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FEATURES Fast Throughput Rate: 1 MSPS Specified for VDD of 2.7 V to 5.25 V Low Power: 4.5 mW Max at 1 MSPS with 3 V Supplies 10.5 mW Max at 1 MSPS with 5 V Supplies Wide Input Bandwidth: 68 dB SNR at 300 kHz Input Frequency Flexible Power/Serial Clock Speed Management No Pipeline Delays High-Speed Serial Interface SPITM/QSPITM/ MICROWIRETM/DSP-Compatible On-Board Reference 2.5 V (AD7495 Only) Standby Mode: 1 A Max 8-Lead SOIC and SOIC Packages APPLICATIONS Battery-Powered Systems Personal Digital Assistants Medical Instruments Mobile Communications Instrumentation and Control Systems Data Acquisition Systems High-Speed Modems Optical Sensors GENERAL DESCRIPTION
VDD VIN REF IN T/H
1 MSPS, 12-Bit ADCs AD7475/AD7495
FUNCTIONAL BLOCK DIAGRAMS
12-BIT SUCCESSIVE APPROXIMATION ADC
SCLK CONTROL LOGIC SDATA CS
AD7475
GND
VDD
VDRIVE
VIN
T/H
12-BIT SUCCESSIVE APPROXIMATION ADC
REF OUT BUF SCLK 2.5V REFERENCE CONTROL LOGIC SDATA CS VDRIVE
The AD7475/AD7495 are 12-bit high-speed, low-power, successive-approximation ADCs. The parts operate from a single 2.7 V to 5.25 V power supply and feature throughput rates up to 1 MSPS. The parts contain a low-noise, wide bandwidth track/hold amplifier that can handle input frequencies in excess of 1 MHz. The conversion process and data acquisition are controlled using CS and the serial clock allowing the devices to interface with microprocessors or DSPs. The input signal is sampled on the falling edge of CS and conversion is also initiated at this point. There are no pipelined delays associated with the part. The AD7475/AD7495 use advanced design techniques to achieve very low power dissipation at high throughput rates. With 3 V supplies and 1 MSPS throughput rate, the AD7475 consumes just 1.5 mA, while the AD7495 consumes 2 mA. With 5 V supplies and 1 MSPS, the current consumption is 2.1 mA for the AD7475 and 2.6 mA for the AD7495. The analog input range for the part is 0 V to REF IN. The 2.5 V reference for the AD7475 is applied externally to the REF IN pin while the AD7495 has an on-board 2.5 V reference. The conversion time is determined by the SCLK frequency.
MICROWIRE is a trademark of National Semiconductor Corporation. SPI and QSPI are trademarks of Motorola, Inc.
AD7495
GND
PRODUCT HIGHLIGHTS
1. High throughput with low power consumption. The AD7475 offers 1 MSPS throughput rates with 4.5 mW power consumption. 2. Single-supply operation with VDRIVE function. The AD7475/ AD7495 operate from a single 2.7 V to 5.25 V supply. The VDRIVE function allows the serial interface to connect directly to either 3 V or 5 V processor systems independent of VDD. 3. Flexible power/serial clock speed management. The conversion rate is determined by the serial clock, allowing the conversion time to be reduced through the serial clock speed increase. The part also features shutdown modes to maximize power efficiency at lower throughput rates. This allows the average power consumption to be reduced while not converting. Power consumption is 1 A when in full shutdown. 4. No pipeline delay. The part features a standard successiveapproximation ADC with accurate control of the sampling instant via a CS input and once off conversion control.
REV. A
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
AD7475/AD7495-SPECIFICATIONS1
AD7475-SPECIFICATIONS1
Parameter DYNAMIC PERFORMANCE Signal to Noise + Distortion Ratio (SINAD) Total Harmonic Distortion (THD) Peak Harmonic or Spurious Noise (SFDR) Intermodulation Distortion (IMD) Second Order Terms Third Order Terms Aperture Delay Aperture Jitter Full Power Bandwidth Full Power Bandwidth DC ACCURACY Resolution Integral Nonlinearity Differential Nonlinearity
(VDD = 2.7 V to 5.25 V, VDRIVE = 2.7 V to 5.25 V, REF IN = 2.5 V, fSCLK = 20 MHz unless otherwise noted; TA = TMIN to TMAX, unless otherwise noted.)
A Version1 68 -75 -76 B Version1 68 -75 -76 Unit dB min dB max dB max Test Conditions/Comments fIN = 300 kHz Sine Wave, fSAMPLE = 1 MSPS fIN = 300 kHz Sine Wave, fSAMPLE = 1 MSPS fIN = 300 kHz Sine Wave, fSAMPLE = 1 MSPS
-78 -78 10 50 8.3 1.3 12 1.5 0.5 +1.5/-0.9 0.5 8 3
-78 -78 10 50 8.3 1.3 12 1 0.5 +1.5/-0.9 0.5 8 3 0 to REF IN 1 20 2.5 1 20 VDRIVE - 1 0.4 1 10
dB typ dB typ ns typ ps typ MHz typ MHz typ Bits LSB max LSB typ LSB max LSB typ LSB max LSB max Volts A max pF typ Volts A max pF typ V min V max A max pF max V min V max A max pF max
@ 3 dB @ 0.1 dB
Offset Error Gain Error ANALOG INPUT Input Voltage Ranges DC Leakage Current Input Capacitance REFERENCE INPUT REF IN Input Voltage Range DC Leakage Current Input Capacitance LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN2 LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL Floating-State Leakage Current Floating-State Output Capacitance2 Output Coding CONVERSION RATE Conversion Time Track/Hold Acquisition Time Throughput Rate POWER REQUIREMENTS VDD VDRIVE IDD3 Normal Mode (Static) Normal Mode (Operational) Partial Power-Down Mode Partial Power-Down Mode Full Power-Down Mode
@ 5 V (typ @ 3 V) @ 25C @ 5 V Guaranteed No Missed Codes to 12 Bits (typ @ 3 V) @ 25C Typically 2.5 LSB
1 20 2.5 1 20
1% for Specified Performance
VDRIVE - 1 0.4 1 10
Typically 10 nA, VIN = 0 V or VDRIVE
VDRIVE - 0.2 0.4 0.4 10 10 10 10 Straight (Natural) Binary 800 300 325 1 2.7/5.25 2.7/5.25 750 2.1 1.5 450 100 1 800 300 325 1 2.7/5.25 2.7/5.25 750 2.1 1.5 450 100 1 -2-
ISOURCE = 200 A; VDRIVE = 2.7 V to 5.25 V ISINK = 200 A
ns max ns max ns max MSPS max V min/max V min/max A typ mA max mA max A typ A max A max
16 SCLK Cycles with SCLK at 20 MHz Sine Wave Input Full-Scale Step Input See Serial Interface Section
Digital I/Ps = 0 V or VDRIVE VDD = 2.7 V to 5.25 V. SCLK On or Off VDD = 4.75 V to 5.25 V. fSAMPLE = 1 MSPS VDD = 2.7 V to 3.6 V. fSAMPLE = 1 MSPS fSAMPLE = 100 kSPS (Static) SCLK On or Off REV. A
AD7475-SPECIFICATIONS (continued)
Parameter POWER REQUIREMENTS (continued) Power Dissipation3 Normal Mode (Operational) Partial Power-Down (Static) Full Power-Down A Version1 B Version1 Unit
AD7475/AD7495
Test Conditions/Comments
10.5 4.5 500 300 5 3
10.5 4.5 500 300 5 3
mW max mW max W max W max W max W max
VDD = 5 V. fSAMPLE = 1 MSPS VDD = 3 V. fSAMPLE = 1 MSPS VDD = 5 V VDD = 3 V VDD = 5 V VDD = 3 V
NOTES 1 Temperature ranges as follows: A, B Versions: -40 C to +85 C. 2 Sample tested @ 25 C to ensure compliance. 3 See Power Versus Throughput Rate section. Specifications subject to change without notice.
