Single-Supply Analog Input Module Reference Design with 16-Bit, 8-Channel ADC for PLC

35 Pages·2015·2.51 MB·English

by  Texas Instruments, Incorporated

LokeshGhulyani
TI Designs - Precision: Verified Design
Single-Supply Analog Input Module Reference Design with
16-Bit, 8-Channel ADC for PLC
TIDesigns–Precision CircuitDescription
TIDesigns–Precisionareanalogsolutionscreatedby
Thisdesignisfor a16-bit,8-channelanaloginput
TI’sanalogexperts.VerifiedDesignsofferthetheory,
modulefor industrialprogrammablelogiccontroller
componentselection,simulation,completeprinted
(PLC) systems. Thecircuit isrealizedwithan8-
circuitboard(PCB)schematicandlayout,billof
channel, 16-bit,successive-approximation-register
materials,andmeasuredperformanceofuseful
(SAR), analog-to-digitalconverter(ADC)with an
circuits. Circuitmodificationsthathelptomeet
integratedprecisionreferenceandanalogfront-end
alternatedesigngoalsarealsodiscussed.
(AFE)circuit.Thedesignexplainsthedesign process
for implementingdifferent voltageranges, different
DesignResources
currentranges, andtemperatureinputsfor industrial
PLCsystems.
DesignArchive Alldesignfiles
TINA-TI SPICEsimulator
ADS8688 Productfolder
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OPA320 Productfolder WEBENCH®DesignCenter
OPA376 Productfolder TIPrecisionDesignsLibrary
AVDD = 5V
1 M: ADS8688
PGA LPF
1 M:
1 M: 1«..8
AVDD VREF 249 Ÿ 1 M: PGA LPF Multiplexer 1A6D bCit
PT100 Rz RG1 RG2
V+ -
RefOut + 4.096V
V-
AGND
AnIMPORTANTNOTICEattheendofthisTIreferencedesignaddressesauthorizeduse,intellectualpropertymattersandother
importantdisclaimersandinformation.
TINA-TIisatrademarkofTexasInstruments,Inc.
WEBENCHisaregisteredtrademarkofTexasInstruments,Inc.
TINA-TIisaregisteredtrademarkofTexasInstruments.
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DesignSummary www.ti.com
1 Design Summary
Theprimaryobjectiveistodesignasingle-supply(5V),8-channel,analoginput module.Thisdesign can
beusedto measurebipolarvoltagerangesof ±10V, ±5V,and ±2.5V,unipolarvoltagerangesof 0Vto
5Vand0Vto10V,abipolarcurrentrangeof ±20mA,andaunipolarcurrent rangeof4mAto20 mA.
Thedesignalsohasaprovisiontomeasuretemperatureusingresistancetemperaturedetectors(RTDs).
Thekeyspecificationsofthedesignare:
• Systemsupplyvoltage:5.5Vdcto40Vdc
• ADCsupplyvoltage(AVDD):5Vdc
• Digital supplyvoltage(DVDD):3.3Vdc
• Responsetime(fortheADC):2µs
• Systeminputsignal:
– Voltageinputs: ±10V,±5V, ±2.5V,0Vto5V,0Vto10V
– Currentinputs:4mAto20mA, ±20-mAdccurrent
– Temperatureinput:Pt100RTD(a200-Ω potentiometerisusedtosimulateanRTD).
ThedesigngoalsandperformancearesummarizedinTable1.Themeasuredacperformanceforvoltage
inputs(±10V)isillustratedinFigure1.
