Thermocouple, Cold-Junction Compensation …

1 ApplicationReportSLOA204 August2014 Thermocouple, cold -JunctionCompensation AnalogApproachShridharAtmaramMoreABSTRAC TT emperatureis the mostcommonlymeasuredphysicalparameterin a varietyof systems,includingautomotive,industrial,a nd varioustemperaturetransducersavailableto addressthe needof widelypopularbecausetheyare inexpensive,havea widerange,are smallin size,and do not thermocoupletemperature-sensingrequireme nts,particularlycoldjunctioncompensation and how to achievethe of of Tables1 SLOA204 August2014 Thermocouple, cold -JunctionCompensation AnalogApproachSubmitDocumentationFeedbac kCopyright 2014,TexasInstrumentsIncorporatedIntrodu ctionto ThermocouplesThermocouplesare a populartype of relativelylow price,widetemperaturerange,lack of requiredexcitation,long-termstability,an d proficiencywith contactmeasurementsmakethesedevicesvery commonin a widerangeof accuracywith a thermocouplecan be moredifficultthana resistancetemperaturedetector(RTD),the low cost and versatilityof a thermocoupleoftenmakeup for this difficultyin ,in contrastwith thermistorsand RTDs,the use of thermocouplesoftensimplifiesapplicationc ircuitrybecausethey requireno , thesedevicesgeneratevoltagewithoutany , however,requirea stablevoltagereferenceand someformof ice-pointor thermocoupleis a cableof two wiresmadefromtwo dissimilarconductors(usuallyalloys)that aresolderedor weldedtogetherat one end,as shownin Figure1.

2 The compositionof the conductorsusedvarieswidelyand dependson the requiredtemperaturerange,accuracy,life span,and ,all thermocoupletypesoperatebasedon the samefundamentaltheory:thethermoelectrico r conductorexperiencesa temperaturegradientfromoneend of the conductorto the other,a voltagepotentialarisesbecausefreeelectro nswithinthe conductordiffuseat differentrates,dependingon higherenergyon the hot side of the conductordiffusemorerapidlythanthe lowerenergyelectronson the net effectis that a buildupof chargeoccursat one end of the conductorand createsa voltagepotentialfromthe hot and cold effectis shownin ThermocoupleJunctionDiagramFigure2. SeebeckEffectDiagramDifferenttypesof metalsexhibitthis effectat varyinglevelsof differenttypesof metalsare pairedtogetherand joinedat a certainpoint(junctionA in Figure1), the differencesin voltageon theend oppositeof the short(junctionsB and C) are proportionalto the temperaturegradientformedfromeitherend of the pair of implicationof this effectis that thermocouplesdo not actuallymeasurean absolutetemperature.

3 They only measurethe temperaturedifferencebetweentwo points,commonlyknownas the hot and cold ,in orderto determinethe temperatureat eitherend of a Thermocouple, the exacttemperatureof the oppositeend mustbe trademarksare the propertyof , cold -JunctionCompensation AnalogApproachSLOA204 August2014 SubmitDocumentationFeedbackCopyright 2014, a classicaldesign,one end of a thermocoupleis kept in an ice bath(junctionsB and C in Figure1) inorderto establisha reality,for mostapplications,providinga true ice pointreferenceis not ,the temperatureof junctionsB and C of the thermocouplearecontinuouslymonitoredand usedas a pointof referenceto calculatethe temperatureat junctionA at theotherend of the knownas the cold junctionsor ice pointfor historicalreasons,althoughthey do not needto be kept cold or referredto asjunctionsbecausethey connectto someformof terminalblockthat transitionsfromthe thermocouplealloysinto the tracesusedon the printedcircuitboard(PCB),whichis transitionbackto copperis whatcreatesthe cold junctionsB and C.

4 Becauseof the law of intermediatemetals,junctionsB and C can be treatedas a singlereferencejunction,providedthat they are held at the sametemperatureor temperatureof the referencejunctionis known,the absolutetemperatureat junctionA can be temperatureat junctionsB and C and thenusingthat temperatureto calculatea secondtemperatureat junctionA is knownas manyapplications,the temperatureof junctionsB and C are measuredusinga diode,thermistor, with any formof cold -junctioncompensation,two conditionsmustbe met to achieveaccuratethermocouplemeasurements: 1. JunctionsB and C mustremainisothermalor be held at the conditioncan beachievedby keepingjunctionsB and C in very closeproximityto eachotherand awayfromanysourcesof heatthat may existon a ,isothermalblocksare usedto keepthe junctionsat the largemassof metaloffersa very goodformof ,maximizingthe copperfill aroundthe junctionsmay be creatinganislandof metalfill on boththe top and bottomlayers,joinedwith periodicallyplacedvias,a simpleisothermalblockcan be this isothermalblockis not affectedby parasiticheatsourcesfromotherareasin the circuit,suchas The isothermaltemperatureof junctionsB and C mustbe closeratemperaturesensor(suchas a diode,RTD,or thermistor)

5 Can be placedto the isothermalblock, currentscan also reducethe accuracyof the performance,TI recommendsensuringthat the cold junctionbe kept withinan enclosureand that air currentsbe kept to a minimumnearthe cold applicationswhereair currentsareunavoidable,a usefulalternativemay be to use someformof shieldingor othermechanicalmethodtocoverthe cold junctionmeasurementunit and connectorblockto protectthe cold importantnoteto rememberis that the orientationof the PCBcan affectthe accuracyofthe thereare heat-generatingelementsphysicallybelowth e coldjunction,for example,inaccuraciescan thermocoupletemperaturemeasurementsinvol vetwo importantfactors:measurementof the thermalgradientfromthe cold junctionavailablefromthe thermocouplebasedon the Seebeckeffectand theactualcold thermocoupleoutputcoefficientis a few V/ C.