AD7495-SPECIFICATIONS1 T
Parameter DYNAMIC PERFORMANCE Signal to Noise + Distortion (SINAD) Total Harmonic Distortion (THD) Peak Harmonic or Spurious Noise (SFDR) Intermodulation Distortion (IMD) Second Order Terms Third Order Terms Aperture Delay Aperture Jitter Full Power Bandwidth Full Power Bandwidth DC ACCURACY Resolution Integral Nonlinearity Differential Nonlinearity Offset Error Gain Error ANALOG INPUT Input Voltage Ranges DC Leakage Current Input Capacitance 68
(VDD = 2.7 V to 5.25 V, VDRIVE = 2.7 V to 5.25 V, fSCLK = 20 MHz unless otherwise noted; TA = TMIN to MAX, unless otherwise noted.)
B Version1 68 -75 -76 -78 -78 10 50 8.3 1.3 12 1 0.5 +1.5/-0.9 0.6 8 7 0 to 2.5 1 20 Unit dB min dB max dB max dB typ dB typ ns typ ps typ MHz typ MHz typ Bits LSB max LSB typ LSB max LSB typ LSB max LSB max Volts A max pF typ V min/max typ ppm/ C typ V min V max A max pF max V min V max A max pF max Test Conditions/Comments fIN = 300 kHz Sine Wave, fSAMPLE = 1 MSPS fIN = 300 kHz Sine Wave, fSAMPLE = 1 MSPS fIN = 300 kHz Sine Wave, fSAMPLE = 1 MSPS
A Version1
-75 -76 -78 -78 10 50 8.3 1.3 12 1.5 0.5 +1.5/-0.9 0.6 8 7 0 to 2.5 1 20
@ 3 dB @ 0.1 dB
@ 5 V (typ @ 3 V) @ 25C @ 5 V Guaranteed No Missed Codes to 12 Bits (typ @ 3 V) @ 25C Typically 2.5 LSB Typically 2.5 LSB
REFERENCE OUTPUT REF OUT Output Voltage 2.4625/2.5375 2.4625/2.5375 REF OUT Impedance 10 10 REF OUT Temperature Coefficient 50 50 LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN2 VDRIVE - 1 0.4 1 10 VDRIVE - 1 0.4 1 10
Typically 10 nA, VIN = 0 V or VDRIVE
LOGIC OUTPUTS Output High Voltage, VOH VDRIVE - 0.2 Output Low Voltage, VOL 0.4 0.4 Floating-State Leakage Current 10 10 10 Floating-State Output Capacitance2 10 Output Coding Straight (Natural) Binary REV. A -3-
ISOURCE = 200 A; VDD = 2.7 V to 5.25 V ISINK = 200 A
AD7475/AD7495-SPECIFICATIONS1
AD7495-SPECIFICATIONS (continued)
Parameter CONVERSION RATE Conversion Time Track/Hold Acquisition Time Throughput Rate POWER REQUIREMENTS VDD VDRIVE IDD Normal Mode (Static) Normal Mode (Operational) Partial Power-Down Mode Partial Power-Down Mode Full Power-Down Mode Power Dissipation3 Normal Mode (Operational) Partial Power-Down (Static) Full Power-Down A Version1 800 300 325 1 2.7/5.25 2.7/5.25 1 2.6 2 650 230 1 13 6 1.15 690 5 3 B Version1 800 300 325 1 2.7/5.25 2.7/5.25 1 2.6 2 650 230 1 13 6 1.15 690 5 3 Unit ns max ns max ns max MSPS max V min/max V min/max mA typ mA max mA max A typ A max A max mW max mW max mW max W max W max W max Digital I/Ps = 0 V or VDRIVE VDD = 2.7 V to 5.25 V. SCLK On or Off VDD = 4.75 V to 5.25 V. fSAMPLE = 1 MSPS VDD = 2.7 V to 3.6 V. fSAMPLE = 1 MSPS fSAMPLE = 100 kSPS (Static) (Static) SCLK On or Off VDD = 5 V. fSAMPLE = 1 MSPS VDD = 3 V. fSAMPLE = 1 MSPS VDD = 5 V VDD = 3 V VDD = 5 V VDD = 3 V Test Conditions/Comments 16 SCLK Cycles with SCLK at 20 MHz Sine Wave Input Full-Scale Step Input See Serial Interface Section
NOTES 1 Temperature ranges as follows: A, B Versions: -40 C to +85 C. 2 Sample tested @ 25 C to ensure compliance. 3 See Power Versus Throughput Rate section. Specifications subject to change without notice.
TIMING SPECIFICATIONS1
Parameter fSCLK 2 tCONVERT tQUIET t2 t3 3 t4 3 t5 t6 t7 t8 4 t9 4 tPOWER-UP 10 20 16 x tSCLK 800 100 10 22 40 0.4 tSCLK 0.4 tSCLK 10 10 45 20 20 650
(VDD = 2.7 V to 5.25 V, VDRIVE = 2.7 V to 5.25 V, REF IN = 2.5 V (AD7475); TA = TMIN to TMAX, unless otherwise noted.)
Unit kHz min MHz max ns max ns min ns min ns max ns max ns min ns min ns min ns min ns max ns max s max s max tSCLK = 1/fSCLK fSCLK = 20 MHz Minimum Quiet Time Required between Conversions CS to SCLK Setup Time Delay from CS Until SDATA 3-State Disabled Data Access Time after SCLK Falling Edge SCLK Low Pulsewidth SCLK High Pulsewidth SCLK to Data Valid Hold Time SCLK Falling Edge to SDATA High Impedance SCLK Falling Edge to SDATA High Impedance CS Rising Edge to SDATA High Impedance Power-Up Time from Full Power-Down AD7475 Power-Up Time from Full Power-Down AD7495 Description
Limit at TMIN, TMAX AD7475/AD7495
NOTES 1 Sample tested at 25 C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of V DRIVE) and timed from a voltage level of 1.6 V. 2 Mark/Space ratio for the SCLK input is 40/60 to 60/40. 3 Measured with the load circuit of Figure 3 and defined as the time required for the output to cross 0.8 V or 2.0 V. 4 t8 and t9 are derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 3. The measured number is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times, t 8 and t9, quoted in the timing characteristics are the true bus relinquish time of the part and are independent of the bus loading. Specifications subject to change without notice.
