset the RDY line high. Placing the part in power-down mode, reduces the total current (AIDD + DIDD) to 31 A max when the part is operated a 5 V and the oscillator allowed to run during power-down mode. With the oscillator shut down, the total IDD is 3 A max at 3 V and 9 A max at 5 V.
Idle Mode
The AD7719 also contains an idle mode. The ADC defaults to this mode on completion of a calibration sequence and on the completion of a conversion when operating in single conversion mode. In idle mode the power consumption of the AD7719 is not reduced below the normal mode dissipation.
ADC Disable Mode
This mode is entered by setting both the AD0EN and AD1EN bits in the Main and Max ADC control registers to 0 and setting the Mode bits (MD2, MD1, MD0) in the Mode register to
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AD7719
should always be carried out before a full-scale calibration. System software should monitor the relevant ADC RDY0/1 bit in the STATUS register to determine end of calibration via a polling sequence or interrupt driven routine.
Grounding and Layout
Since the analog inputs and reference input on the main ADC are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection of the part will remove common-mode noise on these inputs. The analog and digital supplies to the AD7719 are independent and separately pinned out to minimize coupling between the analog and digital sections of the device. The AD7719 can be operated with 5 V analog and 3 V digital supplies or vice versa. The digital filter will provide rejection of broadband noise on the power supplies, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs provided these noise sources do not saturate the analog modulator. As a result, the AD7719 is more immune to noise interference than a conventional highresolution converter. However, because the resolution of the AD7719 is so high, and the noise levels from the AD7719 so low, care must be taken with regard to grounding and layout. The printed circuit board that houses the AD7719 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes that can be easily separated. A minimum etch technique is generally best for ground planes as it gives the best shielding. Although the AD7719 has separate pins for analog and digital ground, the AGND and DGND pins are tied together within the device via the substrate. The user must not tie these pins external to separate ground planes unless the ground planes are connected together near the AD7719. In systems where the AGND and DGND are connected somewhere else in the system, i.e., the system power supply, they should not be connected again at the AD7719 as a ground loop will result. In these situations, it is recommended that the AD7719's AGND and DGND pins be tied to the AGND plane. In any layout, it is important that the user keep in mind the flow of currents in the system ensuring that the return paths for all currents are as close as possible to the paths the currents took to reach their destinations. Avoid forcing digital currents to flow through the AGND sections of the layout. The PWRGND pin is tied internally to AGND on the AD7719. The PWRGND pad internally has a resistance of less then 50 m to the PWRGND pin, while the resistance back to the AGND pad is >3 . This means that 19.5 mA of the maximum specified current (20 mA) will flow to PWRGND with the remaining 0.5 mA flowing to AGND. PWRGND and AGND should be tied together at the AD7719 and it is important to minimize the resistance on the ground return lines. Avoid running digital lines under the device as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7719 to prevent noise coupling. The power supply lines to the AD7719 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches
on the power supply line. Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will reduce the effects of feedthrough through the board. A microstrip technique is by far the best, but is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes while signals are placed on the solder side. Good decoupling is important when using high-resolution ADCs. All analog supplies should be decoupled with 10 F tantalum in parallel with 0.1 F capacitors to AGND. To achieve the best from these decoupling components, they have to be placed as close as possible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 F ceramic capacitors to DGND. In systems where a common supply voltage is used to drive both the AVDD and DVDD of the AD7719, it is recommended that the system's AVDD supply be used. This supply should have the recommended analog supply decoupling capacitors between the AVDD pin of the AD7719 and AGND, and the recommended digital supply decoupling capacitor between the DVDD pin of the AD7719 and DGND.
