RTD Ratiometric Measurements and Filtering Using the ...

1 ApplicationReportSBAA201 ( ) convertersthatareoptimizedforthemeasurem entoftemperaturesensors,includingresista ncetemperaturedetectors(RTDs),thermocoup les, typicalRTDmeasurementapplication,theADS1 148andADS1248areconfiguredina ,makesurethattheanalog-to-digitalconvert er(ADC) a :AlthoughtheADS124824-bitdeviceis referencedthroughoutthisdocument, , (RLEADR emovedforSimplicity).. Four-WireRatiometricConfigurationwithRCF ilters(RLeadRemovedforSimplicity).. March2013 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporated ADCADS1248 Digital FilterPGAAIN0 AIN1 IEXC1 IDAC Current SourceRTDREFP0 REFN0 RBIASRLEADRLEADRLEADRLEADAVSSAGNDN oise and drift of excitation reflected at input path and reference represents the lead wire should be a precision low-drift resistor placed in close proximity to the (RTDs)

2 Workbycorrelatingthechangeoftheresistanc eofa metalsensingelementversusa , resistivesensor,suchasanRTDin dataacquisitionsystems,a constantcurrentsourceexcitationis showsa typicalconfigurationusedtomeasureanRTDin Figure1 , ,generallylimitedto10 ,theRBIAS resistorplacestheRTData voltagegreaterthantheanalognegativesuppl y(AVSS)in orderforthesensortobebiasedin Figure1, resultoftheexcitationcurrentsourcedrifti sreflectedacrossthesensorattheinputpatho fthedevice, ,if theRTDandRBIAS resistancesremainunchanged,thedigitalout putoftheADCis a ratiometricconfigurationprovidesa significantadvantage,wheretheerrorsasa ,whenperformingmeasurementsin a ratiometricconfiguration,thenoiseoftheex citationsourceattheinputsis reflectedtothereferencepathoftheADC;andi n thismanner, March2013 ADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporatedBF3@$=1 2 :41%%/; BF3@$=12 :41 +42.

3 (%+0+%%/2) 3rd-Order ADCPGAGainR1R2 VIN+_enenEMI/ RatiometricMeasurementsInmanyapplication s,inputRClow-passfiltersareemployedtoimp rovetheend-productimmunitytoradiofrequen cyinterference(RFI)andelectromagneticint erference(EMI).However,it is importantthattheinputfilterandthereferen cefilterhavematchedtimeconstantsorthecan cellationofcurrentsourcenoisecandegrade, ,theadditionoflow-passRCfiltersmayprovet obebeneficialinnoisyenvironments,whereth esensorcircuitis ,theerrorsduetotheexcitationsourcedrifta ndnoisemustbeequallyreflectedattheinputs oftheADC, ,theexcitationnoisecancels,resultingin a stable, , genericcircuittopologyfrequentlyusedin ,onedifferentialcapacitor, first-order20- ,deriveEquation1 andEquation2 tocalculatethecornerfrequencies:Differen tial-ModeCornerFrequency:(1)Common-ModeC ornerFrequency.

4 (2)3 SBAA201 March2013 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporated ADCADS1248 Digital FilterPGAAIN0 AIN1 IEXC1 IDAC Current SourceRTDREFP0 REFN0 RBIASAVSSAGNDN oise and drift errors of excitation reflected at input path and reference chosentobe10timeslargerthanthecommon-mod ecapacitors,theresultingdifferentialfilt erprovidesa cornerfrequencythatis (PGA) ,wheretheasymmetricnoiseattenuationcause dbythecommon-modecapacitormismatchis Figure3, a zero-valuetimeconstanttechniqueapproach[ 1] (RLEADR emovedforSimplicity)4 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andSBAA201 March2013 ADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporatedRTDR2R1 R3R4 RTDR2R1R3R4 Simplified Circuit to Find the CIN_CM1 Time ConstantSimplified Circuit to Find the CIN_CM2 Time ConstantRBIASRBIAS-+-+VTESTVTESTBF3@$=12 %+0_&+((:46 RTDR2R1R3R4 RBIAS-+ ,asshownin Figure4.))