-4-
REV. A
AD7475/AD7495
CS
tCONVERT t2
SCLK 1 2 3 4
t6
5 13
B
14 15 16
t7 t3
SDATA THREE-STATE 0 0 0 0 FOUR LEADING ZEROS
t5
DB2 DB1
t4
DB11 DB10
t8
DB0
tQUIET
THREE-STATE
Figure 1. Serial Interface Timing Diagram
Timing Example 1
Having fSCLK = 20 MHz and a throughput of 1 MSPS gives a cycle time of t2 + 12.5(1/fSCLK) + tACQ = 1 s. With t2 = 10 ns min, this leaves tACQ to be 365 ns. This 365 ns satisfies the requirement of 300 ns for tACQ. From Figure 2, tACQ comprises of 2.5(1/fSCLK) + t8 + tQUIET, where t8 = 45 ns. This allows a value of 195 ns for tQUIET, satisfying the minimum requirement of 100 ns.
Timing Example 2
Having fSCLK = 5 MHz and a throughput of 315 KSPS, gives a cycle time of t2 + 12.5(1/fSCLK) + tACQ = 3.174 s.
With t2 = 10 ns min, this leaves tacq to be 664 ns. This 664 ns satisfies the requirement of 300 ns for tACQ. From Figure 2, tACQ is comprised of 2.5(1/fSCLK) + t8 + tQUIET, t8 = 45 ns. This allows a value of 119 ns for tQUIET satisfying the minimum requirement of 100 ns. As in this example and with other slower clock values, the signal may already be acquired before the conversion is complete, but it is still necessary to leave 100 ns minimum tQUIET between conversions. In Example 2 the signal should be fully acquired at approximately Point C in Figure 2.
CS
tCONVERT t2
SCLK 1 2 3 4
t6
5 13
B
14
C
15 16
t5
12.5 (1/fSCLK) 10ns 1/THROUGHPUT
t8
45ns
tQUIET tACQUISITION
Figure 2. Serial Interface Timing Example
200 A
IOL
TO OUTPUT PIN
1.6V CL 50pF 200 A IOH
Figure 3. Load Circuit for Digital Output Timing Specifications
REV. A
-5-
AD7475/AD7495
ABSOLUTE MAXIMUM RATINGS 1
(TA = 25 C unless otherwise noted)
PIN CONFIGURATIONS AD7475 SOIC/ SOIC
VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V VDRIVE to GND . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V Analog Input Voltage to GND . . . . . . . . -0.3 V to VDD + 0.3 V Digital Input Voltage to GND . . . . . . . . . . . . . -0.3 V to +7 V VDRIVE to DVDD . . . . . . . . . . . . . . . . . -0.3 V to DVDD + 0.3 V Digital Output Voltage to GND . . . . . . -0.3 V to VDD + 0.3 V REF IN to GND . . . . . . . . . . . . . . . . . . -0.3 V to VDD + 0.3 V Input Current to Any Pin Except Supplies2 . . . . . . . 10 mA Operating Temperature Range Commercial (A, B Version) . . . . . . . . . . . . -40 C to +85 C Storage Temperature Range . . . . . . . . . . . -65 C to +150 C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150 C SOIC, SOIC Package, Power Dissipation . . . . . . . . 450 mW JA Thermal Impedance . . . . . . . . . . . . . . 157 C/W (SOIC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.9 C/W (SOIC) JC Thermal Impedance . . . . . . . . . . . . . . . 56 C/W (SOIC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.74 C/W (SOIC) Lead Temperature, Soldering Vapor Phase (60 secs) . . . . . . . . . . . . . . . . . . . . . . . . 215 C Infrared (15 secs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 C ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 kV
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 Transient currents of up to 100 mA will not cause SCR latch-up.
REF IN 1 VIN 2
8
VDD
CS TOP VIEW GND 3 (Not to Scale) 6 VDRIVE
7
AD7475
SCLK 4
5
SDATA
AD7495 SOIC/ SOIC
REF OUT 1 VIN 2
8
VDD
CS TOP VIEW GND 3 (Not to Scale) 6 VDRIVE
7
AD7495
SCLK 4
5
SDATA
ORDERING GUIDE
Model AD7495AR AD7495BR AD7495ARM AD7495BRM AD7475AR AD7475BR AD7475ARM AD7475BRM EVAL-AD7495CB3 EVAL-AD7475CB3 EVAL-CONTROL BRD24
Range -40 C to +85 C -40 C to +85 C -40 C to +85 C -40 C to +85 C -40 C to +85 C -40 C to +85 C -40 C to +85 C -40 C to +85 C Evaluation Board Evaluation Board Controller Board
Linearity Error (LSB)1 1.5 1 1.5 1 1.5 1 1.5 1
Package Option2 SO-8 SO-8 RM-8 RM-8 SO-8 SO-8 RM-8 RM-8
Branding Information AD7495AR AD7495BR CCA CCB AD7475AR AD7475BR C9A C9B
NOTES 1 Linearity Error here refers to Integral Linearity Error. 2 SO = SOIC; RM = SOIC. 3 This can be used as a standalone evaluation board or in conjunction with the EVAL-BOARD CONTROLLER for evaluation/demonstration purposes. 4 This EVALUATION BOARD CONTROLLER is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.
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 AD7475/AD7495 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
-6-
REV. A
AD7475/AD7495
PIN FUNCTION DESCRIPTIONS
Pin No. 1
Mnemonic REF IN
Function Reference Input for the AD7475. An external reference must be applied to this input. The voltage range for the external reference is 2.5 V 1% for specified performance. A cap of a least 0.1 F should be placed on the REF IN pin. Reference Output for the AD7495. A minimum 100 nF capacitance is required from this pin to GND. The internal reference can be taken from this pin but buffering is required before it is applied elsewhere in a system. Analog Input. Single-ended analog input channel. The input range is 0 to REF IN. Analog Ground. Ground reference point for all circuitry on the AD7475/AD7495. All analog input signals and any external reference signal should be referred to this GND voltage. Serial Clock. Logic input. SCLK provides the serial clock for accessing data from the part. This clock input is also used as the clock source for the AD7475/AD7495's conversion process. Data Out. Logic Output. The conversion result from the AD7475/AD7495 is provided on this output as a serial data stream. The bits are clocked out on the falling edge of the SCLK input. The data stream consists of four leading zeros followed by the 12 bits of conversion data which is provided MSB first. Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the serial interface of the AD7475/AD7495 will operate. Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the AD7475/AD7495 and also frames the serial data transfer. Power Supply Input. The VDD range for the AD7475/AD7495 is from 2.7 V to 5.25 V.