APPLICATIONS
The AD7719 provides a low-cost, high-resolution analog-todigital function. Because the analog-to-digital function is provided by a sigma-delta architecture, it makes the part more immune to noisy environments, making it ideal for use in sensor measurement and industrial and process control applications. Given the architecture used in the AD7719, where the signal chain is chopped and the device is factory-calibrated at final test, field calibration can be avoided due to the extremely low offset and gain drifts exhibited by this converter. It also provides a programmable gain amplifier, a digital filter, and system calibration options. Thus, it provides far more system-level functionality than off-the-shelf integrating ADCs without the disadvantage of having to supply a high-quality integrating capacitor. In addition, using the AD7719 in a system allows the system designer to achieve a much higher level of resolution because noise performance of the AD7719 is significantly better than that of integrating ADCs. The on-chip PGA allows the AD7719 to handle an analog input voltage range as low as 10 mV full-scale with VREF = 1.25 V. The differential inputs of the part allow this analog input range to have an absolute value anywhere between AGND +100 mV and AVDD - 100 mV. It allows the user to connect the transducer directly to the input of the AD7719. The programmable gain front end on the AD7719 allows the part to handle unipolar analog input ranges from 0 mV to 20 mV to 0 V to 2.5 V and bipolar inputs of 20 mV to 2.5 V. Because the part operates from a single supply these bipolar ranges are with respect to a biased-up differential input. Another key advantage of the AD7719 is that it contains two sigma-delta converters operating in parallel; thus the user does not need to interrupt the main channel when a secondary measurement on a different variable needs to be performed.
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AD7719
Pressure Measurement
One typical application of the AD7719 is pressure measurement. Figure 19 shows the AD7719 used with a pressure transducer, the BP01 from Sensym. The pressure transducer is arranged in a bridge network and gives a differential output voltage between its OUT(+) and OUT(-) terminals. With rated full-scale pressure (in this case 300 mmHg) on the transducer, the differential output voltage is 3 mV/V of the input voltage (i.e., the voltage between its IN(+) and IN(-) terminals). Assuming a 5 V excitation voltage, the full-scale output range from the transducer is 15 mV. The excitation voltage for the bridge can be used to directly provide the reference for the ADC as the reference input range includes the supplies. Alternatively, a suitable resistor divider can be implemented that allows the full dynamic range of the input to be utilized in these application. This implementation is fully ratiometric, so variations in the excitation voltage do not introduce errors in the system. Choosing resistor values of 20 k and 12 k as per Figure 19 give a 1.875 V reference voltage for the AD7719 when the excitation voltage is 5 V.
EXCITATION VOLTAGE = 5V
A second key advantage to using the AD7719 in transducerbased applications is that the on-chip low-side power switch can be fully utilized in low power applications. The low-side power switch is connected in series with the cold side of the bridge. In normal operation the switch is closed and measurements can be taken from the bridge. In applications where power is of concern, the AD7719 can be put in low power mode substantially reducing the power burned in the application. In addition to this, the power switch can be opened while in low power mode thus avoiding the unnecessary burning of power in the front end transducer. When taken back out of power-down and the power switch is closed, the user should ensure that the front end circuitry is fully settled before attempting a read from the AD7719. The circuit in Figure 20 shows a method that utilizes all three pseudo-differential input channels on the AD7719 main channel to temperature-compensate a pressure transducer.
5V AVDD IOUT1 I1
I2
IN+ OUT- OUT+
20k AVDD AIN1 AIN2 DVDD
REFIN(+) 6.25k REFIN(-)
IN- REFIN1(+) 12k REFIN2(-)
IN(+)
AIN2
XTAL1
AD7719
OUT(-)
P1
AD7719
OUT(+) AIN1 AIN4 IN(-) AIN3 250 AGND XTAL2 PRESSURE BRIDGE
DGND AGND PWRGND
Figure 19. Pressure Measurement Using AD7719
Figure 20. Temperature-Compensating a Pressure Transducer
Using the part with a programmed gain of 128 results in the full-scale input span of the AD7719 being 15 mV, which corresponds with the output span from the transducer.
In this application, pseudo-differential input channel AIN1/AIN4 is used to measure the bridge output while pseudo-differential channels AIN2/AIN4 and AIN3/AIN4 measure the voltage across the bridge. The voltage measured across the bridge will vary proportionally with temperature, and the delta in this voltage can be used to temperature-compensate the output of the pressure bridge.