Table1.ComparisonofDesignGoal, Simulation,andMeasuredPerformance
SIMULATED,
PARAMETER GOAL CALCULATED MEASURED
VOLTAGEINPUTS
Totalerror(%FSR),
±0.1% ±0.056% 0.042%
withoutcalibration Forvoltagerangesof±10V
Calibratederror ±0.05% — 0.002%
CURRENTINPUTS
Totalerror(%FSR),
±0.15% ±0.122% 0.028%
withoutcalibration Forcurrentrangesof±20mA
Calibratederror ±0.05% — 0.007%
TEMPERATUREINPUT
Totalerror(°C),
±2°C ±1.56°C 1.4°C
withoutcalibration Fortemperaturerangesof–40°Cto160°C
Calibratederror ±0.5°C — 0.38°C
2 Single-SupplyAnalogInputModuleReferenceDesignwith16-Bit,8-Channel TIDU291A–May2015–RevisedJuly2015
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Transfer characteristics for voltage input
65000
60000
55000
e50000
d
o45000
C
40000

t
u35000
p
ut30000
O
25000

C
20000
D
A15000
10000
5000
0
-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12
Input voltage (V)
Figure1.Transfer Characteristicsforthe ±10-VVoltageInputRange
2 Theory of Operation
2.1 Overview of the Programmable Logic Controller (PLC)
ThePLCisaprogrammablelogiccontrollerusedtocontrolindustrialprocessesandmachines.Abasic
PLCsystemconsistsofananaloginput module,aCPU,andananalogoutput module.Theanaloginput
moduleinterfaceswithsensors(pressure,temperature,flow, andsoforth)andconvertstheanalogsensor
outputintodigital.TheCPUprocessesthesedigitalvaluesandprovidesadigitaloutputthat isconverted
bytheanalogoutputmodulefortheactuatorstocontroltheindustrialprocess.Anexampleblockdiagram
forthePLCisshowninFigure2.
Focus of this
Reference Design
Analog
Analog Input
SENSORS CPU Output Actuators
Module
Module
Figure2.ExampleBlockDiagramforthePLC
Theinputfortheanaloginputmodulecanbeassmallas10mVfromtemperaturesensorsandcan be as
highas±10Vfromactuatorcontrollers.Theanaloginput moduleconsistsofasignal-conditioningcircuit
forfilteringandamplification(orattenuation)ofinput signals,anADCfor conversionofanalogsignalsinto
digital,andacircuitforisolationfromtheCPUandpower-supplycircuit.
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Theoutputsoftheanalogoutputmoduleareeitherthevoltageoutputsof ±10V,±5V,0Vto10V,and
0Vto5Vforcontrollingtherelaysandactuatorsorarecurrent outputsof±20mA,0mAto20mA, and
4mAto20mAforprocesscontrol.Theanalogoutputmoduleconsistsofadigital-to-analogconverter
(DAC)forconvertingdigital valuesfromtheCPUtoanalog, asignal-conditioningcircuit tobringtheoutput
oftheDACtothedesiredvoltageorcurrent output range,andanisolationcircuit for isolationfromthe
CPUandpower-supplycircuit.
2.2 Analog Input Module
ThisTI Designdiscussesthedesignofasingle-supplyanaloginput modulefor thePLC.Thekey
requirementsfortheanaloginputmodulefor thePLCarehigh-voltageinputsandhigh-inputimpedance.
Typically, aresolutionof12to16bitswithanoverallaccuracyof0.1% to0.2% isrequiredfor theanalog
inputmodules.Figure3showstheblockdiagramfor theanaloginput module.
AVDD = 5V
Desired Voltage Input ADS8688
(GRaainn,g Oesff,s eDt CE rproerr)f odremtearnmcein e 2 1 M: PGA LPF ROevseoralullt iSonys, treemsp Aocncsuer atimcye,
the analog front end design for 1 M: 1 determine the ADC,
Voltage Inputs reference and multiplexer.
1«..8
Dpedeseriftroeerrdmm Cainunerc reteh n(eGt aIannipnau,l otO gRf ffasronengt teE esrr,n oDdr )C 3 249 Ÿ 11 MM:: PGA LPF ultiplexer 1A6D bCit
design for Current Inputs AVDD M
VREF
T100 Rz RG1 RG2
P
Type of Temperature Sensor (RTD, V+ -
Thermocouple or NTC), DC RefOut + 4.096V
performance(Gain, Offset Error) and V-
Temperature Input Range determine 4
the analog front design for Temperature AGND
inputs
Figure3.BlockDiagramfortheAnalogInputModule
2.3 Considerations for Selecting the Architecture of the Analog Input Module
TheanaloginputmoduleforthePLCis generallyusedtomeasuretheoutputofpressuresensors,
rotatoryencoders,proximityswitches,temperaturesensors,andsoforth.Thesesensorsareinstalled in
thefieldto measureprocessvariables(suchastemperature,pressure, flow, andsoforth)andtheoutput
ofthesesensorsiseithervoltageorcurrentoutput.Designtheanaloginput moduletomeasurethe
voltageandcurrentoutputsfromthesesenors.
