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16-Bit, 4-Channel/8-Channel, 250 kSPS PulSAR ADC AD7682/AD7689
FEATURES
16-bit resolution with no missing codes 4-channel (AD7682)/8-channel (AD7689) multiplexer with choice of inputs Unipolar single ended Differential (GND sense) Pseudobipolar Throughput: 250 kSPS INL: 0.5 LSB typical, 1.5 LSB maximum (23 ppm or FSR) Dynamic range: 93.8 dB SINAD: 92.5 dB @ 20 kHz THD: -100 dB @ 20 kHz Analog input range: 0 V to VREF with VREF up to VDD Multiple reference types Internal selectable 2.5 V or 4.096 V External buffered (up to 4.096 V) External (up to VDD) Internal temperature sensor Channel sequencer, selectable 1-pole filter, busy indicator No pipeline delay, SAR architecture Single-supply 2.7 V to 5.5 V operation with 1.8 V to 5 V logic interface Serial interface compatible with SPI, MICROWIRE, QSPI, and DSP Power dissipation 3.5 mW @ 2.5 V/200 kSPS 12 mW @ 5 V/250 kSPS Standby current: 50 nA 20-lead 4 mm x 4 mm LFCSP package
FUNCTIONAL BLOCK DIAGRAM
0.5V TO 4.096V 0.1F REFIN BAND GAP REF TEMP SENSOR IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7 COM GND 0.5V TO VDD 22F REF 2.7V TO 5V
VDD 1.8V VIO TO VDD
AD7682/ AD7689
CNV MUX 16-BIT SAR ADC ONE-POLE LPF SPI SERIAL INTERFACE SCK SDO DIN SEQUENCER
07353-001
Figure 1.
Table 1. Multichannel 14-/16-Bit PulSAR(R) ADC
Type 14-Bit 16-Bit 16-Bit Channels 8 4 8 250 kSPS AD7949 AD7682 AD7689 500 kSPS ADC Driver ADA4841-x ADA4841-x ADA4841-x
AD7699
GENERAL DESCRIPTION
The AD7682/AD7689 are 4-channel/8-channel, 16-bit, charge redistribution successive approximation register (SAR) analogto-digital converters (ADCs) that operate from a single power supply, VDD. The AD7682/AD7689 contain all components for use in a multichannel, low power data acquisition system, including a true 16-bit SAR ADC with no missing codes; a 4-channel (AD7682) or 8-channel (AD7689), low crosstalk multiplexer useful for configuring the inputs as single ended (with or without ground sense), differential, or bipolar; an internal low drift reference (selectable 2.5 V or 4.096 V) and buffer; a temperature sensor; a selectable one-pole filter; and a sequencer that is useful when channels are continuously scanned in order. The AD7682/AD7689 use a simple SPI interface for writing to the configuration register and receiving conversion results. The SPI interface uses a separate supply, VIO, which is set to the host logic level. Power dissipation scales with throughput. Each AD7682/AD7689 is housed in a tiny 20-lead LFCSP with operation specified from -40C to +85C.
APPLICATIONS
Battery-powered equipment Medical instruments: ECG/EKG Mobile communications: GPS Personal digital assistants Power line monitoring Data acquisition Seismic data acquisition systems Instrumentation Process control
Rev. 0
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. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2008 Analog Devices, Inc. All rights reserved.
AD7682/AD7689 TABLE OF CONTENTS
Features .............................................................................................. 1 Applications....................................................................................... 1 Functional Block Diagram .............................................................. 1 General Description ......................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Timing Specifications....................................................................... 6 Absolute Maximum Ratings............................................................ 8 ESD Caution.................................................................................. 8 Pin Configurations and Function Descriptions ........................... 9 Typical Performance Characteristics ........................................... 11 Terminology .................................................................................... 14 Theory of Operation ...................................................................... 15 Overview...................................................................................... 15 Converter Operation.................................................................. 15 Transfer Functions...................................................................... 16 Typical Connection Diagrams.................................................. 17 Analog Inputs ............................................................................. 18 Driver Amplifier Choice ........................................................... 19 Voltage Reference Output/Input .............................................. 20 Power Supply............................................................................... 21 Supplying the ADC from the Reference.................................. 21 Digital Interface .............................................................................. 22 Configuration Register, CFG .................................................... 22 Read/Write Spanning Conversion Without a Busy Indicator............................................................................. 24 Read/Write Spanning Conversion with a Busy Indicator..... 25 Application Hints ........................................................................... 26 Layout .......................................................................................... 26 Evaluating AD7682/AD7689 Performance ............................ 26 Outline Dimensions ....................................................................... 27 Ordering Guide .......................................................................... 27
REVISION HISTORY
5/08--Revision 0: Initial Version
Rev. 0 | Page 2 of 28
AD7682/AD7689 SPECIFICATIONS
VDD = 2.5 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, all specifications TMIN to TMAX, unless otherwise noted. Table 2.
Parameter RESOLUTION ANALOG INPUT Voltage Range Absolute Input Voltage Conditions/Comments Min 16 AD7682B/AD7689B Typ Max Unit Bits V V
Analog Input CMRR Leakage Current at 25C Input Impedance 1 THROUGHPUT Conversion Rate Full Bandwidth 2
Unipolar mode Bipolar mode Positive input, unipolar and bipolar modes Negative or COM input, unipolar mode Negative or COM input, bipolar mode fIN = 250 kHz Acquisition phase
0 -VREF/2 -0.1 -0.1 VREF/2 - 0.1
VREF/2 68 1
+VREF +VREF/2 VREF + 0.1 +0.1 VREF/2 + 0.1
dB nA
1/4 Bandwidth2
Transient Response ACCURACY No Missing Codes Integral Linearity Error Differential Linearity Error Transition Noise Gain Error 4 Gain Error Match Gain Error Temperature Drift Offset Error4 Offset Error Match Offset Error Temperature Drift Power Supply Sensitivity
VDD = 4.5 V to 5.5 V VDD = 2.7 V to 4.5 V VDD = 2.3 V to 2.7 V VDD = 4.5 V to 5.5 V VDD = 2.7 V to 4.5 V VDD = 2.3 V to 2.7 V Full-scale step, full bandwidth Full-scale step, 1/4 bandwidth
0 0 0 0 0 0
250 200 190 60 50 47 1.8 14.5
kSPS kSPS kSPS kSPS kSPS kSPS s s Bits LSB 3 LSB LSB LSB LSB ppm/C LSB LSB ppm/C LSB
16 -1.5 -1 REF = VDD = 5 V -30 -2
-2 VDD = 5 V 5%
0.5 0.25 0.5 2 0.5 1 2 0.5 1 1.5
+1.5 +1.5 +30 +2
+2
Rev. 0 | Page 3 of 28
AD7682/AD7689
Parameter AC ACCURACY 5 Dynamic Range Signal-to-Noise Conditions/Comments Min AD7682B/AD7689B Typ Max 93.8 93.5 92.3 88.8 92.5 33.5 91 88.4 -100 110 -125 1.7 2.5 2.500 4.096 1.2 2.3 300 10 15 50 5 2.510 4.106 Unit dB 6 dB dB dB dB dB dB dB dB dB dB MHz ns V V V V A ppm/C ppm/V ppm ms V V A mV mV/C
SINAD
Total Harmonic Distortion Spurious-Free Dynamic Range Channel-to-Channel Crosstalk SAMPLING DYNAMICS -3 dB Input Bandwidth Aperture Delay INTERNAL REFERENCE REF Output Voltage REFIN Output Voltage 7 REF Output Current Temperature Drift Line Regulation Long-Term Drift Turn-On Settling Time EXTERNAL REFERENCE Voltage Range Current Drain TEMPERATURE SENSOR Output Voltage 8 Temperature Sensitivity DIGITAL INPUTS Logic Levels VIL VIH IIL IIH DIGITAL OUTPUTS Data Format 9 Pipeline Delay 10 VOL VOH
fIN = 20 kHz, VREF = 5 V fIN = 20 kHz, VREF = 4.096 V internal REF fIN = 20 kHz, VREF = 2.5 V internal REF fIN = 20 kHz, VREF = 5 V fIN = 20 kHz, VREF = 5 V -60 dB input fIN = 20 kHz, VREF = 4.096 V internal REF fIN = 20 kHz, VREF = 2.5 V internal REF fIN = 20 kHz fIN = 20 kHz fIN = 100 kHz on adjacent channel(s) Selectable VDD = 5 V 2.5 V, @ 25C 4.096 V, @ 25C 2.5 V, @ 25C 4.096 V, @ 25C
92.5 91 87.5 91
90 87
0.425
2.490 4.086
VDD = 5 V 5% 1000 hours CREF = 10 F REF input REFIN input (buffered) 250 kSPS, REF = 5 V @ 25C 0.5 0.5
VDD + 0.3 VDD - 0.2 50 283 1
-0.3 0.7 x VIO -1 -1
+0.3 x VIO VIO + 0.3 +1 +1
V V A A
ISINK = +500 A ISOURCE = -500 A
0.4 VIO - 0.3
V V
Rev. 0 | Page 4 of 28
AD7682/AD7689
Parameter POWER SUPPLIES VDD VIO Standby Current 11, 12 Power Dissipation Conditions/Comments Specified performance Specified performance Operating range VDD and VIO = 5 V, @ 25C VDD = 2.5 V, 100 SPS throughput VDD = 2.5 V, 100 kSPS throughput VDD = 2.5 V, 200 kSPS throughput VDD = 5 V , 250 kSPS throughput VDD = 5 V, 250 kSPS throughput with internal reference Min 2.3 2.3 1.8 50 1.7 1.75 3.5 12.5 15.5 50 -40 AD7682B/AD7689B Typ Max 5.5 VDD + 0.3 VDD + 0.3 Unit V V V nA W mW mW mW mW nJ C
2.1 4.1 15.9 19.2
Energy per Conversion TEMPERATURE RANGE 13 Specified Performance
1 2
TMIN to TMAX
+85
See the Analog Inputs section. The bandwidth is set with the configuration register 3 LSB means least significant bit. With the 5 V input range, one LSB is 76.3 V. 4 See the Terminology section. These specifications include full temperature range variation but not the error contribution from the external reference. 5 With VDD = 5 V, unless otherwise noted. 6 All specifications expressed in decibels are referred to a full-scale input FSR and tested with an input signal at 0.5 dB below full scale, unless otherwise specified. 7 This is the output from the internal band gap. 8 The output voltage is internal and present on a dedicated multiplexer input. 9 Unipolar mode: serial 16-bit straight binary. Bipolar mode: serial 16-bit twos complement. 10 Conversion results available immediately after completed conversion. 11 With all digital inputs forced to VIO or GND as required. 12 During acquisition phase. 13 Contact an Analog Devices, Inc., sales representative for the extended temperature range.