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AD7719
Temperature Measurement
AVDD EMF1 IOUT1 I1 IOUT2 RESISTIVE TRANSDUCER AIN1 EMF2 AIN2 AIN3 A A AIN4 REFIN(+) RREF REFIN(-) MUX1 BUF & PGA 200 A
The AD7719 is also useful in temperature measurement applications, Figure 21 shows an RTD temperature measurement application. In this application, the transducer is an RTD (Resistive Temperature Device), a PT100. The arrangement is a 4-lead RTD configuration. There are voltage drops across the lead resistances RL1 and RL4, but these simply shift the common-mode voltage. There is no voltage drop across lead resistances RL2 and RL3 as the input current to the AD7719 is very low, looking into a high input impedance buffer. RCM is included to shift the analog input voltage to ensure that it lies within the common- mode range (AGND + 100 mV to AVDD - 100 mV) of the ADC. In the application shown, the on-chip 200 A current source provides the excitation current for the PT100 and also generates the reference voltage for the AD7719 via the 12.5 k resistor. Variations in the excitation current do not affect the circuit as both the input voltage and the reference voltage vary ratiometrically with the excitation current. However, the 12.5 k resistor must have a low temperature coefficient to avoid errors in the reference voltage over temperature.
AD7719
Figure 22. Low Resistance Measurement AD7719
5V
errors. DC errors (EMF1 and EMF2) due to parasitic thermocouples produced by differential metal connections (solder and copper track) within the circuit are also eliminated when using this switching arrangement. This excitation is a form of synchronous detection where the sensor is excited with an alternating excitation source and the ADC only measures information in the same phase as the excitation source. The switched polarity current source is developed using the onchip current sources and external phase control switches (A and A) driven from the controller. During the conversion process the AD7719 takes two conversion results, one on each phase. During Phase 1 the on-chip current source is directed to IOUT1 and flows top to bottom through the sensor and switch controlled by A. In Phase 2, the current source is directed to IOUT2 and flows in the opposite direction through the sensor and through switch controlled by A. In all cases the current flows in the same direction through the reference resistor to develop the reference voltage for the ADC. All measurements are ratiometrically derived. The results of both conversions are combined within the microcontroller to produce one output measurement representing the resistance or temperature of the transducer. For example, if the RTD output during Phase 1 is 10 mV, a 1 mV circuit-induced dc error exists due to parasitic thermocouples, the ADC measures 11 mV. During the second phase, the excitation current is reversed and the ADC measures -10 mV from the RTD and again sees 1 mV dc error, giving an ADC output of -9 mV during this phase. These measurements are processed in the controller (11 mV - (-9 mV)/2 = 10 mV) thus removing the dc-induced errors within the system. In the circuit shown in Figure 22, the resistance measurement is made using ratiometric techniques. Resistor RREF, which develops the ADC reference, must be stable over temperature to prevent reference-induced errors in the measurement output.
AVDD DVDD REFIN(-) XTAL1 REFIN(+) RL1 RREF 12.5k 200 A RL2 RTD RL3 RCM RL4 DGND AGND IOUT2 AIN1 DRDY IOUT1 XTAL2
AD7719
AIN2
SCLK DIN DOUT CS PWRGND CONTROLLER
Figure 21. 4-Wire RTD Temperature Masurement Using the AD7719
Figure 22 shows a further enhancement to the circuit shown in Figure 21. Generally dc excitation has been accepted as the normal method of exciting resistive-based sensors like RTD's (Resistance Temperature Detectors) in temperature measurement applications. With dc excitation, the excitation current through the sensor must be large enough so that the smallest temperature/resistance change to be measured results in a voltage change that is larger than the system noise, offset, and drift of the system. The purpose of switching the excitation source is to eliminate dc-induced
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AD7719
3-Wire RTD Configurations
To fully optimize a 3-wire RTD configuration, two identically matched current sources are required. The AD7719, which contains two well-matched current sources, is ideally suited to these applications. One possible 3-wire configuration using the AD7719 is outlined in Figure 23.