Total Harmonic Distortion
REF OUT 2 3 4 5 VIN GND SCLK SDATA
6 7 8
VDRIVE CS VDD
TERMINOLOGY Integral Nonlinearity
This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints of the transfer function are zero scale, a point 1/2 LSB below the first code transition, and full scale, a point 1/2 LSB above the last code transition.
Differential Nonlinearity
Total harmonic distortion (THD) is the ratio of the rms sum of harmonics to the fundamental. For the AD7475/AD7495, it is defined as:
THD(dB ) = 20 log V22 + V32 + V42 + V52 + V62 V1
This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC.
Offset Error
where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonics.
Peak Harmonic or Spurious Noise
This is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal, i.e., AGND + 0.5 LSB.
Gain Error
This is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (i.e., VREF - 1.5 LSB) after the offset error has been adjusted out.
Track/Hold Acquisition Time
Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, it will be a noise peak.
Intermodulation Distortion
The track/hold amplifier returns into track mode on the 13th SCLK rising edge (see Serial Interface section). The Track/Hold Acquisition Time is the minimum time required for the trackand-hold amplifier to remain in track mode for its output to reach and settle to within 0.5 LSB of the applied input signal, given a step change to the input signal.
Signal to (Noise + Distortion) Ratio
With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb where m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are those for which neither m nor n is equal to zero. For example, the second order terms include (fa + fb) and (fa - fb), while the third order terms include (2fa + fb), (2fa - fb), (fa + 2fb) and (fa - 2fb). The AD7475/AD7495 are tested using the CCIF standard where two input frequencies near the top end of the input bandwidth are used. In this case, the second order terms are usually distanced in frequency from the original sine waves while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dBs.
This is the measured ratio of signal to (noise + distortion) at the output of the A/D converter. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal to (noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by: Signal to (Noise + Distortion) = (6.02 N + 1.76) dB Thus for a 12-bit converter, this is 74 dB. REV. A
-7-
AD7475/AD7495
AD7475/AD7495 TYPICAL PERFORMANCE CURVES CIRCUIT INFORMATION
TPC 1 shows a typical FFT plot for the AD7475 at 1 MHz sample rate and 100 kHz input frequency.
-15
-35
SINAD - dB
8192 POINT FFT fSAMPLE = 1MSPS fIN = 100kHz SINAD = 70.46dB THD = -87.7dB SFDR = -89.5dB
The AD7475/AD7495 are fast, micropower, 12-bit, single-supply, A/D converters. The parts can be operated from a 2.7 V to 5.25 V supply. When operated from either a 5 V supply or a 3 V supply, the AD7475/AD7495 are capable of throughput rates of 1 MSPS when provided with a 20 MHz clock. The AD7475/AD7495 provide the user with an on-chip track/ hold, A/D converter, and a serial interface housed in either an 8-lead SOIC or SOIC package, which offers the user considerable space-saving advantages over alternative solutions. The AD7495 also has an on-chip 2.5 V reference. The serial clock input accesses data from the part but also provides the clock source for the successive-approximation A/D converter. The analog input range is 0 V to REF IN for the AD7475 and 0 V to REF OUT for the AD7495. The AD7475/AD7495 also feature power-down options to allow power saving between conversions. The power-down feature is implemented across the standard serial interface as described in the Modes of Operation section.
CONVERTER OPERATION
-55
-75
-95
-115
0
50
100
150
200 250 300 350 FREQUENCY - kHz
400
450
500
TPC 1. AD7475 Dynamic Performance
TPC 2 shows a typical FFT plot for the AD7495 at 1 MHz sample rate and 100 kHz input frequency.
-15
-35
8192 POINT FFT fSAMPLE = 1MSPS fIN = 100kHz SINAD = 69.95dB THD = -89.2dB SFDR = -91.2dB
-55
The AD7475/AD7495 are 12-bit successive approximation analog-to-digital converters based around a capacitive DAC. The AD7475/AD7495 can convert analog input signals in the range 0 V to 2.5 V. Figures 4 and 5 show simplified schematics of the ADC. The ADC comprises of Control Logic, SAR and a Capacitive DAC, which are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into a balanced condition. Figure 4 shows the ADC during its acquisition phase. SW2 is closed and SW1 is in Position A. The comparator is held in a balanced condition and the sampling capacitor acquires the signal on VIN.
CAPACITIVE DAC
SINAD - dB
-75
-95
-115
0
50
100
150
200 250 300 350 FREQUENCY - kHz
400
450
500
VIN
A SW1 B
4k CONTROL LOGIC SW2 COMPARATOR
TPC 2. AD7495 Dynamic Performance
AGND
TPC 3 shows the signal-to-(noise + distortion) ratio performance versus input frequency for various supply voltages while sampling at 1 MSPS with an SCLK of 20 MHz.
71.0
Figure 4. ADC Acquisition Phase
70.5 VDD = VDRIVE = 3.60V
VDD = VDRIVE = 4.75V
SINAD - dB
70.0
When the ADC starts a conversion (see Figure 5), SW2 will open and SW1 will move to position B causing the comparator to become unbalanced. The Control Logic and the Capacitive DAC are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into a balanced condition. When the comparator is rebalanced, the conversion is complete. The Control Logic generates the ADC output code. Figure 6 shows the ADC transfer function.
VDD = VDRIVE = 2.70V 69.5 VDD = VDRIVE = 5.25V 69.0
CAPACITIVE DAC
VIN
A SW1 B
4k CONTROL LOGIC SW2 COMPARATOR
68.5
10
100 INPUT FREQUENCY - kHz
1000
AGND
Figure 5. ADC Conversion Phase
TPC 3. AD7495 SINAD vs. Input Frequency at 1 MSPS
-8-
REV. A
AD7475/AD7495
ADC TRANSFER FUNCTION
The output coding of the AD7475/AD7495 is straight binary. The designed code transitions occur midway between successive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, etc.). The LSB size is = VREF/4096. The ideal transfer characteristic for the AD7475/ AD7495 is shown in Figure 6 below.
5V SUPPLY 0.1 F 10 F
SERIAL INTERFACE
VDD 0V TO 2.5V INPUT VIN
SCLK
AD7475
SDATA CS VDRIVE 0.1 F 10 F
C/ P
GND 111...111 111...110 ADC CODE REF IN
111...000 1LSB = VREF/4096 011...111
0.1 F (MIN)
2.5V AD780
3V SUPPLY
000...010 000...001 000...000 0V 0.5LSB VREF -1.5LSB ANALOG INPUT
Figure 7. AD7475 Typical Connection Diagram
Analog Input
Figure 6. AD7475/AD7495 Transfer Characteristic
TYPICAL CONNECTION DIAGRAM
Figure 7 and Figure 8 show a typical connection diagram for the AD7475 and AD7495 respectively. In both setups the GND pin is connected to the analog ground plane of the system. In Figure 7 REF IN is connected to a decoupled 2.5 V supply from a reference source, the AD780, to provide an analog input range of 0 V to 2.5 V. Although the AD7475 is connected to a VDD of 5 V, the serial interface is connected to a 3 V microprocessor. The VDRIVE pin of the AD7475 is connected to the same 3 V supply of the microprocessor to allow a 3 V logic interface, see Digital Inputs Section. In Figure 8, the REF OUT pin of the AD7495 is connected to a buffer and then applied to a level-shifting circuit used on the analog input to allow a bipolar signal to be applied to the AD7495. A minimum 100 nF capacitance is required on the REF OUT pin to GND. The conversion result from both ADCs is output in a 16-bit word with four leading zeros followed by the MSB of the 12-bit result. For applications where power consumption is of concern, the power-down modes should be used between conversions or bursts of several conversions to improve power performance. See Modes of Operation section of the data sheet.