A24-Vpowerbusisgenerallyavailablefor poweringtheanaloginput module.The24-Vfieldpowerbus
cantypicallyvaryfrom20Vto30Vandcanhavetransientsupto42V.Thenoiseorrippleona24-Vbus
canpotentiallycoupleintoinputthroughtheAFEorthroughtheADC.Therefore, anAFEandanADC
withhighPSRRarepreferredfortheanaloginput moduleandalow-noiseconverter orlinearregulator
withtransientimmunityupto42Visusedfor poweringanaloginput modules.
Withtheincreasingnumberofinputchannelspermodule,thepower andspaceavailableperchannelis
reduced. TheanaloginputmoduleforthePLCmust measureaninput voltageof ±10Vandatraditional
AFErequiresabipolarsupply(±15Vor ±12V)tomeasureasignalof ±10V.Togenerateabipolar
supply,anadditionalpowerconverteris required.Theadditionalpower converter increasesthepowerand
spacerequirementsoftheanaloginputmodule.AnAFEandADCoperatingonasinglesupply(for
example,5V)eliminatestheneedforanadditionalpower converter andhelpsinreducingthespace and
powerrequirementsfortheanaloginputmodule.
Systemdesigngoals(suchasresponsetime,resolution,accuracy, power consumption,andsize)decide
thearchitecturefortheanaloginputmodule.
4 Single-SupplyAnalogInputModuleReferenceDesignwith16-Bit,8-Channel TIDU291A–May2015–RevisedJuly2015
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TheprocessingcycleforaconventionalPLCsystemisshowninFigure4.
Scan Time = Time required to complete one cycle
Scan Physical Inputs
from Analog Input Perform Logic Update Physical
Module & operations & Update Outputs in Analog
Update Inputs in Outputs in Controller Output Module
Controller
Figure4.PLCProcessCycle
2.3.1 ScanTime
Scantimeisthetimedifferencebetweentwoconsecutivereadingsthat thePLCcontrollertakes froma
particularanaloginputchannel.Scantimeistypicallythesumofresponsetimesofindividualblocksof the
PLCsystem.Scantimeisgenerallyintheorderfromafewmillisecondsto100milliseconds.Asthe
throughputperchanneloftheanaloginputmoduledecreases,thescantimeachievedfor thePLCsystem
increases.
2.3.2 ArchitecturesforAnalogInputModule
Therecanbetwoarchitecturesfordesigningtheanaloginput modulefor aPLC:
• SimultaneoussamplingwithaseparateADCfor eachchannel.
Simultaneoussamplingisusefulinsystemsthat requiresignalstobesampledatthesameinstant.
BecauseaseparateADCisusedfor eachchannel, thethroughput(samplespersecond)fromeach
channelisequaltothethroughputof theADCandthetotalthroughput achievedfromthisarchitecture
isthesumofthethroughputoftheADCsoneachchannel. Powermeasurementisacommon
applicationinsimultaneoussampling.Currentandvoltagesignalsaresimultaneouslysampled for
calculatingpower.SimultaneoussamplingrequiresanADCfor eachchannel, whichleadstolarger
circuitsizeandhigherpowerconsumption.
• SingleADCwithmultiplexedinputs.
AsingleADCwithmultiplexedinputssamplesthesignaloneachchannelonebyoneinasequence.
Thetotalthroughputachievedfromthisarchitectureisthethroughput oftheADC.Thethroughputof
theADCisdividedamongallthechannelsandthethroughputperchannelgetsreduced.This
architecturerequiresoneADC,onemultiplexer, andmultipleAFEs.Thisarchitecturegenerallyhas
lowerpowerandsmallercircuitsizethanasimultaneoussamplingarchitecture.