Rev. 0 | Page 5 of 28
AD7682/AD7689 TIMING SPECIFICATIONS
VDD = 4.5 V to 5.5 V, VIO = 2.3 V to VDD, all specifications TMIN to TMAX, unless otherwise noted. Table 3. 1
Parameter Conversion Time: CNV Rising Edge to Data Available Acquisition Time Time Between Conversions CNV Pulse Width Data Write/Read During Conversion SCK Period SCK Low Time SCK High Time SCK Falling Edge to Data Remains Valid SCK Falling Edge to Data Valid Delay VIO Above 4.5 V VIO Above 3 V VIO Above 2.7 V VIO Above 2.3 V CNV Low to SDO D15 MSB Valid VIO Above 4.5 V VIO Above 3 V VIO Above 2.7 V VIO Above 2.3 V CNV High or Last SCK Falling Edge to SDO High Impedance CNV Low to SCK Rising Edge DIN Valid Setup Time from SCK Falling Edge DIN Valid Hold Time from SCK Falling Edge
1
Symbol tCONV tACQ tCYC tCNVH tDATA tSCK tSCKL tSCKH tHSDO tDSDO
Min 1.8 4 10
Typ
Max 2.2
1.4 15 7 7 4 16 17 18 19
Unit s s s ns s ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns
tEN 15 17 18 22 25 10 4 4
tDIS tCLSCK tSDIN tHDIN
See Figure 2 and Figure 3 for load conditions.
Rev. 0 | Page 6 of 28
AD7682/AD7689
VDD = 2.5 V to 4.5 V, VIO = 2.3 V to VDD, all specifications TMIN to TMAX, unless otherwise noted. Table 4. 1
Parameter Conversion Time: CNV Rising Edge to Data Available VDD = 2.7 V to 4.5 V VDD = 2.3 V to 2.7 V Acquisition Time Time Between Conversions VDD = 2.7 V to 4.5 V VDD = 2.3 V to 2.7 V CNV Pulse Width Data Write/Read During Conversion SCK Period SCK Low Time SCK High Time SCK Falling Edge to Data Remains Valid SCK Falling Edge to Data Valid Delay VIO Above 3 V VIO Above 2.7 V VIO Above 2.3 V CNV Low to SDO D15 MSB Valid VIO Above 3 V VIO Above 2.7 V VIO Above 2.3 V CNV High or Last SCK Falling Edge to SDO High Impedance CNV Low to SCK Rising Edge SDI Valid Setup Time from SCK Falling Edge SDI Valid Hold Time from SCK Falling Edge
1
Symbol tCONV tCONV tACQ tCYC tCYC tCNVH tDATA tSCK tSCKL tSCKH tHSDO tDSDO
Min
Typ
Max 3.2 3.4
Unit s s s s s s ns s ns ns ns ns ns ns ns ns ns ns ns ns ns ns
1.8 5 5.2 10 1.4 25 12 12 5 24 30 37
tEN 21 27 35 50 10 5 5
tDIS tCLSCK tSDIN tHDIN
See Figure 2 and Figure 3 for load conditions.
500A
IOL
TO SDO CL 50pF 500A IOH
1.4V
Figure 2. Load Circuit for Digital Interface Timing
70% VIO 30% VIO
tDELAY
2V OR VIO - 0.5V1 0.8V OR 0.5V2
1 2
tDELAY
2V OR VIO - 0.5V1 0.8V OR 0.5V2
07353-003
2V IF VIO ABOVE 2.5V, VIO - 0.5V IF VIO BELOW 2.5V. 0.8V IF VIO ABOVE 2.5V, 0.5V IF VIO BELOW 2.5V.
Figure 3. Voltage Levels for Timing
Rev. 0 | Page 7 of 28
07353-002
AD7682/AD7689 ABSOLUTE MAXIMUM RATINGS
Table 5.
Parameter Analog Inputs INx, 1 COM1 REF, REFIN Supply Voltages VDD, VIO to GND VDD to VIO DIN, CNV, SCK to GND 2 SDO to GND Storage Temperature Range Junction Temperature JA Thermal Impedance (LFCSP) JC Thermal Impedance (LFCSP)
1 2
Rating GND - 0.3 V to VDD + 0.3 V or VDD 130 mA GND - 0.3 V to VDD + 0.3 V -0.3 V to +7 V 7 V -0.3 V to VIO + 0.3 V -0.3 V to VIO + 0.3 V -65C to +150C 150C 47.6C/W 4.4C/W
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 indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ESD CAUTION
See the Analog Inputs section. CNV must be low at power up. See the Power Supply section.
Rev. 0 | Page 8 of 28
AD7682/AD7689 PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
VDD NC IN1 NC IN0 20 19 18 17 16
PIN 1 INDICATOR
VDD REF REFIN GND GND
1 2 3 4 5
AD7682
TOP VIEW (Not to Scale)
NC 6 IN2 7 NC 8 IN3 9 COM 10
15 14 13 12 11
VIO SDO SCK DIN CNV
VDD REF REFIN GND GND
1 2 3 4 5
20 19 18 17 16
PIN 1 INDICATOR
VDD IN3 IN2 IN1 IN0
AD7689
TOP VIEW (Not to Scale)
IN4 6 IN5 7 IN6 8 IN7 9 COM 10
15 14 13 12 11
VIO SDO SCK DIN CNV
NC = NO CONNECT
07353-004
Figure 4. AD7682 20-Lead LFCSP (QFN) Pin Configuration
Figure 5. AD7689 20-Lead LFCSP (QFN) Pin Configuration
Table 6. Pin Function Descriptions
Pin No. 1, 20 AD7682 Mnemonic VDD AD7689 Mnemonic VDD Type 1 P Description Power Supply. Nominally 2.5 V to 5.5 V when using an external reference and decoupled with 10 F and 100 nF capacitors. When using the internal reference for 2.5 V output, the minimum should be 2.7 V. When using the internal reference for 4.096 V output, the minimum should be 4.5 V. Reference Input/Output. See the Voltage Reference Output/Input section. When the internal reference is enabled, this pin produces a selectable system reference = 2.5 V or 4.096 V. When the internal reference is disabled and the buffer is enabled, REF produces a buffered version of the voltage present on the REFIN pin (VDD - 0.3 V maximum) useful when using low cost, low power references. For improved drift performance, connect a precision reference to REF (0.5 V to VDD). For any reference method, this pin needs decoupling with an external 10 F capacitor connected as close to REF as possible. See the Reference Decoupling section. Internal Reference Output/Reference Buffer Input. See the Voltage Reference Output/Input section. When using the internal reference, the internal unbuffered reference voltage is present and needs decoupling with a 0.1F capacitor. When using the internal reference buffer, apply a source between 0.5 V and 4.096 V that is buffered to the REF pin as described above. Power Supply Ground. AD7682: No connection. AD7689: Analog Input Channel 4. AD7682: Analog Input Channel 2. AD7689: Analog Input Channel 5. AD7682: No connection. AD7689: Analog Input Channel 6. AD7682: Analog Input Channel 3. AD7689: Analog Input Channel 7. Common Channel Input. All channels [3:0] or [7:0] can be referenced to a common mode point of 0 V or VREF/2 V. Convert Input. On the rising edge, CNV initiates the conversion. During conversion, if CNV is held high, the busy indictor is enabled. Data Input. This input is used for writing to the 14-bit configuration register. The configuration register can be written to during and after conversion. Serial Data Clock Input. This input is used to clock out the data on ADO and clock in data on DIN in an MSB first fashion.
