温度电阻计算.docx

温度电阻计算.docx

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温度电阻计算

RTDmeasurementscanbedoneusingdiscreteop-ampsanddataconvertersorintegratedfront-endsolutions.TIhassolutionsthatcovereachofthesetopologies.Dependingonvariousparameterssuchascost,boardspace,resolutionandperformanceanengineermayprefertochooseonetopologyoveranother.

RTD:

Op-ampapproach

RTD:

with4-20mAoutput:

XTRapproach:

UsingXTR105

RTD:

AFEapproach:

UsingLMP90100

OP-AMPS

ExcitationwithCurrentSource

Withanexcitationcurrent,theRTDvoltageisdirectlyproportionaltotheRTDresistance.Theoutputvoltageisproportionaltotheexcitationcurrentsoitistemptingtomakethiscurrentlargeenoughtoproduceasubstantialvoltagechangewithtemperature.Butwithincreasedexcitationcurrentcomesincreasedpowerdissipationandself-heatingthatcanaffectmeasurementaccuracy.ThismayrequirecarefulconsiderationofRTDcharacteristicsandaccuracyrequirements.Excitationcurrentsintherangeof100uAto1mAarecommonforaPt100RTD.Outputvoltagechangesbyapproximately38uV/ºCwith100uAexcitation.

ExcitationwithVoltageSourceandResistor

Excitationcurrentcancomefromavoltagesourceandaresistor.Thecurrent,however,isnottrulyconstantinthisconfiguration.AstheRTDresistanceincreaseswithtemperature,theexcitationcurrentdecreases.Thiscanbeaccountedforincalibrationattemperatureextremesbutnonlinearityisincreased,especiallyoverwidemeasurementtemperaturerange.

Still,iftheR1ismadelargecomparedtothechangeinRTDresistancethroughoutthemeasurementrange,resultsmaybesatisfactory.

BasicOpAmpAmplifierCircuit

Withtwomatchedexcitationcurrentsources,theoutputvoltagecanbeadjustedtozeroatthedesiredlow-scaletemperature.Forexample,theRTDvoltagewillbe（100uA）（100Ω）=10mVat0ºC.Toprovide0VoutputatthistemperaturerequiresanegativeoffsetprovidedbyIS2.TheREF200dual100uAcurrentsourceiswellsuitedtothisapplication.

InstrumentationAmplifierCircuit

Aninstrumentationamplifierofferssomeadvantagesoversingleopampcircuits.Here,theoffsettingcircuitry,IS2andR10,doesnotaffectthegainofthecircuit.GainissolelycontrolledbyR7.Furthermore,thecircuitallowsaso-calledthree-wireconnectiontotheRTD.EqualwireresistanceintheconnectionstotheRTDcreateacommon-modevoltagethatisrejectedbytheinstrumentationamplifier.ThisallowslongerconnectionstobemadetotheRTDwithoutcreatingexcessiveerrorduetowireresistance.

RTDmeasurementwithcurrent-loopoutputusingXTR105

Objective:

Designaneasilyconfigurablecircuitthatproducesa4-20mAoutputbasedonanRTDtemperaturerangeof0C–200CusingtheXTR105.

Solution:

Anexampleofthissolutionusingatwo-wireRTDcanbeseeninFigure1oftheXTR105datasheet.

TocompletethisdesignwiththeXTR105theonlyrequirementsaretocalculatethethreeexternalcomponents,Rg,Rz,andRlin1.EquationsforthesecalculationscanbeseeninFigure1andthecalculationsareshownbelow:

RTDTemperatureRange

TheresultingcircuitcanbeseenintheSPICEsimulationbelow.WiththeRTDresistancesettothe0Cvalueof100Ohms,thecircuitoutputis4mA.

WhentheRTDresistanceisatthe200Cvalueof178.845thecircuitoutputis20mA.

Thesimulationsusetheexactcalculatedresistancevaluestoachievetheidealcircuitoutput.Standard1%or0.1%valueswillhavetobeusedinthefinalcircuitandthedeviationsbetweenthecalculatedvaluesandtherealvalueswillappearasanerror.Also,theeffectoftheanaloglinearizationcanbeseenbylookingatthechangeinthecurrents,i_linandi_rtd,astheRTDresistancechangesfromzero-scaletofull-scale.

AnexampleaccuracycalculationforthissystemcanbeseeninTableIIintheXTR105datasheetandisshownbelow:

IfathreewireRTDisused,thecircuitchangesandlooksFigure3intheXTR105datasheet.

TocompletethedesignwiththeXTR105anadditionalresistor,Rlin2,needstobecalculatedalongwithRg,Rz,andRlin1.Althoughcalculationscanbeusedtodeterminetheexactvaluesfortheresistors,Table1intheXTR105datasheetprovidestheuserwiththenearest1%valuesforthefourexternalresistorsbasedontheinitialtemperatureandspan.

Foradesignfrom0C–200Cthe1%valuescanbefoundinthehighlightedcellinthetableaboveandareshownbelow.

Iftheequationsareusedtocalculatetheexactvaluestheresultsareshownbelow.Thedeviationbetweentheactualcalculatedvaluesandthefinalresistorwillappearasanerrorinthesystem.

AFEapproach:

UsingLMP90100

∙DownloadUsingLMP90100PDF