Thesecondarchitectureisselectedforthisdesignreport for itslowerpower consumptionandsmaller
solutionsize.Becausethethroughputper channelinthesecondarchitecturewithasingleADCgets
reduced, anADCwithhigherthroughputisrequiredtomeet thescantimerequirement.ASARADCwith
asufficientthroughput(> 100kSPS,typically)isabletomeet thescantimerequirement ofatypicalPLC
system.
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2.4 Design Considerations for Voltage Inputs
Thekeyrequirementforthevoltageinputsofananaloginput modulefor thePLCisthat theymustaccept
voltagerangesof±10V, ±5V,and±2.5V.Becausetheanaloginput modulehasasingleADCforallthe
inputranges,aprogramablegainamplifier (PGA)isrequiredtoscaleandlevelshiftthesignalsto the
analoginputrangesoftheADC.ThePGAalsoscalestheinput signalstowithinthesupplyvoltage and
makestheoperationoftheanaloginputmodulepossibleonasinglesupply. Figure5 providesthe
simplifiedcircuitofaPGAandEquation1givesthedctransferfunctionofaPGA.
V
b
R
f
R
V i V
1 p + V
out
V
2
R
i R
f
Figure5. SimplifiedCircuitforaPGA
R
f u (cid:14)
V (V -V ) V
out 1 2 b
R
i
where:
• V,V areinputvoltagestothePGA,and
1 2
• V isthebiasvoltagetolevelshifttheoutputofthePGA. (1)
b
TheADCoutputcodecanbecalculatedfromEquation2:
ADC_Output_Code ǻRf u(V1-V2) (cid:14) Vbȼu 1 u(cid:11)2N (cid:16)1(cid:12)
¬ Ri ¼ VFSR
where
• V isfull-scaleinputrangeoftheADCand
FSR
• NistheresolutionoftheADC. (2)
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2.5 Design Considerations for the Current Inputs
Inindustrialprocesscontrol,analog4-mAto20-mAand ±20-mAinputsarewidelyusedfor measuring
processvariablessuchaspressureandtemperature.Thebenefitsofthecurrent looparethat the
accuracyofthesignalisnotaffectedbyvoltagedropsininterconnectingwiring.Even ifthereissignificant
voltagedropresultingfromthewireresistance,thecurrent-looptransmittermaintainsthepropercurrent
uptoitsmaximumvoltagecapability.Aprecisionresistorisrequiredtoconvert thecurrentintovoltage so
thatitcanbemeasuredbyanADC.ThevalueoftheprecisionresistorcanbecalculatedfromEquation3.
R V
R i u FSR
S
R I
f S(cid:16)max
where
• V isthefull-scaleinputrangeoftheADC,
FSR
• I isthefull-scalevalueofthecurrent,and
S-max
• V isthefull-scaleinputoftheADC. (3)
FSR
AsimplifiedcircuitforcurrentmeasurementsisshowninFigure6.Thedctransferfunctionfor current
inputisstatedinEquation4andtheADCoutputcodecanbecalculatedfromEquation5.
V
b
Is IIN Ri Rf
Rs Is - IIN Vp + Vout
R
i R
f
Figure6.SimplifiedCircuitforCurrentMeasurements
R
f (cid:11) (cid:12)
V u I (cid:16)I uR V
s s(cid:14)
out R IN b
i
where:
• V isthefull-scaleinputrangeoftheADC,
FSR
• I isthefull-scalecurrent,and
S
• R isthesenseresistorformeasuringcurrent. (4)
s
ADC_OUTPUT _CODE ¨§Rf u(cid:11)I (cid:16)I (cid:12)uR V ¸·u 1 u(cid:11)2N (cid:16)1(cid:12)
s IN s(cid:14)
© Ri b¹ V
FSR
where
• V isthefull-scaleinputrangeoftheADCand
FSR
• NistheresolutionoftheADC. (5)
Processvariablessuchaspressureandtemperaturearenearlystaticinnature.Thedcspecifications
(gainerror,offseterror,andINLerror)oftheanaloginput modulebecomecriticalindesigningtheanalog
inputmoduleformeasuringthesevariables.Gainerror,offseterror,andINLerrorcontributetotheoverall
accuracyoftheanaloginputmodule.Asanexample,thecurrentoutput(±20mA)fromapressuresensor
mustbemeasuredwithanaccuracyof20 µAfor anoverallaccuracyof ±0.1%oftheanaloginputmodule.