2
REF
REF
AI/O
3
REFIN
REFIN
AI/O
4, 5 6 7 8 9 10 11 12 13
GND NC IN2 NC IN3 COM CNV DIN SCK
GND IN4 IN5 IN6 IN7 COM CNV DIN SCK
P AI AI AI AI AI DI DI DI
Rev. 0 | Page 9 of 28
07353-005
AD7682/AD7689
Pin No. 14 AD7682 Mnemonic SDO AD7689 Mnemonic SDO Type 1 DO Description Serial Data Output. The conversion result is output on this pin synchronized to SCK. In unipolar modes, conversion results are straight binary; in bipolar modes, conversion results are twos complement. Input/Output Interface Digital Power. Nominally at the same supply as the host interface (1.8 V, 2.5 V, 3 V, or 5 V). Analog Input Channel 0. AD7682: No connection. AD7689: Analog Input Channel 1. AD7682: Analog Input Channel 1. AD7689: Analog Input Channel 2. AD7682: No connection. AD7689: Analog Input Channel 3.
15 16 17 18 19
1
VIO IN0 NC IN1 NC
VIO IN0 IN1 IN2 IN3
P AI AI AI AI
AI = analog input, AI/O = analog input/output, DI = digital input, DO = digital output, and P = power.
Rev. 0 | Page 10 of 28
AD7682/AD7689 TYPICAL PERFORMANCE CHARACTERISTICS
VDD = 2.5 V to 5.5 V, VREF = 2.5 V to 5 V, VIO = 2.3 V to VDD
1.5 1.5
1.0
1.0
0.5 DNL (LSB)
07353-009
INL (LSB)
0.5
0
0
-0.5 -0.5
-1.0
0
16,384
32,768 CODES
49,152
65,536
0
16,384
32,768 CODES
49,152
65,536
Figure 6. Integral Nonlinearity vs. Code, VREF = VDD = 5 V
200k 180k 160k 140k 135,326 124,689 = 0.50 VREF = VDD = 5V
Figure 9. Differential Nonlinearity vs. Code, VREF = VDD = 5 V
160k 140k 120k 100k 135,207 = 0.78 VREF = VDD = 2.5V
COUNTS
COUNTS
120k 100k 80k 60k
80k 63,257 60k 40k 51,778
40k 20k
07353-007
20k 0 0 487 619 CODE IN HEX 0 0 8001 0 8002 0 1 78
6649 7FFB 7FFC 7FFD 7FFE 7FFF 8000 CODE IN HEX
4090 8001
7FFA 7FFB 7FFC 7FFD 7FFE 7FFF 8000
8002
8003
Figure 7. Histogram of a DC Input at Code Center
0 -20 AMPLITUDE (dB of Full-Scale) -40 -60 -80 -100 -120 -140 -160
07353-008
Figure 10. Histogram of a DC Input at Code Center
0 VREF = VDD = 2.5V fs= 200kSPS fIN = 19.9kHz SNR = 88.0dB SINAD = 87.0dB THD = -89dB SFDR = 89dB SECOND HARMONIC = -105dB THIRD HARMONIC = -90dB
AMPLITUDE (dB of Full-Scale)
VREF = VDD = 5V fS= 250kSPS fIN = 19.9kHz SNR = 92.9dB SINAD = 92.4dB THD = -102dB SFDR = 103dB SECOND HARMONIC = -111dB THIRD HARMONIC = -104dB
-20 -40 -60 -80 -100 -120 -140 -160
0
25
50
75
100
125
0
25
50 FREQUENCY (kHz)
75
100
FREQUENCY (kHz)
Figure 8. 20 kHz FFT, VREF = VDD = 5 V
Figure 11. 20 kHz FFT, VREF = VDD = 2.5 V
Rev. 0 | Page 11 of 28
07353-011
-180
-180
07353-010
0
60
1
07353-006
-1.5
-1.0
AD7682/AD7689
100 95 90 85
100 95 90 85 80 75 70 65
07353-041
80 75 70 65 60 VDD = VREF VDD = VREF VDD = VREF VDD = VREF = 5V, -0.5dB = 5V, -10dB = 2.5V, -0.5dB = 2.5V, -10dB
SINAD (dB)
SNR (dB)
VDD = VREF VDD = VREF VDD = VREF VDD = VREF
= 5V, -0.5dB = 5V, -10dB = 2.5V, -0.5dB = 2.5V, -10dB
07353-012 07353-017 07353-016
0
50
100 FREQUENCY (kHz)
150
200
60
0
50
100 FREQUENCY (kHz)
150
200
Figure 12. SNR vs. Frequency
96 94 92
SNR, SINAD (dB)
Figure 15. SINAD vs. Frequency
17.0 130 125 120 16.0
ENOB (Bits)
-60 -65 -70 SFDR = 2kHz -75 -80
SNR @ 2kHz SINAD @ 2kHz SNR @ 20kHz SINAD @ 20kHz ENOB @ 2kHz ENOB @ 20kHz
16.5
115 110
SFDR (dB)
90 88 86 84 82 80 1.0
15.5 15.0 14.5 14.0 13.5
100 95 90 85 80 75
SFDR = 20kHz THD = 20kHz
-90 -95
-100 -105 THD = 2kHz -110 -115 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 -120 5.5
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
REFERENCE VOLTAGE (V)
07353-013
13.0
70 1.0
REFERENCE VOLTAGE (V)
Figure 13. SNR, SINAD, and ENOB vs. Reference Voltage
96
Figure 16. SFDR and THD vs. Reference Voltage
-90
fIN = 20kHz
94 VDD = VREF = 5V
fIN = 20kHz
-95
92
VDD = VREF = 5V
SNR (dB)
THD (dB)
90
-100
VDD = VREF = 2.5V
88
VDD = VREF = 2.5V
-105
86
-35
-15
5
25
45
65
85
105
125
07353-014
84 -55
-110 -55
-35
-15
5
25
45
65
85
105
125
TEMPERATURE (C)
TEMPERATURE (C)
Figure 14. SNR vs. Temperature
Figure 17. THD vs. Temperature
Rev. 0 | Page 12 of 28
THD (dB)
105
-85
AD7682/AD7689
-60 3000 2750 2500 -80 THD (dB) 2.5V INTERNAL REF 4.096V INTERNAL REF INTERNAL BUFFER, TEMP ON INTERNAL BUFFER, TEMP OFF EXTERNAL REF, TEMP ON EXTERNAL REF, TEMP OFF VIO 100
fS = 200kSPS
90 80 70 60 50 40 30
07353-021
-70
VDD CURRENT (A)
2250 2000 1750 1500 1250
-90
-100 VDD = VREF VDD = VREF VDD = VREF VDD = VREF 0 50 100 FREQUENCY (kHz) = 5V, -0.5dB = 2.5V, -0.5dB = 2.5V, -10dB = 5V, -10dB 150 200
07353-015
-110
-120
1000 2.5
3.0
3.5
4.0 VDD SUPPLY (V)
4.5
5.0
20 5.5
Figure 18. THD vs. Frequency
95 94 93 92
Figure 21. Operating Currents vs. Supply
3000 2750 180
fIN = 20kHz
VDD = VREF = 5V
fS = 200kSPS
VDD = 5V, INTERNAL 4.096V REF
160 140 120
2500
VDD CURRENT (A)
SNR (dB)
91 90 89 88 87 86 -8 -6 -4 -2 0
07353-018
2250 2000 1750 1500 1250 VIO -35 -15 5 25 45 65 85 105 1000 -55 VDD = 2.5, EXTERNAL REF VDD = 5V, EXTERNAL REF
100 80 60 40 20 125
VDD = VREF = 2.5V
INPUT LEVEL (dB)
TEMPERATURE (C)
Figure 19. SNR vs. Input Level
3
25
Figure 22. Operating Currents vs. Temperature
OFFSET ERROR AND GAIN ERROR (LSB)
2
20
VDD = 2.5V, 85C
TDSDO DELAY (ns)
1
15 VDD = 2.5V, 25C 10 VDD = 5V, 85C 5 VDD = 5V, 25C VDD = 3.3V, 25C
07353-023
0
-1 UNIPOLAR ZERO UNIPOLAR GAIN BIPOLAR ZERO BIPOLAR GAIN -35 -15 5 25 45 65 85 105 125
07353-020
-2
VDD = 3.3V, 85C
-3 -55
0
0
20
TEMPERATURE (C)
40 60 80 SDO CAPACITIVE LOAD (pF)
100
120
Figure 20. Offset and Gain Errors vs. Temperature
Figure 23. tDSDO Delay vs. SDO Capacitance Load and Supply
Rev. 0 | Page 13 of 28
07353-022
85 -10
VIO CURRENT (A)
AD7682/AD7689 TERMINOLOGY
Least Significant Bit (LSB) The LSB is the smallest increment that can be represented by a converter. For an analog-to-digital converter with N bits of resolution, the LSB expressed in volts is Signal-to-Noise Ratio (SNR) SNR is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the Nyquist frequency, excluding harmonics and dc. The value for SNR is expressed in decibels. Signal-to-(Noise + Distortion) Ratio (SINAD) SINAD is the ratio of the rms value of the actual input signal to the rms sum of all other spectral components below the Nyquist frequency, including harmonics but excluding dc. The value for SINAD is expressed in decibels. Total Harmonic Distortion (THD) THD is the ratio of the rms sum of the first five harmonic components to the rms value of a full-scale input signal and is expressed in decibels. Spurious-Free Dynamic Range (SFDR) SFDR is the difference, in decibels, between the rms amplitude of the input signal and the peak spurious signal. Effective Number of Bits (ENOB) ENOB is a measurement of the resolution with a sine wave input. It is related to SINAD by the formula ENOB = (SINADdB - 1.76)/6.02 and is expressed in bits. Channel-to-Channel Crosstalk Channel-to-channel crosstalk is a measure of the level of crosstalk between any two adjacent channels. It is measured by applying a dc to the channel under test and applying a full-scale, 100 kHz sine wave signal to the adjacent channel(s). The crosstalk is the amount of signal that leaks into the test channel and is expressed in decibels. Reference Voltage Temperature Coefficient Reference voltage temperature coefficient is derived from the typical shift of output voltage at 25C on a sample of parts at the maximum and minimum reference output voltage (VREF) measured at TMIN, T (25C), and TMAX. It is expressed in ppm/C as
