Analog Engineer's Circuit Cookbook: ADCs

113 Pages·2017·6.13 MB·English

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Circuit Cookbook: ADCs C
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Analog Engineer’s Circuit Cookbook:
ADCs
First Edition
Edited by:
Art Kay, Luis Chioye and Dale Li
Special thanks for technical contribution:
Peggy Liska
Cynthia Sosa
Reed Kaczmarek
Bryan McKay
Manuel Chavez
Analog Engineer’s
Circuit Cookbook: ADCs
(First Edition)
Message from the editors:
The Analog Engineer’s Circuit Cookbook: ADCs provides analog-to-digital converter (ADC) sub-circuit ideas that can be
quickly adapted to meet your specific system needs. Each circuit is presented as a “definition-by-example.” They include
step-by-step instructions, like a recipe, with formulas enabling you to adapt the circuit to meet your design goals. Additionally,
all circuits are verified with SPICE simulations and include links to the corresponding TINA-TITM SPICE circuits.
We’ve provided at least one recommended ADC for each circuit, but you can swap it with another device if you’ve found one
that’s a better fit for your design. You can search our large portfolio of ADCs at www.ti.com/ADCs.
Our circuits require a basic understanding of amplifier and data converter concepts. If you’re new to data converter design,
we highly recommend completing our TI Precision Labs (TIPL) training series. TIPL includes courses on introductory topics,
such as device architecture, as well as advanced, application-specific problem-solving, using both theory and practical
knowledge. Check out our curriculum for op amps, ADCs and more at: www.ti.com/precisionlabs.
We plan to update this e-book with new ADC circuit building blocks and encourage you to see if your version is the latest at
www.ti.com/circuitcookbooks. If you have feedback on any of our existing circuits or would like to request additional ADC
circuits for the next edition of this e-book, please contact us at [email protected].
We hope you find our collection of ADC circuits helpful in developing your designs!
Additional resources to explore

TI Precision Labs ADC Parametric Quick Search
ti.com/precisionlabs ti.com/ADC-search
• On-demand courses and tutorials ranging from introductory to • Find your next precision or high-speed ADC
advanced concepts that focus on application-specific problem solving
• Hands-on labs and evaluation modules (EVM) available DIY Amplifier Circuit Evaluation Module (DIYAMP-EVM)
ti.com/DIYAMP-EVM
- TIPL Op Amps experimentation platform, ti.com/TIPL-amp-evm
• Single-channel circuit evaluation module providing SC70, SOT23
- TIPL SAR ADC experimentation platform, ti.com/TIPL-adc-evm
and SOIC package options in 12 popular amplifier configurations
Analog Engineer’s Pocket Reference
Dual-Channel DIY Amplifier Circuit Evaluation Module
ti.com/analogrefguide
(DUAL-DIYAMP-EVM)
• PCB, analog and mixed-signal design formulae; includes conversions,
ti.com/dual-diyamp-evm
tables and equations
• Dual-channel circuit evaluation module in an SOIC-8 package with
• e-book, iTunes app and hardcopy available
10 popular amplifier configurations
The Signal e-book
TINA-TI simulation software
ti.com/signalbook
ti.com/tool/tina-ti
• Short, bite-sized lessons on on op-amp design topics, such as offset
• Complete SPICE simulator for DC, AC, transient and noise analysis
voltage, input bias current, stability, noise and more
• Includes schematic entry and post-processor for waveform math
Analog Wire Blog
Analog Engineer’s Calculator
ti.com/analogwire
ti.com/analogcalc
• Technical blogs written by analog experts that include tips, tricks and
• ADC and amplifier design tools, noise and stability analysis,
design techniques
PCB and sensor tools
TI Designs
TI E2E™ Community
ti.com/tidesigns
ti.com/e2e
• Ready-to-use reference designs with theory, calculations,
• Support forums for all TI products
simulations schematics, PCB files and bench test results
The platform bar is a trademark of Texas Instruments. © 2018 Texas Instruments Incorporated.