Figure 9 shows an equivalent circuit of the analog input structure of the AD7475/AD7495. The two diodes D1 and D2 provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 200 mV. This will cause these diodes to become forward-biased and start conducting current into the substrate. 20 mA is the maximum current these diodes can conduct without causing irreversible damage to the part. The capacitor C1 in Figure 9 is typically about 4 pF and can primarily be attributed to pin capacitance. The resistor R1 is a lumped component made up of the on resistance of a switch. This resistor is typically about 100 . The capacitor C2 is the ADC sampling capacitor and has a capacitance of 16 pF typically. For ac applications, removing high frequency components from the analog input signal is recommended by use of an RC low-pass filter on the relevant analog input pin. In applications where harmonic distortion and signal to noise ratio are critical, the analog input should be driven from a low impedance source. Large source impedances will significantly affect the ac performance of the ADC. This may necessitate the use of an input buffer amplifier. The choice of the op amp will be a function of the particular application.
VDD D1 VIN C1 4pF D2 CONVERSION PHASE-SWITCH OPEN TRACK PHASE-SWITCH CLOSED C2 16pF
R1
Figure 9. Equivalent Analog Input Circuit
0.1 F V R 3R R GND REF OUT R 0V TO 2.5V INPUT VDD VIN
10 F
5V SUPPLY
SERIAL INTERFACE
SCLK
0V
V
AD7495
SDATA CS VDRIVE 0.1 F 10 F
C/ P
0.1 F (MIN)
3V SUPPLY
Figure 8. AD7495 Typical Connection Diagram
REV. A
-9-
AD7475/AD7495
When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance will depend on the amount of total harmonic distortion (THD) that can be tolerated. The THD will increase as the source impedance increases and performance will degrade. Figure 10 shows a graph of the total harmonic distortion versus source impedance for various analog input frequencies.
-10 -20 -30 -40 fIN = 10kHz fIN = 500kHz fIN = 200kHz -60 -70 fIN = 100kHz -80 -90
allows the ADC to easily interface to both 3 V and 5 V processors. For example, if the AD7475/AD7495 were operated with a VDD of 5 V, and the VDRIVE pin could be powered from a 3 V supply. The AD7475/AD7495 has better dynamic performance with a VDD of 5 V while still being able to interface to 3 V digital parts. Care should be taken to ensure VDRIVE does not exceed VDD by more than 0.3 V. (See Absolute Maximum Ratings.)
Reference Section
THD - dB
An external reference source should be used to supply the 2.5 V reference to the AD7475. Errors in the reference source will result in gain errors in the AD7475 transfer function and will add the specified full-scale errors on the part. A capacitor of at least 0.1 F should be placed on the REF IN pin. Suitable reference sources for the AD7475 include the AD780, the AD680, and the AD1852. The AD7495 contains an on-chip 2.5 V reference. As shown in Figure 12, the voltage that appears at the REF OUT pin is internally buffered before being applied to the ADC, the output impedance of this buffer is typically 10 . The reference is capable of sourcing up to 2 mA. The REF OUT pin should be decoupled to AGND using a 100 nF or greater capacitor. If the 2.5 V internal reference is to be used to drive another device that is capable of glitching the reference at critical times, then the reference will have to be buffered before driving the device. To ensure optimum performance of the AD7495 it is recommended that the Internal Reference not be over driven. If the use of an external reference is required the AD7475 should be used.
V REF OUT 25
-50
1
10 100 1000 SOURCE IMPEDANCE - Ohms
10000
Figure 10. THD vs. Source Impedance for Various Analog Input Frequencies
Figure 11 shows a graph of total harmonic distortion versus analog Input frequency for various supply voltages while sampling at 1 MSPS with an SCLK of 20 MHz.
-75 -77 -79 -81 VDD = VDRIVE = 5.25V VDD = VDRIVE = 2.70V
160k
40k
Figure 12. AD7495 Reference Circuit
VDD = VDRIVE = 3.60V
THD - dB
-83 -85 -87 VDD = VDRIVE = 4.75V -89 -91 -93 -95 10 100 INPUT FREQUENCY - kHz 1000
MODES OF OPERATION
Figure 11. THD vs. Analog Input Frequency for Various Supply Voltages
Digital Inputs
The mode of operation of the AD7475/AD7495 is selected by controlling the (logic) state of the CS signal during a conversion. There are three possible modes of operation, Normal Mode, Partial Power-Down Mode, and Full Power-Down Mode. The point at which CS is pulled high after the conversion has been initiated will determine which power-down mode, if any, the device will enter. Similarly, if already in a power-down mode, CS can control whether the device will return to Normal operation or remain in power-down. These modes of operation are designed to provide flexible power management options. These options can be chosen to optimize the power dissipation/throughput rate ratio for differing application requirements.
Normal Mode
The digital inputs applied to the AD7475/AD7495 are not limited by the maximum ratings which limit the analog inputs. Instead, the digital inputs applied can go to 7 V and are not restricted by the VDD + 0.3 V limit as on the analog inputs. Another advantage of SCLK and CS not being restricted by the VDD + 0.3 V limit is the fact that power supply sequencing issues are avoided. If CS or SCLK are applied before VDD, there is no risk of latch-up as there would be on the analog inputs if a signal greater than 0.3 V were applied prior to VDD.
VDRIVE
This mode is intended for fastest throughput rate performance as the user does not have to worry about any power-up times with the AD7475/AD7495 remaining fully powered all the time. Figure 13 shows the general diagram of the operation of the AD7475/AD7495 in this mode. The conversion is initiated on the falling edge of CS as described in the Serial Interface section. To ensure the part remains fully powered up at all times, CS must remain low until at least 10 SCLK falling edges have elapsed after the falling edge of CS. If CS is brought high any time after the 10th SCLK falling edge, but -10- REV. A
The AD7475/AD7495 also has the VDRIVE feature. VDRIVE controls the voltage at which the serial interface operates. VDRIVE
AD7475/AD7495
CS 1 SCLK 10 16
SDATA
FOUR LEADING ZEROS + CONVERSION RESULT
Figure 13. Normal Mode Operation
CS 1 SCLK 2 10 16
Figure 14. Entering Partial Power-Down Mode
THE PART BEGINS TO POWER UP CS A1 SCLK 10 16 1 16 THE PART IS FULLY POWERED UP
SDATA
INVALID DATA
VALID DATA
Figure 15. Exiting Partial Power-Down Mode
before the 16th SCLK falling edge, the part will remain powered up but the conversion will be terminated and SDATA will go back into three-state. Sixteen serial clock cycles are required to complete the conversion and access the conversion result. CS may idle high until the next conversion or may idle low until sometime prior to the next conversion (effectively idling CS low). Once a data transfer is complete (SDATA has returned to three-state), another conversion can be initiated after the quiet time, tQUIET, has elapsed by bringing CS low again.