TCVREF (ppm/ C ) = VREF ( Max ) - VREF ( Min) VREF (25C ) x (TMAX - TMIN ) x 10 6
LSB (V) =
VREF 2N
Integral Nonlinearity Error (INL) INL refers to the deviation of each individual code from a line drawn from negative full scale through positive full scale. The point used as negative full scale occurs 1/2 LSB before the first code transition. Positive full scale is defined as a level 11/2 LSB beyond the last code transition. The deviation is measured from the middle of each code to the true straight line (see Figure 25). Differential Nonlinearity Error (DNL) In an ideal ADC, code transitions are 1 LSB apart. DNL is the maximum deviation from this ideal value. It is often specified in terms of resolution for which no missing codes are guaranteed. Offset Error The first transition should occur at a level 1/2 LSB above analog ground. The unipolar offset error is the deviation of the actual transition from that point. Gain Error The last transition (from 111 ... 10 to 111 ... 11) should occur for an analog voltage 11/2 LSB below the nominal full scale. The gain error is the deviation in LSB (or percentage of full-scale range) of the actual level of the last transition from the ideal level after the offset error is adjusted out. Closely related is the full-scale error (also in LSB or percentage of full-scale range), which includes the effects of the offset error. Aperture Delay Aperture delay is the measure of the acquisition performance. It is the time between the rising edge of the CNV input and the point at which the input signal is held for a conversion. Transient Response Transient response is the time required for the ADC to accurately acquire its input after a full-scale step function is applied. Dynamic Range Dynamic range is the ratio of the rms value of the full scale to the total rms noise measured with the inputs shorted together. The value for dynamic range is expressed in decibels.
where: VREF (Max) = maximum VREF at TMIN, T (25C), or TMAX. VREF (Min) = minimum VREF at TMIN, T (25C), or TMAX. VREF (25C) = VREF at 25C. TMAX = +85C. TMIN = -40C.
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AD7682/AD7689 THEORY OF OPERATION
INx+ SWITCHES CONTROL MSB 32,768C REF COMP GND 32,768C 16,384C MSB 4C 2C C C LSB SW- CNV 16,384C 4C 2C C C CONTROL LOGIC OUTPUT CODE LSB SW+ BUSY
INx- OR COM
Figure 24. ADC Simplified Schematic
OVERVIEW
The AD7682/AD7689 are 4-channel/8-channel, 16-bit, charge redistribution successive approximation register (SAR) analog-todigital converters (ADCs). These devices are capable of converting 250,000 samples per second (250 kSPS) and power down between conversions. For example, when operating with an external reference at 1 kSPS, they consumes 17 W typically, ideal for battery-powered applications. The AD7682/AD7689 contain all of the components for use in a multichannel, low power data acquisition system, including * * * * * * 16-bit SAR ADC with no missing codes 4-channel/8-channel, low crosstalk multiplexer Internal low drift reference and buffer Temperature sensor Selectable one-pole filter Channel sequencer
CONVERTER OPERATION
The AD7682/AD7689 are successive approximation ADCs based on a charge redistribution DAC. Figure 24 shows the simplified schematic of the ADC. The capacitive DAC consists of two identical arrays of 16 binary-weighted capacitors, which are connected to the two comparator inputs. During the acquisition phase, terminals of the array tied to the comparator input are connected to GND via SW+ and SW-. All independent switches are connected to the analog inputs. Thus, the capacitor arrays are used as sampling capacitors and acquire the analog signal on the INx+ and INx- (or COM) inputs. When the acquisition phase is complete and the CNV input goes high, a conversion phase is initiated. When the conversion phase begins, SW+ and SW- are opened first. The two capacitor arrays are then disconnected from the inputs and connected to the GND input. Therefore, the differential voltage between the INx+ and INx- (or COM) inputs captured at the end of the acquisition phase is applied to the comparator inputs, causing the comparator to become unbalanced. By switching each element of the capacitor array between GND and CAP, the comparator input varies by binary-weighted voltage steps (VREF/2, VREF/4, ... VREF/32,768). The control logic toggles these switches, starting with the MSB, to bring the comparator back into a balanced condition. After the completion of this process, the part returns to the acquisition phase, and the control logic generates the ADC output code and a busy signal indicator. Because the AD7682/AD7689 have an on-board conversion clock, the serial clock, SCK, is not required for the conversion process.
These components are configured through an SPI-compatible, 14-bit register. Conversion results, also SPI compatible, can be read after or during conversions with the option for reading back the current configuration. The AD7682/AD7689 provide the user with an on-chip trackand-hold and do not exhibit pipeline delay or latency. The AD7682/AD7689 are specified from 2.3 V to 5.5 V and can be interfaced to any 1.8 V to 5 V digital logic family. They are housed in a 20-lead, 4 mm x 4 mm LFCSP that combines space savings and allows flexible configurations. They are pin-for-pin compatible with the 16-bit AD7699 and 14-bit AD7949.
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AD7682/AD7689
TRANSFER FUNCTIONS
With the inputs configured for unipolar range (single ended, COM with ground sense, or paired differentially with INx- as ground sense), the data output is straight binary. With the inputs configured for bipolar range (COM = VREF/2 or paired differentially with INx- = VREF/2), the data outputs are twos complement. The ideal transfer characteristic for the AD7682/AD7689 is shown in Figure 25 and for both unipolar and bipolar ranges with the internal 4.096 V reference.
TWOS STRAIGHT COMPLEMENT BINARY 011...111 011...110 011...101 111...111 111...110 111...101
ADC CODE
100...010 100...001 100...000
000...010 000...001 000...000 -FSR -FSR + 1LSB +FSR - 1LSB +FSR - 1.5LSB ANALOG INPUT
-FSR + 0.5LSB
Figure 25. ADC Ideal Transfer Function
Table 7. Output Codes and Ideal Input Voltages
Description FSR - 1 LSB Midscale + 1 LSB Midscale Midscale - 1 LSB -FSR + 1 LSB -FSR
1 2
Unipolar Analog Input1 VREF = 4.096 V 4.095938 V 2.048063 V 2.048 V 2.047938 V 62.5 V 0V
Digital Output Code (Straight Binary Hex) 0xFFFF 0x8001 0x8000 0x7FFF 0x0001 0x0000
Bipolar Analog Input2 VREF = 4.096 V 2.047938 V 62.5 V 0V -62.5 V -2.047938 V -2.048 V
Digital Output Code (Twos Complement Hex) 0x7FFF 0x0001 0x00004 0xFFFF3 0x8001 0x8000
With COM or INx- = 0 V or all INx referenced to GND. With COM or INx- = VREF /2. 3 This is also the code for an overranged analog input ((INx+) - (INx-), or COM, above VREF - VGND). 4 This is also the code for an underranged analog input ((INx+) - (INx-), or COM, below VGND).