Fordc(static)conditions,themajorportionoferroriscontributedbythegainerror,theoffseterror,and
INL.Totalerrorforstaticconditionsiscalculatedastheroot ofsumofsquaresofgainerror,offseterror,
andINL.
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2.6 Design Considerations for the Temperature Input
RTDsaresensorsthatareusedtomeasuretemperaturebycorrelatingresistanceoftheelement withthe
temperature.AccordingtotheIEC751/ITS-90standard,theresistanceofaplatinum resistance
temperaturedetectorcanbederivedfromEquation6.
For T = -200°C to 0°C
R R u[1(cid:14)AuT(cid:14)BuT2]
T 0
For T = 0°C to 850°C
R R u[1(cid:14)AuT(cid:14)BuT2 (cid:14)C(1-T)3]
T 0
Where
x(cid:3) R is RTD resistance at temperature T.
T
x(cid:3) R is RTD resistance at 0°C.
0
x(cid:3) A = 3.9083 x 10-3 °C-1
x(cid:3) B = -5.775 x 10-7 °C-2
x(cid:3) C = -4.182 x 10-12 °C-4 (6)
AsimplifiedschematicfortemperaturemeasurementusinganRTDisshowninFigure7.
Instrumentation
Amplifier C Level Shift and
Noise
Op-Amp ADC Driver
REF/2 + R
3 R
2
INA
RTD + V R Op-Amp V R
INA 1 + OPA FLT VOUT
C
3 R
ISET 1 CFLT
Current
Source RSET R3 REF/2 RFLT
R
2
Figure7.SimplifiedSchematicforTemperatureMeasurementusinganRTD
AcurrentsourceisrequiredforexcitingtheRTD.Alow-noise,low-offset,andlow-drift operational
amplifier isrequiredtomakeaprecisecurrentsourcefor excitingtheRTD.ThevalueofI iskeptat
SET
1mAtolimittheself-heatingoftheRTD.Theexcitation currentcanbederivedfromEquation7.
V
I REF/2
SET R
SET (7)
Alow-noise,high-inputimpedanceinstrumentationamplifierisrequiredtoamplifythevoltageacrossthe
RTD. ThegainrequiredforthisstagecanbederivedfromEquation8.
V
G FSR
INA (cid:11) (cid:12)
I uR
SET RTD(cid:16)MAX
where:
• I istheexcitationcurrentfortheRTD,
SET
• R isthemaximumresistanceoftheRTDfortheselectedtemperaturerange,and
RTD-MAX
• V isthefull-scaleinputrangeoftheADC. (8)
FSR
Equation9statesthedctransferfunctionfor theinstrumentationamplifierstage:
V I uR uG
INA SET RTD INA
where
• G isthegainoftheinstrumentationamplifier (9)
INA
Forthetemperaturerangeof –40°Cto160°C:
R =84.27Ω atT= –40°C.
RTD-MIN
R =161.05Ω atT=160°C.
RTD-MAX
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Becausetheanaloginputmoduleisdesignedtooperateonasinglesupply, theoutputofthe
instrumentationamplifiercanonlyswingbetween0Vandthesupplyvoltage(AVDD).AnADCwith a
single-endedinputandfull-scaleinputrangelessthantheAVDDsupplyisrequiredfor temperature
measurementinthisanalogmodule.
Theoutputoftheinstrumentationgainstagecoversonlyafractionofthefull-scaleinput rangeofthe
ADC. Therefore,theoutputoftheinstrumentationgainstagemust belevelshiftedandamplifiedto match
thefull-scaleinputrangeoftheADC.Equation10statesthedctransferfunctionfor thisstage.
