AN-225IC Temperature Sensor Provides …

1 ApplicationReportSNOA663B April1979 ,thesecircuitsareaccurateto C overa 0 C to70 C temperaturerangeusingJ orK ,a thermocoupleamplifierusinganLM335forcold -junctioncompensationhasbeendescribedfor whichworstcaseerrorcanbeaslowas1 Cper40 Cchangein (a) (b) Thermocouple( C/ V)..72 LinearApproximationstoTypeS April1979 RevisedApril2013AN-225 ICTemperatureSensorProvidesThermocoupleC old-JunctionCompensationSubmitDocumentat ionFeedbackCopyright 1979 2013, ,thermocouplesarestilla thermocoupleis madebyjoiningwiresoftwodifferentmetalsas shownin Figure1. Theoutputvoltageis knownastheSeebeckcoefficientandrangesfro m5 V/ C to50 V/ C (TM TREF) thermocoupleis sensitivetoa temperaturedifference, thetemperatureatthereferencejunctionmust beknownin ordertomakea tokeepthereferencejunctionin C, moreconvenientapproach,knownascold-junct ioncompensation,is toadda compensatingvoltagetothethermocoupleoutp utsothatthereferencejunctionappearstobea t0 thisvoltageis madeproportionaltotemperaturewiththesame constantofproportionalityasthethermocoup le,changesin ,whichhasa ,is a naturalchoicetousein zenerdiodewitha breakdownvoltageproportionaltoabsolutete mperatureat10mV/ ,becausetheLM135extrapolatestozerooutput at0 K, thetoleranceofthethermocoupleitself, typically %ofreadingforJ, K,andT typesor %forS andR types,sothata measurementof1000 C , isdifficulttomatchthethermocouple' linearabsolutetemperaturesensor,soa bestfit thisnonlinearityisplottedasa typeis themostlinear,whiletheS typeis.

2 Compensationerroris a functionofboththermocouplenonlinearityan dalsothevariationinambienttemperature,si ncethestraight-lineapproximationtothethe rmocouplecharacteristicis April1979 RevisedApril2013 CompensationSubmitDocumentationFeedbackC opyright 1979 2013, ,increasederrorresultsif, duetocomponentinaccuracies, ,thecompletecircuitmustcontainresistorsa nda voltagereferencein ,butthetemperaturecoefficientoftheseexte rnalcomponentsis usuallythelimitingfactor(unlessthisdrift is measuredandtrimmedout).3 CircuitDescriptionA single-supplycircuitis shownin Figure3. R3andR4dividedownthe10mV/ K voltagetobuckoutthe0 C ,adjustR1sothatV1= <5 C T,where<5 C is theSeebeckcoefficientandT is theambienttemperaturein degreesKelvin. Then,adjustR2sothatV1 V2is ,Vishaybulkmetalorprecisionmetalfilmtype switha 1%toleranceandatemperaturecoefficientof 5 ppm/ C lowTCR,theseresistorsexhibitlowthermalem fwhentheleadsareatdifferenttemperatures, rangingfrom3 V/ C V/ C especiallyimportantwhenusingS orR typethermocouplesthatoutputonly6 V/ temperaturecoefficientof 25ppm/ C orbetteranda 1% notimportantsothattheirTCRis ,thetrimpotsshouldbeofa ,manymodernsingle-turnpotsarejustaseasyt osettowithin , ,goodsingle-turncermetpots(suchasAllenBr adleytypeE,Westonseries840,orCTSseries36 0)shouldbeconsideredasgoodcandidatesforh igh-resolutiontrimapplications,competing withthemoreobvious(butslightlymoreexpens ive)multi-turntrimpotssuchasAllenBradley typeRTorMT,Westontype850, roomtemperatureadjustment,drifterrorwill beonly C at70 C and C at0 (typeK)

3 Thermocoupleis ,a compensationaccuracyof C canbeobtainedovera0 C 70 C andT typethermocouples,theambienttemperaturem ustbekeptwithina morelimitedrange,suchas15 C to50 ,moreaccuratecompensationovera April1979 RevisedApril2013AN-225 ICTemperatureSensorProvidesThermocoupleC old-JunctionCompensationSubmitDocumentat ionFeedbackCopyright 1979 2013, C resistors,thecircuitcanachieve C errorovera 10 C thermalemfof20 V for1 C temperaturedifferencein leadtemperatureasopposedto2 V/ C V/ C ,with50ppm/ C obtainableforthickfilmand5 ppm/ C ,onecanuseseriesandparallelconnectionsof resistorsononeormoresubstrates.(1) (1)SeeAppendixA ( )( )( V/ C) (1)(2)A circuitforusewithgroundedthermocouplesis shownin Figure4. If dualsuppliesareavailable,thiscircuitis preferabletothatofFigure3 sinceit ,shortouttheLM329 BandadjustR5sothatVo= <5 C T,where<5 C is theSeebeckcoefficientofthethermocouplean dT is goodgroundingsystemis essentialhere,foranygrounddifferentialwi llappearin serieswiththethermocoupleoutput.