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AD7682/AD7689
TYPICAL CONNECTION DIAGRAMS
10F2 100nF 5V 1.8V TO VDD
100nF
100nF
V+
0V TO VREF ADA4841-x 3 V- V+ INx IN0
REF
REFIN VDD
VIO
AD7682/AD7689
DIN SCK SDO CNV MOSI SCK MISO SS
0V TO VREF ADA4841-x 3 0V OR VREF /2 V- COM
GND
NOTES: 1. INTERNAL REFERENCE SHOWN. SEE VOLTAGE REFERENCE OUTPUT/INPUT SECTION FOR REFERENCE SELECTION. 2. CREF IS USUALLY A 10F CERAMIC CAPACITOR (X5R). 3. SEE DRIVER AMPLIFIER CHOICE SECTION FOR ADDITIONAL RECOMMENDED AMPLIFIERS. 4. SEE THE DIGITAL INTERFACE SECTION FOR CONFIGURING AND READING CONVERSION DATA.
Figure 26. Typical Application Diagram with Multiple Supplies
5V 1.8V TO VDD
V+
10F2
100nF
100nF
100nF
REF ADA4841-x 3 IN0 V+ INx
REFIN VDD
VIO
AD7682/AD7689
DIN SCK SDO MOSI SCK MISO SS
ADA4841-x 3
CNV
VREF p-p VREF /2
COM
GND
NOTES: 1. INTERNAL REFERENCE SHOWN. SEE VOLTAGE REFERENCE OUTPUT/INPUT SECTION FOR REFERENCE SELECTION. 2. CREF IS USUALLY A 10F CERAMIC CAPACITOR (X5R). 3. SEE DRIVER AMPLIFIER CHOICE SECTION FOR ADDITIONAL RECOMMENDED AMPLIFIERS. 4. SEE THE DIGITAL INTERFACE SECTION FOR CONFIGURING AND READING CONVERSION DATA.
07353-028 07353-029
Figure 27. Typical Application Diagram Using Bipolar Input
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AD7682/AD7689
Unipolar or Bipolar
Figure 26 shows an example of the recommended connection diagram for the AD7682/AD7689 when multiple supplies are available.
CMRR (dB)
70 65 60 55 50 45 40 35
07353-031
Bipolar Single Supply
Figure 27 shows an example of a system with a bipolar input using single supplies with the internal reference (optional different VIO supply). This circuit is also useful when the amplifier/signal conditioning circuit is remotely located with some common mode present. Note that for any input configuration, the inputs INx are unipolar and always referenced to GND (no negative voltages even in bipolar range). For this circuit, a rail-to-rail input/output amplifier can be used; however, the offset voltage vs. input common-mode range should be noted and taken into consideration (1 LSB = 62.5 V with VREF = 4.096 V). Note that the conversion results are in twos complement format when using the bipolar input configuration. Refer to the AN-581 Application Note for additional details about using single-supply amplifiers.
30
1
10
100 FREQUENCY (kHz)
1k
10k
Figure 29. Analog Input CMRR vs. Frequency
ANALOG INPUTS
Input Structure
Figure 28 shows an equivalent circuit of the input structure of the AD7682/AD7689. The two diodes, D1 and D2, provide ESD protection for the analog inputs, IN[7:0] and COM. Care must be taken to ensure that the analog input signal does not exceed the supply rails by more than 0.3 V because this causes the diodes to become forward biased and to start conducting current. These diodes can handle a forward-biased current of 130 mA maximum. For instance, these conditions may eventually occur when the input buffer supplies are different from VDD. In such a case, for example, an input buffer with a short circuit, the current limitation can be used to protect the part.
VDD INx+ OR INx- OR COM GND D1 CPIN D2
07353-030
During the acquisition phase, the impedance of the analog inputs can be modeled as a parallel combination of the capacitor, CPIN, and the network formed by the series connection of RIN and CIN. CPIN is primarily the pin capacitance. RIN is typically 3.5 k and is a lumped component made up of serial resistors and the on resistance of the switches. CIN is typically 27 pF and is mainly the ADC sampling capacitor.
Selectable Low Pass Filter
During the conversion phase, where the switches are opened, the input impedance is limited to CPIN. While the AD7682/ AD7689 are acquiring, RIN and CIN make a one-pole, low-pass filter that reduces undesirable aliasing effects and limits the noise from the driving circuitry. The low-pass filter can be programmed for the full bandwidth or 1/4 of the bandwidth with CFG[6] as shown in Table 9. Note that the converters throughout must also be reduced by 1/4 when using the filter. If the maximum throughput is used with the BW set to 1/4, the converter acquisition time, tACQ, will be violated, resulting in increased THD.
Input Configurations
Figure 30 shows the different methods for configuring the analog inputs with the configuration register (CFG[12:10]). Refer to the Configuration Register, CFG, section for more details.
RIN
CIN
Figure 28. Equivalent Analog Input Circuit
This analog input structure allows the sampling of the true differential signal between INx+ and COM or INx+ and INx-. (COM or INx- = GND 0.1 V or VREF 0.1 V). By using these differential inputs, signals common to both inputs are rejected, as shown in Figure 29.
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AD7682/AD7689
CH0+ CH1+ CH2+ CH3+ CH4+ CH5+ CH6+ CH7+ IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7 COM GND A--8 CHANNELS, SINGLE ENDED CH0+ CH1+ CH2+ CH3+ CH4+ CH5+ CH6+ CH7+ COM- IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7 COM GND B--8 CHANNELS, COMMON REFERNCE
channel pairs are always paired IN (even) = INx+ and IN (odd) = INx- regardless of CFG[7]. To enable the sequencer, CFG[2:1] are written to for initializing the sequencer. After CFG[13:0] are updated, DIN must be held low while reading data out (at least for Bit 13), or the CFG will begin updating again. While operating in a sequence, the CFG can be changed by writing 012 to CFG[2:1]. However, if changing CFG11 (paired or single channel) or CFG[9:7] (last channel in sequence), the sequence reinitializes and converts IN0 (or IN1) after CFG is updated.
Examples
CH0+ (-) CH0- (+) CH1+ (-) CH1- (+) CH2+ (-) CH2- (+) CH3+ (-) CH3- (+) IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7 COM GND C--4 CHANNELS, DIFFERENTIAL CH0+ (-) CH0- (+) CH1+ (-) CH1- (+) CH2+ CH3+ CH4+ CH5+ COM- IN0 IN1 IN2 IN3 IN4 IN5 IN6 IN7 COM GND D--COMBINATION
07353-032
Only the bits for input and sequencer are highlighted. As a first example, scan all IN[7:0] referenced to COM = GND with temperature sensor.
13 CFG 12 1 11 10 INCC 1 0 9 87 INx 111 6 BW 5 43 REF 21 SEQ 10 0 RB -
As a second example, scan three paired channels without temperature sensor and referenced to VREF/2.
13 CFG 12 0 11 10 INCC 0 X 9 87 INx 10X 6 BW 5 43 REF 21 SEQ 11 0 RB -
Figure 30. Multiplexed Analog Input Configuraitons
Source Resistance
When the source impedance of the driving circuit is low, the AD7682/AD7689 can be driven directly. Large source impedances significantly affect the ac performance, especially total harmonic distortion (THD). The dc performances are less sensitive to the input impedance. The maximum source impedance depends on the amount of THD that can be tolerated. The THD degrades as a function of the source impedance and the maximum input frequency.
The analog inputs can be configured as * * Figure 30A, single ended referenced to system ground; CFG[12:10] = 1112. Figure 30B, bipolar differential with a common reference point; COM = VREF/2; CFG[12:10] = 0102. Unipolar differential with COM connected to a ground sense; CFG[12:10] = 1102. Figure 30C, bipolar differential pairs with INx- referenced to VREF/2; CFG[12:10] = 00X2. Unipolar differential pairs with INx- referenced to a ground sense; CFG[12:10] = 10X2. In this configuration, the INx+ is identified by the channel in CFG[9:7]. Example: for IN0 = IN1+ and IN1 = IN1-, CFG[9:7] = 0002; for IN1 = IN1+ and IN0 = IN1-, CFG[9:7] = 0012. Figure 30D, inputs configured in any of the above combinations (showing that the AD7682/AD7689 can be configured dynamically).