6 For a highaccuracysystemthe processingblocksare required,whichare able to havevery high resolutiontoresolveeffectivelyin otheroptionis to gain the outputthrougha low-noiseand optionis preferredbecauseof the systemcomplexityreductionfor gainof the amplifiercan be adjustedto the requiredslopeand spanof the very importantbecausethe erroris directlyaddedtothe temperaturegradientand cannotbe beachievedusinga localdiode,RTD,thermistor,or not havea linearoutputand are linearfor a very popularbecauseof theirlinearoutput,relativelylow cost,and high device-basedtemperaturesensorsusuallyhav eslopein the rangeof 5 mV/ C or 10 mV/ measurementsystemwith an analog-to-digitalconverter(ADC)and controllercan measuretheindividualsignals(thatis, the thermocoupleamplifiedsignaland cold junctionsignal).

7 The controllercanthentake careof differentslopesof the thermocouplesignaland cold junctionsignalin the softwaretoarriveat the cold -junctioncompensatedtemperaturegradi entand,thus in turn,the hot August2014 Thermocouple, cold -JunctionCompensation AnalogApproachSubmitDocumentationFeedbac kCopyright 2014, systemsometimesmay needto performa cold junctioncompensationin the analogdomaininsteadof the digitaldomainbecauseof varioussystemrestrictions(suchas the ADCor ADCchannelsnot beingavailable,SW overheads,systemcomplexitycost,and so forth).Figure3 helpsachievethe requiredoutputvoltagefor a helpsdesignthe variouscomponentvaluesaccordingto the outputrange,temperaturerange,type of Thermocouple, and so discussesthe designcalculationsin can be updatedwithcalculatedvaluesto verifythe designis a two-stageimplementation,whereeachstageis implementedusingan instrumentationamplifier(INA)that has very low offsetat high gains,low offsetdrift,and low exampleof suchan INA is the INA333has a supplyvoltagerangeof up to 5 V.

8 Forhigheroutputvoltageswings,choosean INA with cold -junctiontemperaturemeasurement,an IC-basedanalogoutputdevice(suchas the LM35) devicehas linearoutputwith a 10-mV/ C TINA-TIsimulationdesignin Figure3 implementsthe cold -junctioncompensationmethoddiscussed in this TINA simulationdesignis availableat is replacedby the thermocouplechosenin the final is replacedby a cold -junctionmeasurementdevice,suchas the and V6 functionas positiveand negativereferencesfor TINA-TISimulation4 Thermocouple, cold -JunctionCompensation AnalogApproachSLOA204 August2014 SubmitDocumentationFeedbackCopyright 2014, designcalculationslistedin this sectioncalculatethe passivecomponentvaluesusedin the excelsheetis also madeavailableto performthe excelsheetis XLS utilityincludesthe J-typethermocouplegradienttableand the calculationsfor the the type of thermocoupleusedin the end application,the tablemustbe Typeof thermocouple = J ThermocoupleSeebeckcoefficient= V/ C Hot-junctiontemperaturerange= 100 C to 400 C cold -junctiontemperature= 25 C cold -junctiontemperaturegradient= 10 mV/ C Outputvoltagerange= 0 V to 2.

9 Choosea suitableslopeV/C for the stage1 valueless thanthe mV/ C was chosenin this hot-junctiontemperature the , 125 C to 375 hot-junctiongradient the , to cold -junctiontemperature cold -junctiontemperaturegradient= 250 cold -junctiontemperaturegradient/ stage1 outputslopechosen= total resistanceof R1 + R3 as a startingpointfor the CJCattenuation= 10000 .R1 = (R1 + R3) / CJCattenuation= 1000 .R3 = total resistance R1 = 9000 .Stage1 gain = stage1 outputslope/ Seebeckcoefficient= = 100 k / (stage1 gain 1) = 5504 .This formulais applicablefor the you chooseanotherINA,use the formulaspecifiedinthat INA output= (thermocoupleoutputvoltage stage1 gain)+ (CJCoutputvoltage/ CJCoutputattenuation).Stage1 output= 100 mV to 400 gain = (outputvoltagerange)/ (stage1 outputrange)= = 100 k / (stage2 gain 1) = 33333.

10 This formulais applicablefor the you chooseanotherINA,use the formulaspecifiedinthat invertinginput= stage1 minimumoutput= 100 a positiveoffsetis needed,choosea positiveVref higherthanthe a negativeoffsetisneeded,choosea August2014 Thermocouple, cold -JunctionCompensation AnalogApproachSubmitDocumentationFeedbac kCopyright 2014, total resistanceof R5 + R6 or R8 + R9 as a startingpointfor the Vref attenuation= 10000 .R6 = (R5 + R6) invertingvoltage/ Vref = 400 .R5 = total resistance R6 = 9600 .The designcreatedwith thesevalueswas simulatedand the simulationresultsare as shownin SimulationDesign4 ConclusionFigure3 can be usedfor thermocouplecold-junctioncompensationwhi chcan be modifiedfor a giventemperaturerangeof measurementand a givenoutputvoltagerangeexpectedby followingthe simpledesignequationsprovidedin Section3.