Table of Contents
Low-Power/Cost-Optimized Circuits Low-Level Sensor Input Circuits
Driving a SAR ADC Directly without a Front-End Buffer Circuit Low-Input Bias Current Front End SAR ADC Circuit .................65
(Low-Power, Low-Sampling-Speed DAQ) ....................................5
Circuit for Driving a Switched-Capacitor SAR ADC with an
Low-Power Sensor Measurements: 3.3-V, 1-ksps, 12-Bit, Instrumentation Amplifier ...........................................................70
Single-Ended, Dual-Supply Circuit .............................................10
Circuit for Driving a Switched-Capacitor SAR ADC with a
Low-Power Sensor Measurements: 3.3-V, 1-ksps, 12-Bit, Buffered Instrumentation Amplifier .............................................77
Single-Ended, Single-Supply Circuit ..........................................16
Input Protection, Filtering and Isolation Circuits
Level Translation Input Drive Circuits
Reducing Effects of External RC Filter Circuit on Gain and Drift
High-Voltage Battery Monitor Circuit: ±20 V, 0-10 kHz, 18-Bit Error for Integrated Analog Front Ends (AFEs): ±10 V, up to
Fully Differential ..........................................................................23 200 kHz, 16-Bit ...........................................................................85
Single-Ended-to-Differential Circuit Using an Op Amp and Fully Antialiasing Filter Circuit Design for Single-Ended ADC Input
Differential Amplifier (FDA) for Bipolar Signals ...........................30 Using Fixed Cutoff Frequency ....................................................91
High-Input Impedance, True Differential, Analog Front End (AFE) Digitally-Isolated ADS8689 Circuit Design ...............................100
Attenuator Circuit for SAR ADCs ................................................37
Circuit to Increase Input Range on an Integrated Analog Front
Commonly Used Auxiliary Circuits
End (AFE) SAR ADC ...................................................................45
Powering a Dual-Supply Op Amp Circuit with One LDO .........104
Circuit for Driving High-Voltage SAR ADCs for High-Voltage,
True Differential Signal Acquisition .............................................50
Isolated Power Supply, Low-Noise Circuit: 5 V, 100 mA ..........108
High-Current Battery Monitor Circuit: 0-10 A, 0-10 kHz,
18-Bit ..........................................................................................59
Want more circuits?
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•Browse a complete list of op amp Circuit Cookbook: ADCs Code Vin
and ADC circuits
Visit ti.com/circuitcookbooks
Analog Engineer's Circuit: ADCs
SBAA256–January2018
Driving a SAR ADC Directly Without a Front-End Buffer
Circuit (Low-Power, Low-Sampling-Speed DAQ)
AbhijeetGodbole
Design Description
Thisdesignexplainshow sensoroutputscanbedirectlyinterfacedwithaSARADCinput.Inapplications
suchasEnvironmentalSensors,GasDetectors,andSmokeorFireDetectors,theinputisveryslow-
movingandthesensoroutputvoltageissampledatfairlyslowerspeeds(10kspsorso).Insuchorsimilar
systems,thesensoroutputcanbedirectlyinterfacedwiththeSARADCinputwithouttheneedfora driver
amplifiertoachieveasmallform-factor,low-costdesign.
InterfacingSensorOutputDirectlytoa SARADC
ThefollowingfigureshowsatypicalapplicationdiagramforinterfacingasensordirectlytoaSARADC
inputwithouttheuseofadriveramplifier.Thesensor blockhighlightstheTheveninequivalentofasensor
output.Voltagesource,V ,istheThevenin-equivalentvoltageandsourceresistanceR istheThevenin-
TH TH
equivalentimpedance.MostsensordatasheetsprovidetheTheveninmodelofthesensorfromwhichthe
valueoftheseriesimpedancecanbeeasilycalculated.
V
TH ½ LSB
V (
CSH
t)
SENSOR
V0 SAR ADC
tAC
RTH t0 Q S RSW
W
+ C
V SH
TH –
C
FLT
SBAA256–January2018 5 Driving a SAR ADC Directly Without a Front-End Buffer
SubmitDocumentationFeedback Circuit (Low-Power, Low-Sampling-Speed DAQ)
Copyright©2018,TexasInstrumentsIncorporated
www.ti.com
Specifications
Parameter Calculated Simulated Measured
TransientADCInputSettling <0.5LSB 36.24µV N/A
Error <100.5µV
StepInputFullScaleRange 3.15V 3.15V 3.14978
InputSourceImpedance(R ) 10kΩ 10kΩ 10.01kΩ
TH
FilterCapacitorValue(C ) 680pF 680pF N/A
FLT
ADCSamplingSpeed 10ksps 10ksps 10ksps
DesignNote
1. Determinesourceimpedanceofinputsignal.CalculatetheRCtimeconstantoftheinputsource
impedanceandfiltercapacitor(knownvalue).
2. Determinetheminimumacquisitiontimerequired fortheinputsignaltosettleforagivensource
impedanceandthefiltercapacitorcombination.