4 (1)*R3thruR6are1%,5 ppm/ C.(10ppm/ C tracking.)(2) R7is 1%,25ppm/ April1979 RevisedApril2013 CompensationSubmitDocumentationFeedbackC opyright 1979 2013, 10mV/ C outputfrom0 C to1300 C is seenin Figure5. Thetrimmingprocedureis asfollows:firstshortouttheLM329B, (equaltotheinputoffsetvoltagetimesthevol tagegain).Thenapplya 50mVinputvoltageandadjusttheGAINADJUST potuntiltheoutputvoltageis , K ,removetheshortacrosstheLM329 BandadjusttheZEROADJUST potsothattheoutputvoltageequals10mV/ C timestheambienttemperaturein ( )Coefficient( V/ C) April1979 RevisedApril2013AN-225 ICTemperatureSensorProvidesThermocoupleC old-JunctionCompensationSubmitDocumentat ionFeedbackCopyright 1979 2013, (1)(2)Allfixedresistors 1%,25ppm/ C 0 C to1300 C rangeduetothermocouplenonlinearityis under1 C for0 C to300 C butis ashighas17 C theoutputis digitized,correctionfactorscanbestoredin a ROMandaddedin V/ C whichwillresultin a 1 C errorforevery8 C V/ C maximumoffsetvoltagedriftwillreducethise rrorto1 C per40 desired,thistemperaturedriftcanbetrimmed outwithonlyonetemperaturecyclebyfollowin gtheproceduredetailedin orFigure7 V/ C , thelowlevelthermocoupleoutputis , easilyachievedbyterminatingbothleadstoid enticallysizedcopperpadsandkeepingthemaw ayfromthermalgradientscausedbycomponents thatgeneratesignificantheat.

5 (1)*R2andR3are1%,10ppm/ C.(20ppm/ C tracking.)(2) R1andR6are1%,50ppm/ April1979 RevisedApril2013 CompensationSubmitDocumentationFeedbackC opyright 1979 2013, (a)MethodsforSensingTemperatureofReferen ceJunction* (b) Thermocouple( C/ V) CError( C) CError( C)10 April1979 RevisedApril2013AN-225 ICTemperatureSensorProvidesThermocoupleC old-JunctionCompensationSubmitDocumentat ionFeedbackCopyright 1979 2013,TexasInstrumentsIncorporatedAppendi xA , 24-hourbakeat85 C is , movingairorhouseit in a box, thermocouple,thereis nouniquevalueofitsSeebeckcoefficient<5 ,onemustapproximatethethermocouplefuncti onwithastraightlineanddetermine<5 C fromtheline' graph,theerrorofthelineapproximationis notsogross,sothata ,sothata firstcut,onecanpickthesepointsatthe and theerrorbecomestoolargeateithertemperatu reextreme, foundthatminimizeserroroverthetemperatur erange,useitsslopeastheSeebeckcoefficien tvaluewhendesigninga typeS thermocoupleis shownin Table2.

6 Notethatpickingthetwointercepts(zeroerro rpoints)closetogetherresultsin lesserrorovera narrowertemperaturerange.(1)(1)A collectionofthermocoupletablesusefulfort hispurposeis foundin theOmegaTemperatureMeasurementHandbookpu blishedbyOmegaEngineering,Stamford, ThermocoupleApproximation#1 ZeroErrorat25 CApproximation#2 ZeroErrorat30 Cand60 Cand50 CTypeSLinearError(1)LinearError(1)Centig radeThermocoupleTemperatureOutput( V)Approx. V CApprox. V C0 0 17 17 16 16 5 2715 12 16 11 10 5546 9 47 8 15 8478 6 78 6 20 113110 3 110 3 25 14214200142 1 30 1730035 40 45 50 2990055 330 1 60 36536500362 3 65 398397 1 394 4 70 432429 3 425 7 <5 C = V/ C C errorfor20 C < T<5 C = V/ C C errorfor25 C < T< 70 C< 50 C(1)Erroris thedifferencebetweenlinearapproximationa ndactualthermocoupleoutputin C, April1979 RevisedApril2013 CompensationSubmitDocumentationFeedbackC opyright 1979 2013, TechniqueforTrimmingOutOffsetDrift6 AppendixB TechniqueforTrimmingOutOffsetDriftShorto utthethermocoupleinputandmeasurethecircu itoutputvoltageat25 C andat70 , asshown.

7 (1)Next,shortouttheLM329 BandadjusttheTCADJ potsothatVOUT= (20mV/ K ) 298 K at25 246mVat25 C (246timesthe25 C outputofanice-point-referencedthermocoup le).Thisprocedurecompensatesforallsource sofdrift,includingresistorTC,referencedr ift( 20ppm/ CmaximumfortheLM329B) (SNOA728)AN-222 SuperMatchedBipolarTransistorPairSetsNew StandardsforDriftandNoise(SNOA626)9 SNOA663B April1979 RevisedApril2013AN-225 ICTemperatureSensorProvidesThermocoupleC old-JunctionCompensationSubmitDocumentat ionFeedbackCopyright 1979 2013,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,enhancements,improvementsandotherchang estoitssemiconductorproductsandservicesp erJESD46,latestissue,andtodiscontinueany productorserviceperJESD48, (alsoreferredtohereinas components ) aresoldsubjecttoTI s ,in accordancewiththewarrantyin TI s ,testingofallparametersofeachcomponentis productsandapplications, ,eitherexpressorimplied,is grantedunderanypatentright,copyright,mas kworkright,orotherintellectualpropertyri ghtrelatingtoanycombination,machine,orpr ocessin licensetousesuchproductsorservicesora licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, is solelyresponsibleforcompliancewithallleg al,regulatoryandsafety-relatedrequiremen tsconcerningitsproducts,andanyuseofTIcom ponentsin itsapplications, hasallthenecessaryexpertisetocreateandim plementsafeguardswhichanticipatedangerou sconsequencesoffailures,monitorfailuresa ndtheirconsequences, , ,TI s goalis.

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