R
V (cid:11)V -V (cid:12)u 2
INA REF/2
OPA R
1
where:
• V istheoutputvoltageoftheoperationalamplifierand
OPA
• V istheoutputvoltageoftheinstrumentationamplifier. (10)
INA
Equation11statesthecompletetransferequationfor thetemperatureinput andtheADCoutputcodefor
thetemperatureinputcanbecalculatedfromEquation12.
R ª V 1 V º
V 2 u«G u REF u uR (cid:16) REF »
OUT INA RTD
R1 ¬ 2 RSET 2 ¼ (11)
R ª V 1 V º 1 (cid:11) (cid:12)
ADC_Output_Code 2 u«GINA u REF u uRRTD (cid:16) REF »u u 2N (cid:16)1
R 2 R 2 V
1 ¬ SET ¼ FSR (12)
TheREF /2voltageisgeneratedbyaresistordividerfromtheV voltageandbufferedwithan
REF
operationalamplifiertoprovidealow-impedanceREF/2sourcefor drivingtheinvertinginput oftheADC
driver.Figure8providestheschematicfor theREF/2source.
R Op-Amp
REF 4 +
REF/2
R
4
REF/2
Source
Figure8.SchematicfortheREF/2Source
ThedesignconsiderationsfordrivingaSARADCcanbefoundinsection2ofTIDesignTIDU181.
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2.6.1 NoiseAnalysis
ThelaststageforthecircuitillustratedinFigure7 isalevelshiftandADCdriverstage.Theoperational
amplifier isconfiguredforadifferentialinput andasingle-endedoutput.Thetotalintegratednoise at the
outputoftheoperationalamplifierconfiguredasadifferentialamplifiercanbeestimatedbyEquation13.
R (cid:14)R R (cid:14)R R R V
V [{2˜(I R) 2(cid:14) (e ˜( 1 2))2(cid:14)8kTR( 1 2)}uENB(cid:14)(E ˜ 2)2(cid:14)(E ˜ 2)2(cid:14)( 1/f_AMP_PP)2]
AMP-NOISE n 2 n R 2 R INAOUT R REF/2 R 6.6
1 1 1 1
where:
• V isthetotalintegratedRMSnoiseattheoutputoftheoperationalamplifier,
AMP-NOISE
• I isthewhitecurrentnoisespecificationoftheoperationalamplifier(A/√Hz),
n
• V isthewhitevoltagenoisespecificationoftheoperationalamplifier(V/√Hz),
n
• E isthetotalnoiseattheoutputoftheinstrumentationamplifier(VRMS),
INAOUT
• E isthetotalnoiseattheREF/2output(VRMS),
REF/2
• V istheflickernoisespecificationoftheoperationalamplifierspecifiedaspeak-to-peaknoise(V ),
1/f_AMP_PP PP
and
• ENBistheeffectivenoisebandwidth.
1 Œ
ENB ˜
2Œ ˜2˜R ˜C 2
FLT FLT (13)
Detailsfornoiseanalysisinoperationalamplifierscanbefoundinapplicationreport SLVA043.
ForanADCwithaninputrangeofV ,theRMSvalueoftheinput-referrednoisecanbefound fromthe
FSR
specifiedvalueofSNRinthedatasheetbyusingEquation14.
SNR(cid:11)dB(cid:12)
V (cid:16)
V FSR u10 20
n_ADC_RMS
2 2
where:
• V istheRMSnoiseattheinputoftheADCand
n_ADC_RMS
• V isthefull-scaleinputrangeoftheADC. (14)
FSR
Thetotalnoiseforatemperaturemeasurementsystemistheroot sumsquare(RSS)ofthenoise at the
outputoftheamplifierandtheinput-referrednoiseoftheADC.Thesystemnoisefor atemperatureinput
canbecalculatedusingEquation15.
V [V 2 (cid:14) V 2]

SYS-NOISE AMP-NOISE n_ADC_RMS (15)
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