3. SelectCOGcapacitorstominimizedistortion.
4. Use0.1%20ppm/°Cfilmresistorsorbetterforgoodgaindriftandtominimizedistortion.
Driving a SAR ADC Directly Without a Front-End Buffer 6 SBAA256–January2018
Circuit (Low-Power, Low-Sampling-Speed DAQ) SubmitDocumentationFeedback
Copyright©2018,TexasInstrumentsIncorporated
www.ti.com
ComponentSelectionfor ADCInputSettling
SARADCscanbedirectlyinterfacedwithsensorswhentheanaloginputsourceiscapableofdriving the
switchedcapacitorloadofaSARADCandsettlingtheanaloginputsignaltowithin ½ofanLSBwithin
theacquisitiontimeoftheSARADC.Toachieve this,theexternalRCfilter(R andC )mustsettle
TH FLT
withintheacquisitiontime(t )oftheADC.TherelationshipbetweentheADCacquisitiontimeandRC
ACQ
timeconstantoftheexternalfilteris:
t ≥k·Ԏ
ACQ FLT
where
• Ԏ =R ·C
FLT TH FLT
• kisthesinglepoletimeconstantforNbitADC
Thefollowingdesignexamplevaluesaregiveninthetableonpage1:
R =10kΩ
TH
C =680pF
FLT
K=11(Singlepoletimeconstantmultiplierfor14-bitADC) – Moreinformationisfoundonpage96and
page97oftheAnalogEngineer’sPocketReference.
Minimumacquisitiontimerequiredforproper settlingiscalculatedusingthisequation:
t ≥11·10kΩ·680pF=74.80µs
ACQ
FormoreinformationonSARADCsandfrontend design forSARADCs,refertoIntroductiontoSAR
ADCFront-EndComponentSelection.
TransientInputSettlingSimulationusingTI-TINA
ThefollowingfigureshowsthesettlingofanADS7056 ADCgivena3.15-VDCinputsignal.Thistypeof
simulationshowsthatthesampleandholdkickbackcircuitisproperlyselected.RefertoRefinetheRfilt
andCfiltValuesintheTIPrecisionLabs-ADCstrainingvideoseriesfordetailedtheoryonthissubject.
SBAA256–January2018 7 Driving a SAR ADC Directly Without a Front-End Buffer
SubmitDocumentationFeedback Circuit (Low-Power, Low-Sampling-Speed DAQ)
Copyright©2018,TexasInstrumentsIncorporated
www.ti.com
Increasing AcquisitionTimeofSARADCforInputSignalSettling
TheacquisitiontimeofaSARADCcan be increased byreducingthethroughputinthefollowingways:
1. ReducingtheSCLKfrequencytoreducethethroughput.
2. KeepingtheSCLKfixedatthehighestpermissiblevalueandincreasingtheCShightime.
ThefollowingtableliststheacquisitiontimefortheprevioustwocasesfortheADS7056 SARADC
operatingat10kspsthroughput(tcycle=100µs).Case2providesalongeracquisitiontimefortheinput
signaltosettlebecauseoftheincreasedfrequencyoftheSCLKgivenafixedconversionandcycle time.
AcquisitionTime
Case SCLK t ConversionTime(18·t )
cycle SCLK (t –t )
cycle conv
1 0.24MHz 100µs 74.988µs 25.01µs
2 60MHz 100µs 0.3µs 99.70µs
Thefollowingtableshowsaperformancecomparison betweenan8-,10-,12-,and14-bitADCwith
respecttosamplingspeedandeffectivenumber ofbits(ENOB)whenasensoroutputwithanoutput
impedance of10kΩ isdirectlyinterfacedwiththeADCinput.Asexpected,theENOBdegradeswithhigher
samplingratesbecausetheacquisitiontimedecreases.
ADS7040(8-bitADC) ADS7041(10-bitADC) ADS7056(14-bitADC)
Sampling ADS7042(12-bitADC)
Speed(ksps) ENOCBFLT(R=T1H.=5n1F0)kΩ, ENOCBFLT(R=TH1.=5n1F0)kΩ, ENOB(RTH=10kΩ,CFLT=1.5nF) ENOB(RT6H8=0p1F0)kΩ,CFLT=
10 7.93 9.87 10 12.05
100 7.92 9.85 9.97 10.99
500 7.88 9.68 9.95 8.00
PerformanceAchievedatDifferentThroughput RateswithDifferentSourceimpedance
ThefollowingfigureprovidestheENOBachievedfromtheADS7056atdifferentthroughoutwithdifferent
inputimpedances.NotethatalltheresultsforweretakenwithoutanADCdriveramplifier.