Partial Power-Down Mode
This mode is intended for use in applications where slower throughput rates are required; either the ADC is powered down between each conversion, or a series of conversions may be performed at a high throughput rate and then the ADC is powered down for a relatively long duration between these bursts of several conversions. When the AD7475 is in partial power-down, all analog circuitry is powered down except for the bias current generator; and, in the case of the AD7495, all analog circuitry is powered down except for the on-chip reference and reference buffer. To enter partial power-down, the conversion process must be interrupted by bringing CS high anywhere after the second falling edge of SCLK and before the tenth falling edge of SCLK as shown in Figure 14. Once CS has been brought high in this window of SCLKs, the part will enter partial power-down, and the conversion that was initiated by the falling edge of CS will be terminated, and SDATA will go back into three-state. If CS is brought high before the second SCLK falling edge, the part will remain in Normal Mode and will not power down. This will avoid accidental power-down due to glitches on the CS line. In order to exit this mode of operation and power the AD7475/ AD7495 up again, a dummy conversion is performed. On the REV. A
falling edge of CS the device will begin to power up, and will continue to power up as long as CS is held low until after the falling edge of the tenth SCLK. The device will be fully powered up once 16 SCLKs have elapsed, and valid data will result from the next conversion as shown in Figure 15. If CS is brought high before the second falling edge of SCLK, the AD7475/AD7495 will go back into partial power-down again. This avoids accidental power-up due to glitches on the CS line; although the device may begin to power up on the falling edge of CS, it will power down again on the rising edge of CS. If in partial power-down and CS is brought high between the second and tenth falling edges of SCLK, the device will enter full power-down mode.
Power-Up Time
The power-up time of the AD7475/AD7495 from partial powerdown is typically 1 s, which means that with any frequency of SCLK up to 20 MHz, one dummy cycle will always be sufficient to allow the device to power up from partial power-down. Once the dummy cycle is complete, the ADC will be fully powered up and the input signal will be acquired properly. The quiet time tQUIET must still be allowed from the point where the bus goes back into three-state after the dummy conversion, to the next falling edge of CS. When running at 1 MSPS throughput rate, the AD7475/AD7495 will power up and acquire a signal within 0.5 LSB in one dummy cycle, i.e., 1 s. When powering up from the power-down mode with a dummy cycle, as in Figure 15, the track-and-hold that was in hold mode while the part was powered down, returns to track mode after the first SCLK edge the part receives after the falling edge of CS. This is shown as Point A in Figure 15. Although at any SCLK frequency one dummy cycle is sufficient to power the device up and acquire VIN, it does not necessarily mean that a full dummy -11-
AD7475/AD7495
THE PART ENTERS PARTIAL POWER-DOWN CS 1 SCLK 2 10 16 1 2 10 16 THE PART BEGINS TO POWER UP THE PART ENTERS FULL POWER-DOWN
SDATA
INVALID DATA
THREE-STATE INVALID DATA
THREE-STATE
Figure 16. Entering Full Power-Down Mode
THE PART BEGINS TO POWER UP THE PART IS FULLY POWERED UP
t POWER-UP
CS 1 SCLK 10 16 1 16
SDATA
INVALID DATA
VALID DATA
Figure 17. Exiting Full Power-Down Mode
cycle of 16 SCLKs must always elapse to power up the device and fully acquire VIN; 1 s will be sufficient to power the device up and acquire the input signal. If, for example, a 5 MHz SCLK frequency was applied to the ADC, the cycle time would be 3.2 s. In one dummy cycle, 3.2 s, the part would be powered up and V IN fully acquired. However, after 1 s with a 5 MHz SCLK, only 5 SCLK cycles would have elapsed. At this stage, the ADC would be fully powered up and the signal acquired. So, in this case the CS can be brought high after the tenth SCLK falling edge and brought low again after a time tQUIET to initiate the conversion.
Full Power-Down Mode
long as CS is held low until after the falling edge of the tenth SCLK. The power-up time is longer than one dummy conversion cycle however, and this time, tPOWER-UP, must elapse before a conversion can be initiated as shown in Figure 17. (See Timing Specifications.) When power supplies are first applied to the AD7475/AD7495, the ADC may power up in either of the power-down modes or normal mode. Because of this, it is best to allow a dummy cycle to elapse to ensure the part is fully powered up before attempting a valid conversion. Likewise, if it is intended to keep the part in the partial power-down mode immediately after the supplies are applied, then two dummy cycles must be initiated. The first dummy cycle must hold CS low until after the tenth SCLK falling edge, Figure 13; in the second cycle CS must be brought high before the tenth SCLK edge but after the second SCLK falling edge, Figure 14. Alternatively, if it is intended to place the part in full power-down mode when the supplies have been applied, then three dummy cycles must be initiated. The first dummy cycle must hold CS low until after the tenth SCLK edge, Figure 13; the second and third dummy cycle place the part in full power-down, Figure 16. See Modes of Operation section. Once supplies are applied to the AD7475/AD7495, enough time must be allowed, for the AD7475, for the external reference to power up and charge the reference capacitor to its final value. For the AD7495, enough time should be allowed for the internal reference buffer to charge the reference capacitor. Then, to place the AD7475/AD7495 in normal mode, a dummy cycle, 1 s, should be initiated. If the first valid conversion is then performed directly after the dummy conversion, care must be taken to ensure that adequate acquisition time has been allowed. As mentioned earlier, when powering up from the power-down mode, the part will return to track upon the first SCLK edge applied after the falling edge of CS. However, when the ADC powers up initially after supplies are applied, the track-and-hold will already be in track. This means (assuming one has the facility to monitor the ADC supply current) if the ADC powers up REV. A
This mode is intended for use in applications where slower throughput rates are required than that in the partial power-down mode, as power up from a full power-down would not be complete in just one dummy conversion. This mode is more suited to applications where a series of conversions performed at a relatively high throughput rate would be followed by a long period of inactivity and hence power-down. When the AD7475/AD7495 is in full power-down, all analog circuitry is powered down. Full power-down is entered in a way similar to partial power-down, except the timing sequence shown in Figure 14 must be executed twice. The conversion process must be interrupted in a similar fashion by bringing CS high anywhere after the second falling edge of SCLK and before the tenth falling edge of SCLK. The device will enter partial power-down at this point. To reach full power-down, the next conversion cycle must be interrupted in the same way as shown in Figure 16. Once CS has been brought high in this window of SCLKs, then the part will power down completely. NOTE: It is not necessary to complete the 16 SCLKs once CS has been brought high to enter a power-down mode. To exit full power-down, and power the AD7475/AD7495 up again, a dummy conversion is performed as when powering up from partial power-down. On the falling edge of CS the device will begin to power up, and will continue to power up as
-12-
AD7475/AD7495
in the desired mode of operation, and thus a dummy cycle is not required to change mode, then neither is a dummy cycle required to place the track-and-hold into track. If no current monitoring facility is available, the relevant dummy cycle(s) should be performed to ensure the part is in the required mode.