*
DRIVER AMPLIFIER CHOICE
Although the AD7682/AD7689 are easy to drive, the driver amplifier must meet the following requirements: * The noise generated by the driver amplifier must be kept as low as possible to preserve the SNR and transition noise performance of the AD7682/AD7689. Note that the AD7682/ AD7689 have a noise much lower than most of the other 16-bit ADCs and, therefore, can be driven by a noisier amplifier to meet a given system noise specification. The noise from the amplifier is filtered by the AD7682/AD7689 analog input circuit low-pass filter made by RIN and CIN or by an external filter, if one is used. Because the typical noise of the AD7682/AD7689 is 35 V rms (with VREF = 5 V), the SNR degradation due to the amplifier is
*
Sequencer
The AD7682/AD7689 include a channel sequencer useful for scanning channels in a IN0 to INx fashion. Channels are scanned as singles or pairs, with or without the temperature sensor, after the last channel is sequenced. The sequencer starts with IN0 and finishes with INx set in CFG[9:7]. For paired channels, the channels are paired depending on the last channel set in CFG[9:7]. Note that the
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AD7682/AD7689
SNRLOSS 35 = 20 log 35 2 + f -3dB ( Ne N ) 2 2
AD7682/AD7689 and is thus useful for performing a system calibration. Note that, when using the temperature sensor, the output is straight binary referenced from the AD7682/AD7689 GND pin. The internal reference is temperature-compensated to within 15 mV. The reference is trimmed to provide a typical drift of 3 ppm/C.
where: f-3dB is the input bandwidth in megahertz of the AD7682/AD7689 (1.7 MHz in full BW or 425 kHz in 1/4 BW) or is the cutoff frequency of an input filter, if one is used. N is the noise gain of the amplifier (for example, 1 in buffer configuration). eN is the equivalent input noise voltage of the op amp, in nV/Hz. * For ac applications, the driver should have a THD performance commensurate with the AD7682/AD7689. Figure 18 shows THD vs. frequency for the AD7682/AD7689. For multichannel, multiplexed applications on each input or input pair, the driver amplifier and the AD7682/ AD7689 analog input circuit must settle a full-scale step onto the capacitor array at a 16-bit level (0.0015%). In the amplifier data sheet, settling at 0.1% to 0.01% is more commonly specified. This may differ significantly from the settling time at a 16-bit level and should be verified prior to driver selection.
External Reference and Internal Buffer
For improved drift performance, an external reference can be used with the internal buffer. The external reference is connected to REFIN, and the output is produced on the REF pin. An external reference can be used with the internal buffer with or without the temperature sensor enabled. Refer to Table 9 for the register details. With the buffer enabled, the gain is unity and is limited to an input/output of 4.096 V. The internal reference buffer is useful in multiconverter applications because a buffer is typically required in these applications. In addition, a low power reference can be used because the internal buffer provides the necessary performance to drive the SAR architecture of the AD7682/AD7689.
*
External Reference
In any of the six voltage reference schemes, an external reference can be connected directly on the REF pin because the output impedance of REF is >5 k. To reduce power consumption, the reference and buffer can be powered down independently or together for the lowest power consumption. However, for applications requiring the use of the temperature sensor, the reference must be active. Refer to Table 9 for register details. For improved drift performance, an external reference such as the ADR43x or ADR44x is recommended.
Table 8. Recommended Driver Amplifiers
Amplifier ADA4841-x AD8655 AD8021 AD8022 OP184 AD8605, AD8615 Typical Application Very low noise, small, and low power 5 V single supply, low noise Very low noise and high frequency Low noise and high frequency Low power, low noise, and low frequency 5 V single supply, low power
Reference Decoupling
Whether using an internal or external reference, the AD7682/ AD7689 voltage reference output/input, REF, has a dynamic input impedance and should therefore be driven by a low impedance source with efficient decoupling between the REF and GND pins. This decoupling depends on the choice of the voltage reference but usually consists of a low ESR capacitor connected to REF and GND with minimum parasitic inductance. A 10 F (X5R, 1206 size) ceramic chip capacitor is appropriate when using the internal reference, the ADR43x/ADR44x external reference, or a low impedance buffer such as the AD8031 or the AD8605. The placement of the reference decoupling capacitor is also important to the performance of the AD7682/AD7689, as explained in the Layout section. The decoupling capacitor should be mounted on the same side as the ADC at the REF pin with a thick PCB trace. The GND should also connect to the reference decoupling capacitor with the shortest distance and to the analog ground plane with several vias.
VOLTAGE REFERENCE OUTPUT/INPUT
The AD7682/AD7689 allow the choice of a very low temperature drift internal voltage reference, an external reference, or an external buffered reference. The internal reference of the AD7682/AD7689 provide excellent performance and can be used in almost all applications. There are six possible choices of voltage reference schemes briefly described in Table 9 with more details in each of the following sections.
Internal Reference/Temperature Sensor
The internal reference can be set for either 2.5 V or a 4.096 V output as detailed in Table 9. With the internal reference enabled, the band gap voltage is also present on the REFIN pin, which requires an external 0.1 F capacitor. Because the current output of REFIN is limited, it can be used as a source if followed by a suitable buffer, such as the AD8605. Enabling the reference also enables the internal temperature sensor, which measures the internal temperature of the
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AD7682/AD7689
If desired, smaller reference decoupling capacitor values down to 2.2 F can be used with a minimal impact on performance, especially on DNL. Regardless, there is no need for an additional lower value ceramic decoupling capacitor (for example, 100 nF) between the REF and GND pins. For applications that use multiple AD7682/AD7689s or other PulSAR devices, it is more effective to use the internal reference buffer to buffer the external reference voltage, thus reducing SAR conversion crosstalk. The voltage reference temperature coefficient (TC) directly impacts full scale; therefore, in applications where full-scale accuracy matters, care must be taken with the TC. For instance, a 15 ppm/C TC of the reference changes full scale by 1 LSB/C.
10000 1000
OPERATING CURRENT (A)
VDD = 5V, INTERNAL REF
100 10 1
VDD = 5V, EXTERNAL REF VDD = 2.5V, EXTERNAL REF
VIO
0.1 0.010 0.001
10
100
POWER SUPPLY
The AD7682/AD7689 use two power supply pins: an analog and digital core supply (VDD) and a digital input/output interface supply (VIO). VIO allows direct interface with any logic between 1.8 V and VDD. To reduce the supplies needed, the VIO and VDD pins can be tied together. The AD7682/AD7689 are independent of power supply sequencing between VIO and VDD. The only restriction is that CNV must be low when powering up the AD7682/AD7689. Additionally, it is very insensitive to power supply variations over a wide frequency range, as shown in Figure 31.
75 70 65 60
PSSR (dB)
1k 10k SAMPLING RATE (sps)
100k
1M
Figure 32. Operating Currents vs. Sampling Rate
SUPPLYING THE ADC FROM THE REFERENCE
For simplified applications, the AD7682/AD7689, with their low operating current, can be supplied directly using the reference circuit as shown in Figure 33. The reference line can be driven by * * * The system power supply directly A reference voltage with enough current output capability, such as the ADR43x/ADR44x A reference buffer, such as the AD8605, which can also filter the system power supply, as shown in Figure 33
5V
5V 10
55 50 45 40 35 1 10 100 FREQUENCY (kHz) 1k 10k
07353-034
5V
10k 1F
AD8605
1
10F
1F
0.1F
0.1F
REF
VDD
VIO
AD7689
07353-035
30
1OPTIONAL
REFERENCE BUFFER AND FILTER.
Figure 33. Example of an Application Circuit
Figure 31. PSRR vs. Frequency
The AD7682/AD7689 power down automatically at the end of each conversion phase; therefore, the operating currents and power scale linearly with the sampling rate. This makes the part ideal for low sampling rates (even of a few hertz) and low battery-powered applications.
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AD7682/AD7689 DIGITAL INTERFACE
The AD7682/AD7689 use a simple 4-wire interface and are compatible with SPI, MICROWIRETM, QSPITM, digital hosts, and DSPs, for example, Blackfin(R) ADSP-BF53x, SHARC(R), ADSP219x, and ADSP-218x. The interface uses the CNV, DIN, SCK, and SDO signals and allows CNV, which initiates the conversion, to be independent of the readback timing. This is useful in low jitter sampling or simultaneous sampling applications. A 14-bit register, CFG[13:0], is used to configure the ADC for the channel to be converted, the reference selection, and other components, which are detailed in the Configuration Register, CFG, section. When CNV is low, reading/writing can occur during conversion, acquisition, and spanning conversion (acquisition plus conversion), as detailed in the following sections. The CFG word is updated on the first 14 SCK rising edges, and conversion results are read back on the first 15 (or 16 if busy mode is selected) SCK falling edges. If the CFG readback is enabled, an additional 14 SCK falling edges are required to read back the CFG word associated with the conversion results with the CFG MSB following the LSB of the conversion result. A discontinuous SCK is recommended because the part is selected with CNV low, and SCK activity begins to write a new configuration word and clock out data. Note that in the following sections, the timing diagrams indicate digital activity (SCK, CNV, DIN, SDO) during the conversion. However, due to the possibility of performance degradation, digital activity should occur only prior to the safe data reading/writing time, tDATA, because the AD7682/AD7689 provide error correction circuitry that can correct for an incorrect bit during this time. From tDATA to tCONV, there is no error correction and conversion results may be corrupted. The user should configure the AD7682/AD7689 and initiate the busy indicator (if desired) prior to tDATA. It is also possible to corrupt the sample by having SCK or DIN transitions near the sampling instant. Therefore, it is recommended to keep the digital pins quiet for approximately 30 ns before and 10 ns after the rising edge of CNV, using a discontinuous SCK whenever possible to avoid any potential performance degradation. The SCK frequency required is calculated by
f SCK
Number _ SCK _ Edges t DATA
The time between tDATA and tCONV is a safe time when digital activity should not occur, or sensitive bit decisions may be corrupt.