12.5
12
11.5
s)
Bit
B ( 11
O
N
E
10.5
33Ohm, 680pF
330Ohm, 680pF
10 3.3kOhm, 680pF
10kOhm, 680pF
20kOhm, 680pF
9.5
2 22 42 62 82 100
Sampling Speed(kSPS)
D039
Driving a SAR ADC Directly Without a Front-End Buffer 8 SBAA256–January2018
Circuit (Low-Power, Low-Sampling-Speed DAQ) SubmitDocumentationFeedback
Copyright © 2018, Texas Instruments Incorporated
www.ti.com
DesignFeaturedDevices:
Device KeyFeatures Link OtherPossibleDevices
8-bitresolution,SPI,1-Mspssamplerate,single-
ADS7040 endedinput,AVDD/Vrefinputrange1.6Vto http://www.ti.com/product/ADS7040 SimilarDevices
3.6V.
10-bitresolution,SPI,1Mspssamplerate,
ADS7041 single-endedinput,AVDD/Vrefinputrange1.6V http://www.ti.com/product/ADS7041 SimilarDevices
to3.6V.
12-bitresolution,SPI,1-Mspssamplerate,
ADS7042 single-endedinput,AVDD/Vrefinputrange1.6V http://www.ti.com/product/ADS7042 SimilarDevices
to3.6V.
14-bitresolution,SPI,2.5-Mspssamplerate,
ADS7056 single-endedinput,AVDD/Vrefinputrange1.6V http://www.ti.com/product/ADS7056 SimilarDevices
to3.6V.
NOTE: TheADS7042andADS7056usetheAVDDasthereferenceinput.Ahigh-PSRRLDO,such
astheTPS7A47,shouldbeusedasthepowersupply.
Linkto Keyfiles
SourceFilesforInterfacingSensorOutputDirectlywithSARADCs (http://www.ti.com/lit/zip/sbac178)
FordirectsupportfromTIEngineersusetheE2Ecommunity:
e2e.ti.com
OtherLinks
www.ti.com/adcs
www.ti.com/opamp
SBAA256–January2018 9 Driving a SAR ADC Directly Without a Front-End Buffer
SubmitDocumentationFeedback Circuit (Low-Power, Low-Sampling-Speed DAQ)
Copyright©2018,TexasInstrumentsIncorporated
Analog Engineer's Circuit: ADCs
SBAA251–November2017
Low-Power Sensor Measurements: 3.3-V, 1-ksps, 12-bit,
Single-Ended, Dual-Supply Circuit
ReedKaczmarek
Input ADCInput DigitalOutputADS7042
V =0V AIN_P=0V,AIN_M=0V 000 or0
inMin H 10
V =3.3V AIN_P=3.3V,AIN_M=0V FFF or4096
inMax H 10
PowerSupplies
AVDD V V
ee dd
3.3V –0.3V 4.5V
DesignDescription
Thisdesignshowsanlow-poweramplifierbeing usedtodriveaSARADCthatconsumesonlynWof
powerduringoperation.Thisdesignisintendedforsystemscollectingsensordataandrequirealow-
powersignalchainwhichonlyburnssingle-digitµWofpower.PIRsensors,gassensors,andglucose
monitorsareafewexamplesofpower-sensitivesystemsthatbenefitfromthisSARADCdesign.The
valuesinthecomponentselectionsectioncanbeadjustedtoallowfordifferentdatathroughputratesand
differentbandwidthamplifiers.Low-PowerSensorMeasurements:3.3V,1 ksps,12-bitSingle-Ended,
SingleSupplyshowsasimplifiedversionofthiscircuitwherethenegativesupplyisgrounded.The –0.3-V
negativesupplyinthisexampleisusedtoachievethebestpossiblelinearinputsignalrange.SeeSAR
ADCPowerScalingforadetaileddescriptionoftrade-offsinlow-powerSARdesign.
AVDD 3.3V
Vdd 4.5V
0.1(cid:29)F
0.1(cid:29)F
-
+ Rfilt
(cid:159)
200k
AVDD
LPV811 AinP
+
ADS7042
C
+ filt AinM
510pF
Vee -0.3V
V
SENSOR
Copyright © 2017, Texas Instruments Incorporated
SBAA251–November2017 10 Low-Power Sensor Measurements: 3.3-V, 1-ksps,
SubmitDocumentationFeedback 12-bit, Single-Ended, Dual-Supply Circuit
Copyright©2017,TexasInstrumentsIncorporated