5V
AVDD DVDD IOUT1 200 A REFIN(+) 12.5k REFIN(-) RL1 AIN1 RTD RL2 RL3 RCM AGND DGND AIN2 IOUT2 200 A DRDY SCLK DIN DOUT CS CONTROLLER XTAL2 XTAL1
AD7719
Figure 23. 3-Wire RTD Configuration Using the AD7719
In this 3-wire configuration, the lead resistances will result in errors if only one current source is used as the 200 A will flow through RL1, developing a voltage error between AIN1 and AIN2. In the scheme outlined below, the second RTD current source is used to compensate for the error introduced by the 200 A flowing through RL1. The second RTD current flows through RL2. Assuming RL1 and RL2 are equal (the leads would normally be of the same material and of equal length), and IOUT1 and IOUT2 match, the error voltage across RL2 equals the error voltage across RL1 and no error voltage is developed between AIN1 and AIN2. Twice the voltage is developed across RL3 but, since this is a common-mode voltage, it will not introduce errors. RCM is included so the current flowing through the combination of RL3 and RCM develops enough voltage that the analog input voltage seen by the AD7719 is within the common-mode range of the ADC. The reference voltage for the AD7719 is also generated using one of these matched current sources. This reference voltage is developed across the 12.5 k resistor as shown, and applied to the differential reference inputs of the AD7719. This scheme ensures that the analog input voltage span remains ratiometric to the reference voltage. Any errors in the analog input voltage due to the temperature drift of the RTD current source is compensated for by the variation in the reference voltage. The typical drift matching between the two RTD current sources is less than 1 ppm/C. The voltage on either IOUT pin can go to within 0.6 V of the AVDD supply.
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AD7719
Smart Transmitters
Smart transmitters are another key design-in area for the AD7719. The dual sigma-delta converter, single supply operation, 3-wire interface capabilities, and small package size are all of benefit is in smart transmitters. Here, the entire smart transmitter must operate from the 4 mA-to-20 mA loop. Tolerances in the loop mean that the amount of current available to power the transmitter is as low as 3.5 mA. Figure 24 shows a block diagram of a smart transmitter which includes the AD7719.
Not shown in Figure 24 is the isolated power source required to power the front end. The advantages of the AD7719 in these applications is the dual channel operation, meaning that the user does not have to interrupt the main channel when measuring secondary variables, and therefore does not have the latency associated with the settling times of the digital filter. The fact that the AD7719 is factory-calibrated means that in the majority of applications, the user will not have to perform any field calibration given the excellent offset and gain drift performance of the device as a result of the signal chain chopping employed in the signal chain.
DN25D 3.3V 10 F 0.1 F 1.25V DVDD MAIN VARIABLES AIN1 AIN2 AIN3 AIN4 REFIN(-) AVDD REFIN2 REFIN(+) 0.1 F VCC CLOCK MICROCONTROLLER GND AIN5 COM CS DOUT SCLK DIN DGND AGND COM C1 C2 C3 LOOP RTN LOOP POWER LATCH DATA 1000pF 4.7 F REF OUT1 REF OUT2 REF IN BOOST VCC LV COMP DRIVE 1k 10 F 0.01 F
0.01 F
AD7719
AIN5 SECONDARY VARIABLES
AD421
Figure 24. Smart Transmitter Employing the AD7719
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AD7719
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
28-Lead Plastic SOIC (R-28)
0.7125 (18.10) 0.6969 (17.70)
28
15
0.2992 (7.60) 0.2914 (7.40)
1 14
0.4193 (10.65) 0.3937 (10.00)
PIN 1
0.1043 (2.65) 0.0926 (2.35)
0.0291 (0.74) 0.0098 (0.25)
45
0.0118 (0.30) 0.0040 (0.10)
0.0500 (1.27) BSC
8 0 0.0192 (0.49) SEATING 0.0125 (0.32) 0.0138 (0.35) PLANE 0.0091 (0.23)
0.0500 (1.27) 0.0157 (0.40)
28-Lead Plastic TSSOP (RU-28)
0.386 (9.80) 0.378 (9.60)
28
15
0.177 (4.50) 0.169 (4.30) 0.256 (6.50) 0.246 (6.25)
1 14
PIN 1 0.006 (0.15) 0.002 (0.05) 0.0433 (1.10) MAX
SEATING PLANE
0.0256 (0.65) BSC
0.0118 (0.30) 0.0075 (0.19)
0.0079 (0.20) 0.0035 (0.090)
8 0
0.028 (0.70) 0.020 (0.50)
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PRINTED IN U.S.A.
C02460-1-7/01(0)

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