Reading/Writing During Acquisition, Any Speed Hosts
When reading/writing during acquisition (n), conversion results are for the previous (n - 1) conversion, and writing is for the (n + 1) acquisition. For the maximum throughput, the only time restriction is that the reading/writing take place during the tACQ (min) time. For slow throughputs, the time restriction is dictated by throughput required by the user, and the host is free to run at any speed. Thus for slow hosts, data access must take place during the acquisition phase.
Reading/Writing Spanning Conversion, Any Speed Host
When reading/writing spanning conversion, the data access starts at the current acquisition (n) and spans into the conversion (n). Conversion results are for the previous (n - 1) conversion, and writing the CFG is for the next (n + 1) acquisition and conversion. Similar to reading/writing during conversion, reading/writing should only occur up to tDATA. For the maximum throughput, the only time restriction is that reading/writing take place during the tACQ (min) + tDATA time. For slow throughputs, the time restriction is dictated by the user's required throughput, and the host is free to run at any speed. Similar to the reading/writing during acquisition, for slow hosts, the data access must take place during the acquisition phase with additional time into the conversion. Note that data access spanning conversion requires the CNV to be driven high to initiate a new conversion, and data access is not allowed when CNV is high. Thus, the host must perform two bursts of data access when using this method.
CONFIGURATION REGISTER, CFG
The AD7682/AD7689 use a 14-bit configuration register (CFG[13:0]) as detailed in Table 9 for configuring the inputs, channel to be converted, one-pole filter bandwidth, reference, and channel sequencer. The CFG is latched (MSB first) on DIN with 14 SCK rising edges. The CFG update is edge dependent, allowing for asynchronous or synchronous hosts.
Reading/Writing During Conversion, Fast Hosts
When reading/writing during conversion (n), conversion results are for the previous (n - 1) conversion, and writing the CFG is for the next (n + 1) acquisition and conversion. After the CNV is brought high to initiate conversion, it must be brought low again to allow reading/writing during conversion. Reading/writing should only occur up to tDATA and, because this time is limited, the host must use a fast SCK.
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AD7682/AD7689
The register can be written to during conversion, during acquisition, or spanning acquisition/conversion and is updated at the end of conversion, tCONV (max). There is always a one deep delay when writing CFG. Note that, at power up, the CFG is undefined and two dummy conversions are required to update the register. To preload the CFG with a factory setting, hold DIN high for two conversions. Thus CFG[13:0] = 0x3FFF. This sets the AD7682/AD7689 for
13 CFG 12 INCC 11 INCC 10 INCC 9 INx 8 INx 7 INx
* * * *
IN[7:0] unipolar referenced to GND, sequenced in order Full bandwidth for one-pole filter Internal reference/temperature sensor disabled, buffer enabled No readback of CFG
Table 9 summarizes the configuration register bit details. See the Theory of Operation section for more details.
6 BW 5 REF 4 REF 3 REF 2 SEQ 1 SEQ 0 RB
Table 9. Configuration Register Description
Bit(s) [13] Name CFG Description Configuration update. 0 = Keep current configuration settings. 1 = Overwrite contents of register. Input channel configuration. Selection of pseudobipolar, pseudodifferential, pairs, single-ended or temperature sensor. Refer to the Input Configurations section. Bit 12 Bit 11 Bit 10 Function 0 0 X Bipolar differential pairs; INx- referenced to VREF/2 0.1 V. 0 1 0 Bipolar; INx referenced to COM = VREF/2 0.1 V. 0 1 1 Temperature sensor. 1 0 X Unipolar differential pairs; INx- referenced to GND 0.1 mV. 1 1 0 Unipolar, IN0 to IN7 referenced to COM = GND 0.1 V (GND sense). 1 1 1 Unipolar, IN0 to IN7 referenced to GND. Input channel selection in binary fashion. AD7682 AD7689 Bit 9 Bit 8 Bit 7 Channel Bit 9 Bit 8 Bit 7 Channel 0 0 X IN0 0 0 0 IN0 0 1 X IN1 0 0 1 IN1 1 0 X IN2 ... ... ... 1 1 X IN3 1 1 1 IN7 Select bandwidth for low-pass filter. Refer to the Selectable Low Pass Filter section. 0 = 1/4 of BW, uses an additional series resistor to further bandwidth limit the noise. Maximum throughout must be reduced to 1/4 also. 1 = Full BW. Reference/buffer selection. Selection of internal, external, external buffered, and enabling of the on-chip temperature sensor. Refer to the Voltage Reference Output/Input section. Bit 5 Bit 4 Bit 3 Function 0 0 0 Internal reference, REF = 2.5 V output. 0 0 1 Internal reference, REF = 4.096 V output. 0 1 0 External reference, temperature enabled. 0 1 1 External reference, internal buffer, temperature enabled. 1 1 0 External reference, temperature disabled. 1 1 1 External reference, internal buffer, temperature disabled. Channel sequencer. Allows for scanning channels in an IN0 to INx fashion. Refer to the Sequencer section. Bit 2 Bit 1 Function 0 0 Disable sequencer. 0 1 Update configuration during sequence. 1 0 Scan IN0 to INx (set in CFG[9:7]), then temperature. 1 1 Scan IN0 to INx (set in CFG[9:7]). Read back the CFG register. 0 = Read back current configuration at end of data. 1 = Do not read back contents of configuration.
[12:10]
INCC
[9:7]
INx
[6]
BW
[5:3]
REF
[2:1]
SEQ
0
RB
Rev. 0 | Page 23 of 28
AD7682/AD7689
READ/WRITE SPANNING CONVERSION WITHOUT A BUSY INDICATOR
This mode is used when the AD7682/AD7689 are connected to any host using an SPI, serial port, or FPGA. The connection diagram is shown in Figure 34, and the corresponding timing is given in Figure 35. For SPI, the host should use CPHA = CPOL = 0. Reading/writing spanning conversion is shown, which covers all three modes detailed in the Digital Interface section. A rising edge on CNV initiates a conversion, forces SDO to high impedance, and ignores data present on DIN. After a conversion is initiated, it continues until completion irrespective of the state of CNV. CNV must be returned high before the safe data transfer time, tDATA, and then held high beyond the conversion time, tCONV, to avoid generation of the busy signal indicator. After the conversion is complete, the AD7682/AD7689 enter the acquisition phase and power down. When the host brings CNV low after tCONV (max), the MSB is enabled on SDO. The host also must enable the MSB of CFG at this time (if necessary)
AD7682/ AD7689
to begin the CFG update. While CNV is low, both a CFG update and a data readback take place. The first 14 SCK rising edges are used to update the CFG, and the first 15 SCK falling edges clock out the conversion results starting with MSB - 1. The restriction for both configuring and reading is that they both occur before the tDATA time of the next conversion elapses. All 14 bits of CFG[13:0] must be written, or they are ignored. In addition, if the 16-bit conversion result is not read back before tDATA elapses, it is lost. The SDO data is valid on both SCK edges. Although the rising edge can be used to capture the data, a digital host using the SCK falling edge allows a faster reading rate, provided it has an acceptable hold time. After the 16th (or 30th) SCK falling edge, or when CNV goes high (whichever occurs first), SDO returns to high impedance. If CFG readback is enabled, the CFG associated with the conversion result (n - 1) is read back MSB first following the LSB of the conversion result. A total of 30 SCK falling edges is required to return SDO to high impedance if this is enabled.
DIGITAL HOST
SS MISO MOSI SCK
07353-036
CNV
SDO DIN SCK
FOR SPI USE CPHA = 0, CPOL = 0.