POWER VERSUS THROUGHPUT RATE
By using the partial power-down mode on the AD7475/AD7495 when not converting, the average power consumption of the ADC decreases at lower throughput rates. Figure 18 shows how, as the throughput rate is reduced, the part remains in its partial power-down state longer and the average power consumption over time drops accordingly.
100 AD7495 5V SCLK = 20MHz
remaining 8 s where the part is in partial power-down. With a throughput rate of 100 kSPS, the average power dissipated during each conversion cycle is (2/10) (6 mW) + (8/10) (0.69 mW) = 1.752 mW. Figure 18 shows the power versus throughput rate when using the partial power-down mode between conversions with both 5 V and 3 V supplies for both the AD7475 and AD7495. For the AD7475, partial power-down current is lower than that of the AD7495. Full power-down mode is intended for use in applications with slower throughput rates than required for the partial powerdown mode. It is necessary to leave 650 s for the AD7495 to be fully powered up from full power-down before initiating a conversion. Current consumptions between conversions is typically less than 1 A. Figure 19 shows a typical graph of current versus throughput for the AD7495 while operating in different modes. At slower throughput rates, e.g., 10 SPS to 1 kSPS, the AD7495 was operated in Full Power-Down mode. As the throughput rate increased, up to 100 kSPS, the AD7495 was operated in Partial Power-Down mode, with the part being powered down between conversions. With throughput rates from 100 kSPS to 1 MSPS, the part operated in Normal mode, remaining fully powered up at all times.
2.0 1.8 VDD = 5V
10
AD7475 5V SCLK = 20MHz
POWER - mW
1 AD7475 3V SCLK = 20MHz 0.1
AD7495 3V SCLK = 20MHz
0.01
0.001 0 50 100 150 200 250 THROUGHPUT - kSPS 300 350
1.6 1.4
Figure 18. AD7495 Power vs. Throughput for Partial Power-Down
CURRENT - mA
1.2 1.0 0.8 0.6 0.4 0.2 0 10 FULL POWER-DOWN PARTIAL POWER-DOWN
For example if the AD7495 is operated in a continuous sampling mode with a throughput rate of 100 kSPS and an SCLK of 20 MHz (VDD = 5 V), and the device is placed in partial powerdown mode between conversions, then the power consumption is calculated as follows. The maximum power dissipation during normal operation is 13 mW (VDD = 5 V). If the power-up time from partial power-down is one dummy cycle, i.e., 1 s, and the remaining conversion time is another cycle, i.e., 1 s, then the AD7495 can be said to dissipate 13 mW for 2 s during each conversion cycle. For the remainder of the conversion cycle, 8 s, the part remains in partial power-down mode. The AD7495 can be said to dissipate 1.15 mW for the remaining 8 s of the conversion cycle. If the throughput rate is 100 kSPS, the cycle time is 10 s and the average power dissipated during each cycle is (2/10) (13 mW) + (8/10) (1.15 mW) = 3.52 mW. If VDD = 3 V, SCLK = 20 MHz and the device is again in partial powerdown mode between conversions, the power dissipated during normal operation is 6 mW. The AD7495 can be said to dissipate 6 mW for 2 s during each conversion cycle and 0.69 mW for the
CS
NORMAL
100
1k 10k THROUGHPUT - SPS
100k
1M
Figure 19. Typical AD7495 Current vs. Throughput
SERIAL INTERFACE
Figure 20 shows the detailed timing diagram for serial interfacing to the AD7475/AD7495. The serial clock provides the conversion clock and also controls the transfer of information from the AD7475/AD7495 during conversion. CS initiates the data transfer and conversion process. The falling edge of CS puts the track and hold into hold mode, takes the bus out of three-state, and the analog input is sampled at this point.
tCONVERT t2
SCLK 1 2 3 4
t6
5 13
B
14 15 16
t7 t3
0 SDATA THREE-STATE 0 0 0 FOUR LEADING ZEROS
t5
DB2 DB1
t4
DB11 DB10
t8
DB0
tQUIET
THREE-STATE
Figure 20. Serial Interface Timing Diagram
REV. A
-13-
AD7475/AD7495
CS
tCONVERT t2
SCLK 1 2 3 4
t6
5 13
B
14 15 16
t7 t3
0 SDATA THREE-STATE 0 0 0 FOUR LEADING ZEROS
t9
DB2
t4
DB11 DB10
tQUIET
THREE-STATE
Figure 21. Serial Interface Timing Diagram--Conversion Termination
The conversion is also initiated at this point and will require 16 SCLK cycles to complete. Once 13 SCLK falling edges have elapsed, the track and hold will go back into track on the next SCLK rising edge as shown in Figure 20 at Point B. On the 16th SCLK falling edge the SDATA line will go back into threestate. If the rising edge of CS occurs before 16 SCLKs have elapsed, the conversion will be terminated and the SDATA line will go back into three-state, as shown in Figure 21, otherwise SDATA returns to three-state on the 16th SCLK falling edge as shown in Figure 20. Sixteen serial clock cycles are required to perform the conversion process and to access data from the AD7475/AD7495. CS going low provides the first leading zero to be read in by the microcontroller or DSP. The remaining data is then clocked out by subsequent SCLK falling edges beginning with the 2nd leading zero, thus the first falling clock edge on the serial clock has the second leading zero provided. The final bit in the data transfer is valid on the sixteenth falling edge, having being clocked out on the previous (15th) falling edge. In applications with a slower SCLK, it may be possible to read in data on each SCLK rising edge, although the first leading zero will still have to be read on the first SCLK falling edge after the CS falling edge. Therefore, the first rising edge of SCLK after the CS falling edge would provide the second leading zero and the 15th rising SCLK edge would have DB0 provided. This method may not work with most Micros/DSPs, but could possibly be used with FPGAs and ASICs.
MICROPROCESSOR INTERFACING
The connection diagram is shown in Figure 22. It should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the TMS320C5x/C54x provide equidistant sampling. The VDRIVE pin of the AD7475/ AD7495 takes the same supply voltage as that of the TMS320C5x/ C54x. This allows the ADC to operate at a higher voltage than the serial interface, i.e., TMS320C5x/C54x, if necessary.