Figure 34. Connection Diagram for the AD7682/AD7689 Without a Busy Indicator
> tCONV
tCYC tCONV tCNVH
RETURN CNV HIGH FOR NO BUSY
tCONV tDATA
tDATA
RETURN CNV HIGH FOR NO BUSY
CNV
tACQ
ACQUISITION (n - 1) CONVERSION (n - 1) (QUIET TIME) UPDATE (n) CFG/SDO SCK 14 15 16/ 30 1 2 14 15 ACQUISITION (n) CONVERSION (n) SEE NOTE 16/ 30 (QUIET TIME)
ACQUISITION (n + 1)
UPDATE (n + 1) CFG/SDO
tCLSCK
CFG MSB
tSDIN
CFG MSB - 1
tHDIN
CFG LSB X X SEE NOTE
LSB + 1
DIN
CFG LSB
X
X
tEN END CFG (n)
SDO
LSB + 1
tEN
LSB MSB
BEGIN CFG (n + 1) tHSDO
tDSDO
tEN
END CFG (n + 1) LSB
MSB - 1
tDIS
END DATA (n - 2)
tDIS
BEGIN DATA (n - 1)
tDIS
END DATA (n - 1)
tDIS
07353-037
NOTES: 1. THE LSB IS FOR CONVERSION RESULTS OR THE CONFIGURATION REGISTER CFG (n - 1) IF 15 SCK FALLING EDGES = LSB OF CONVERSION RESULTS. 29 SCK FALLING EDGES = LSB OF CONFIGURATION REGISTER. ON THE 16TH OR 30TH SCK FALLING EDGE, SDO IS DRIVEN TO HIGH IMPENDANCE.
Figure 35. Serial Interface Timing for the AD7682/AD7689 Without a Busy Indicator
Rev. 0 | Page 24 of 28
AD7682/AD7689
READ/WRITE SPANNING CONVERSION WITH A BUSY INDICATOR
This mode is used when the AD7682/AD7689 are connected to any host using an SPI, serial port, or FPGA with an interrupt input. The connection diagram is shown in Figure 36, and the corresponding timing is given in Figure 37. For SPI, the host should use CPHA = CPOL = 1. Reading/writing spanning conversion is shown, which covers all three modes detailed in the Digital Interface section. A rising edge on CNV initiates a conversion, forces SDO to high impedance, and ignores data present on DIN. After a conversion is initiated, it continues until completion irrespective of the state of CNV. CNV must be returned low before the safe data transfer time, tDATA, and then held low beyond the conversion time, tCONV, to generate the busy signal indicator. When the conversion is complete, SDO transitions from high impedance to low with a pull-up to VIO, which can be used to interrupt the host to begin data transfer. After the conversion is complete, the AD7682/AD7689 enter the acquisition phase and power down. The host must enable the MSB of CFG at this time (if necessary) to begin the CFG
VIO
update. While CNV is low, both a CFG update and a data readback take place. The first 14 SCK rising edges are used to update the CFG, and the first 16 SCK falling edges clock out the conversion results starting with the MSB. The restriction for both configuring and reading is that they both occur before the tDATA time elapses for the next conversion. All 14 bits of CFG[13:0] must be written or they are ignored. Also, if the 16-bit conversion result is not read back before tDATA elapses, it is lost. The SDO data is valid on both SCK edges. Although the rising edge can be used to capture the data, a digital host using the SCK falling edge allows a faster reading rate, provided it has an acceptable hold time. After the optional 17th SCK falling edge, SDO returns to high impedance. Note that, if the optional SCK falling edge is not used, the busy feature cannot be detected if the LSB for the conversion is low. If CFG readback is enabled, the CFG associated with the conversion result (n - 1) is read back MSB first following the LSB of the conversion result. A total of 31 SCK falling edges is required to return SDO to high impedance if this is enabled.
AD7682/ AD7689
DIGITAL HOST
MISO IRQ SS MOSI SCK
07353-038
SDO
CNV DIN SCK
FOR SPI USE CPHA = 1, CPOL = 1.
Figure 36. Connection Diagram for the AD7682/AD7689 with a Busy Indicator
tCYC tDATA tACQ tCNVH tDATA tCONV
CNV
CONVERSION (n - 1)
CONVERSION (n - 1)
(QUIET TIME) UPDATE (n) CFG/SDO
ACQUISITION (n)
CONVERSION (n) SEE NOTE
(QUIET TIME)
ACQUISITION (n + 1)
UPDATE (n + 1) CFG/SDO
SCK
15
16
17/ 31
1
2
15
16
tSDIN
DIN X X X
tHDIN
X X
17/ 31
CFG CFG MSB MSB -1
X
END CFG (n) SDO LSB +1 END DATA (n - 2) LSB
tDIS
BEIGN CFG (n + 1) MSB MSB -1
tHSDO tDSDO
tEN
END CFG (n + 1) LSB +1 LSB
tDIS
tEN
BEGIN DATA (n - 1)
tDIS
END DATA (n - 1) SEE NOTE
tEN
Figure 37. Serial Interface Timing for the AD7682/AD7689 with a Busy Indicator
Rev. 0 | Page 25 of 28
07353-039
NOTES: 1. THE LSB IS FOR CONVERSION RESULTS OR THE CONFIGURATION REGISTER CFG (n - 1) IF 16 SCK FALLING EDGES = LSB OF CONVERSION RESULTS. 30 SCK FALLING EDGES = LSB OF CONFIGURATION REGISTER. ON THE 17TH OR 31st SCK FALLING EDGE, SDO IS DRIVEN TO HIGH IMPENDANCE. OTHERWISE, THE LSB REMAINS ACTIVE UNTIL THE BUSY INDICATOR IS DRIVEN LOW.
AD7682/AD7689 APPLICATION HINTS
LAYOUT
The printed circuit board that houses the AD7682/AD7689 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. The pinout of the AD7682/AD7689, with all its analog signals on the left side and all its digital signals on the right side, eases this task. Avoid running digital lines under the device because these couple noise onto the die unless a ground plane under the AD7682/AD7689 is used as a shield. Fast switching signals, such as CNV or clocks, should not run near analog signal paths. Crossover of digital and analog signals should be avoided. At least one ground plane should be used. It can be common or split between the digital and analog sections. In the latter case, the planes should be joined underneath the AD7682/AD7689. The AD7682/AD7689 voltage reference input REF has a dynamic input impedance and should be decoupled with minimal parasitic inductances. This is done by placing the reference decoupling ceramic capacitor close to, ideally right up against, the REF and GND pins and connecting them with wide, low impedance traces. Finally, the power supplies VDD and VIO of the AD7682/ AD7689 should be decoupled with ceramic capacitors, typically 100 nF, placed close to the AD7682/AD7689 and connected using short, wide traces to provide low impedance paths and reduce the effect of glitches on the power supply lines.
EVALUATING AD7682/AD7689 PERFORMANCE
Other recommended layouts for the AD7682/AD7689 are outlined in the documentation of the evaluation board for the AD7682/AD7689 (EVAL-AD7682CBZ/EVAL-AD7689CBZ). The evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from a PC via the evaluation controller board, EVAL-CONTROL BRD3.
Rev. 0 | Page 26 of 28
AD7682/AD7689 OUTLINE DIMENSIONS
4.00 BSC SQ 0.60 MAX
15 16 20 1
0.60 MAX
PIN 1 INDICATOR
2.65 2.50 SQ 2.35
5
PIN 1 INDICATOR
3.75 BSC SQ
0.50 BSC
EXPOSED PAD
(BOTTOM VIEW)
TOP VIEW 0.80 MAX 0.65 TYP
0.50 0.40 0.30
11
10
6
0.25 MIN
1.00 0.85 0.80 SEATING PLANE
12 MAX
COMPLIANT TO JEDEC STANDARDS MO-220-VGGD-1
Figure 38. 20-Lead Lead Frame Chip Scale Package (LFCSP_VQ) 4 mm x 4 mm Body, Very Thin Quad (CP-20-4) Dimensions shown in millimeters
ORDERING GUIDE
Model AD7682BCPZ1 AD7682BCPZRL71 AD7689ACPZ 1 AD7689ACPZRL71 AD7689BCPZ1 AD7689BCPZRL71 EVAL-AD7682CBZ1 EVAL-AD7689CBZ1 EVAL-CONTROL BRD3 2
1 2
Integral Nonlinearity 2 LSB max 2 LSB max 6 LSB max 6 LSB max 2 LSB max 2 LSB max
No Missing Code 16 bits 16 bits 15 bits 15 bits 16 bits 16 bits
Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C
012508-B
0.30 0.23 0.18
0.05 MAX 0.02 NOM COPLANARITY 0.08 0.20 REF
Package Description 20-Lead QFN (LFCSP_VQ) 20-Lead QFN (LFCSP_VQ) 20-Lead QFN (LFCSP_VQ) 20-Lead QFN (LFCSP_VQ) 20-Lead QFN (LFCSP_VQ) 20-Lead QFN (LFCSP_VQ) Evaluation Board Evaluation Board Controller Board
Package Option CP-20-4 CP-20-4 CP-20-4 CP-20-4 CP-20-4 CP-20-4
Ordering Quantity Tray, 490 Reel, 1,500 Tray, 490 Reel, 1,500 Tray, 490 Reel, 1,500
Z = RoHS Compliant Part. This controller board allows a PC to control and communicate with all Analog Devices evaluation boards whose model numbers end in CB.
Rev. 0 | Page 27 of 28
AD7682/AD7689 NOTES
(c)2008 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D07353-0-5/08(0)
Rev. 0 | Page 28 of 28

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