AD7475/AD7495*
SCLK TMS320C5x/C54x* CLKX CLKR SDATA CS VDRIVE DR FBX FSR VDD
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 22. Interfacing to the TMS320C5x/C54x
AD7475/AD7495 to ADSP-21xx
The ADSP-21xx family of DSPs are interfaced directly to the AD7475/AD7495 without any glue logic required. The VDRIVE pin of the AD7475/AD7495 takes the same supply voltage as that of the ADSP-21xx. This allows the ADC to operate at a higher voltage than the serial interface, i.e., ADSP-21xx, if necessary. The SPORT control register should be set up as follows: TFSW = RFSW = 1, Alternate Framing INVRFS = INVTFS = 1, Active Low Frame Signal DTYPE = 00, Right Justify Data SLEN = 1111, 16-Bit Data Words ISCLK = 1, Internal Serial Clock TFSR = RFSR = 1, Frame Every Word IRFS = 0, ITFS = 1. To implement the power-down modes SLEN should be set to 1001 to issue an 8-bit SCLK burst. The connection diagram is shown in Figure 23. The ADSP21xx has the TFS and RFS of the SPORT tied together, with TFS set as an output and RFS set as an input. The DSP operates in Alternate Framing Mode and the SPORT control register is set up as described. The Frame synchronizations signal generated on the TFS is tied to CS and as with all signal processing applications equidistant sampling is necessary. However, in this example, the timer interrupt is used to control the sampling rate of the ADC and under certain conditions, equidistant sampling may not be achieved.
The serial interface on the AD7475/AD7495 allows the parts to be directly connected to a range of many different microprocessors. This section explains how to interface the AD7475/AD7495 with some of the more common microcontroller and DSP serial interface protocols.
AD7475/AD7495 to TMS320C5x/C54x
The serial interface on the TMS320C5x/C54x uses a continuous serial clock and frame synchronization signals to synchronize the data transfer operations with peripheral devices like the AD7475/ AD7495. The CS input allows easy interfacing between the TMS320C5x/C54x and the AD7475/AD7495 without any glue logic required. The serial port of the TMS320C5x/C54x is set up to operate in burst mode with internal CLKX (Tx serial clock) and FSX (Tx frame sync). The serial port control register (SPC) must have the following setup: FO = 0, FSM = 1, MCM = 1 and TXM = 1. The format bit, FO, may be set to 1 to set the word length to 8 bits, in order to implement the power-down modes on the AD7475/AD7495.
-14-
REV. A
AD7475/AD7495
AD7475/AD7495*
SCLK SDATA CS VDRIVE ADSP-21xx* SCLK DR RFS TFS
implement the power-down modes on the AD7475/AD7495 then the word length can be changed to eight bits by setting bits WL1 = 0 and WL0 = 0 in CRA. It should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the DSP56xxx provide equidistant sampling. The VDRIVE pin of the AD7475/AD7495 takes the same supply voltage as that of the DSP56xxx. This allows the ADC to operate at a voltage higher than the serial interface, i.e., DSP56xxx, if necessary.
AD7475/AD7495*
SCLK SDATA CS VDRIVE DSP56xxx* SCLK SRD SC2
*ADDITIONAL PINS OMITTED FOR CLARITY
VDD
Figure 23. Interfacing to the ADSP-21xx
The Timer registers etc., are loaded with a value that will provide an interrupt at the required sample interval. When an interrupt is received, a value is transmitted with TFS/DT (ADC control word). The TFS is used to control the RFS and hence the reading of data. The frequency of the serial clock is set in the SCLKDIV register. When the instruction to transmit with TFS is given, (i.e., AX0 = TX0), the state of the SCLK is checked. The DSP will wait until the SCLK has gone high, low, and high before transmission will start. If the timer and SCLK values are chosen such that the instruction to transmit occurs on or near the rising edge of SCLK, the data may be transmitted or it may wait until the next clock edge. For example, the ADSP-2111 has a master clock frequency of 16 MHz. If the SCLKDIV register is loaded with the value 3, an SCLK of 2 MHz is obtained, and eight master clock periods will elapse for every 1 SCLK period. If the timer registers are loaded with the value 803, 100.5 SCLKs will occur between interrupts and subsequently between transmit instructions. This situation will result in nonequidistant sampling as the transmit instruction is occurring on a SCLK edge. If the number of SCLKs between interrupts is a whole integer figure of N, equidistant sampling will be implemented by the DSP.
AD7475/AD7495 to DSP56xxx
*ADDITIONAL PINS OMITTED FOR CLARITY
VDD
Figure 24. Interfacing to the DSP56xxx
AD7475/AD7495 to MC68HC16
The Serial Peripheral Interface (SPI) on the MC68HC16 is configured for Master Mode (MSTR = 1), Clock Polarity Bit (CPOL) = 1 and the Clock Phase Bit (CPHA) = 0. The SPI is configured by writing to the SPI Control Register (SPCR), see 68HC16 user manual. The serial transfer will take place as a 16-bit operation when the SIZE bit in the SPCR register is set to SIZE = 1. To implement the power-down modes with an 8-bit transfer set SIZE = 0. A connection diagram is shown in Figure 25. The VDRIVE pin of the AD7475/AD7495 takes the same supply voltage as that of the MC68HC16. This allows the ADC to operate at a higher voltage than the serial interface, i.e., MC68HC16, if necessary.
AD7475/AD7495*
SCLK SDATA CS VDRIVE MC68HC16* SCLK/PCM2 MISO/PMC0 SS/PMC3
The connection diagram in Figure 24 shows how the AD7475/ AD7495 can be connected to the SSI (Synchronous Serial Interface) of the DSP56xxx family of DSPs from Motorola. The SSI is operated in Synchronous Mode (SYN bit in CRB = 1) with internally generated 1-bit clock period frame sync for both Tx and Rx (bits FSL1 = 1 and FSL0 = 0 in CRB). Set the word length to 16 by setting bits WL1 = 1 and WL0 = 0 in CRA. To
*ADDITIONAL PINS OMITTED FOR CLARITY
VDD
Figure 25. Interfacing to the MC68HC16
REV. A
-15-
AD7475/AD7495
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead SOIC (SO-8)
C01684-2.5-4/01(A)
0.1968 (5.00) 0.1890 (4.80)
8 5 4
0.1574 (4.00) 0.1497 (3.80) PIN 1
1
0.2440 (6.20) 0.2284 (5.80)
0.0500 (1.27) BSC 0.0098 (0.25) 0.0040 (0.10) SEATING PLANE 0.0688 (1.75) 0.0532 (1.35) 0.020 (0.51) 0.013 (0.33) 8 0.0098 (0.25) 0 0.0075 (0.19)
0.0196 (0.50) 0.0099 (0.25)
45
0.050 (1.27) 0.016 (0.40)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
8-Lead microSOIC (RM-8)
0.122 (3.10) 0.114 (2.90)
8
5
0.122 (3.10) 0.114 (2.90)
1 4
0.199 (5.05) 0.187 (4.75)
PIN 1 0.0256 (0.65) BSC 0.120 (3.05) 0.112 (2.84) 0.006 (0.15) 0.002 (0.05) 0.018 (0.46) SEATING 0.008 (0.20) PLANE 0.043 (1.09) 0.037 (0.94) 0.011 (0.28) 0.003 (0.08) 0.120 (3.05) 0.112 (2.84) 33 27
0.028 (0.71) 0.016 (0.41)
-16-
REV. A
PRINTED IN U.S.A.

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