5 ReplacedifferentialinputcapacitorCIN_DIF F witha RTD+ R1+ ,theRCconstantseenbythisfilteris approximatelyCIN_DIFF(RTD+ R1+ R2),resultingin anapproximatecornerfrequencyof:(3)Thesam eapproachis usedtodeterminethecornerfrequenciesofthe common-modefilters, March2013 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporatedBF3@$=1 2 %4'(_%/2:44; BF3@$=12 %4'(_%/1(43 +4$+#5 ) BF3@$=12 %4'(_&+(((43 +4$+#5 +44) BF3@$=12 %+0_%/2(42 +4$+#5) BF3@$=12 %+0_%/1(41 +46&+4$+#5) Low-PassFilterDesignConsiderationsin testsource,theresistanceseenbyCIN_CM1is R1+ RTD+ RBIAS, yieldingacornerfrequencyof:(4)Insimilarf ashion,thecornerfrequencyprovidedbyCIN_C M2is givenas:(5)TheresistanceoftheRTDsensorch angeswithtemperaturemeasurement, alsocausesa mismatchonthecornerfrequenciesoftheinput common-modefilters,buttheimpactofnoiseca ncellationcausedbythecommon-modefiltersi s ,thecornerfrequenciesforthedifferentialr eferencepathcircuitmaybecalculatedas:(6) Andthecommon-modefiltersatthereferencepa thmaybecalculatedas.)))))

6 (7)(8)Althoughit is notalwayspossibletoexactlymatchthecorner frequenciesofallthefilters,a goodcompromiseis toattempttobalancethecornerfrequenciesof theinputpathdifferentialfilterandtherefe rencepathdifferentialfilterbecausethesef iltershavea dominanteffectin theRCfilterdesignis low-noise, fewk ; however, typicallyin ,thethermalnoisecontributionoftheresisto rsis negligiblewhentheresistorvaluesarekeptbe low20k .Amongceramicsurface-mountcapacitors,COG (NPO) COG(NPO)ceramiccapacitorsprovidesthemost stableelectricalpropertiesovervoltage,fr equency, March2013 ADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporated ADCPGA = 8 V/VData Rate = 20 SPSD igital FilterPGAAIN0 AIN1 IEXC1 IDAC Current SourceVariable ResistorREFP0 REFN0 RBIAS2 k AVSSAGNDR3R4 ADS1248No filter used at the input pathCREF set to 10 F, 1 F, and F5 k 5 k IDAC = 1000 theRatiometricConfiguration3 NoisePerformanceUsingtheADS1248intheRati ometricConfigurationUsingmatchedRCfilter sattheinputpathandthereferencepathresult sin a setupusingunmatchedfilters, theratiometriccircuit.

7 Themeasurementsareperformedapplyinga low-passfilterin thereferencepathwhilenofilteris low-passfilteratthereferencewithcornerfr equenciesof130Hz,13Hz, , setto1000 A,producinga voltagereferenceof2 V acrossthe2-k configuredata datarateof20 SPSwithPGAgainof8 V/V,allowinga thiscasewassimulatedusinga to250 in ordertoproduceaninputvoltageof0 March2013 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporatedCREF = 10 F CREF =1 F CREF = F Input Referred Noise ( VRMS) Input Voltage (V) C003 NoisePerformanceUsingtheADS1248in showsexperimentalnoisemeasurementsusingt heADS1248withthedifferentreferenceinputf ilters, pattern, nofilterin theADCinputpath;therefore,thenoiseproduc edbytheexcitationsourceis , ;therefore,ratiometricnoisecancellationi s approximately350nVrmswhenthedeviceis configuredwithaPGAgainof8 10 F,wherea thereferencepath,theincreasednoiseduetot hemismatchedRCfiltersis smaller(CREF= F),theincreasingnoiseversusinputlevelis VREF.

8 Astheinputsignalincreases,uncorrelatedno isepresentoneitherVINorVREF resultsin ,thenoiseis March2013 ADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporatedBF3@$=1 2 51J((2G + 5G + 5G ) BF3@$=12 51J((46&+ + ) ADCADS1248 Digital FilterPGAAIN0 AIN1 IEXC1 IDAC Current SourceVariable ResistorREFP0 REFN0 RBIASAVSSAGND R1R2R3R45 nF51 nF5 k k 5 nF51 nF 5 nF5 k 5 k IDAC = 1000 A2 k PGA = 8 V/VData Rate = 20 , Figure8 showsa Four-WireRatiometricConfigurationwithRCF ilters(RLeadRemovedforSimplicity)Theresi stanceattheinputis sweptfrom0 to250 toproducea in , thedifferential-modefiltercornerfrequenc yattheinputpathcanbecalculatedas:(9)Thed ifferential-modeinputcornerfrequencychan gesverylittle(258 Hzto253Hz)asthesensorresistancechangesfr om0 to250 (full-scale).))

9 UsingEquation6, thedifferentialmodefiltercornerfrequency atthereferencepathcanbecalculatedas:(10) Thedifferentialmodefiltercornerfrequency atthereferencepathis March2013 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013, Input Referred Noise ( VRMS) Input Voltage (V) C001 NoisePerformanceUsingtheADS1248in Vrmswhenthedeviceis configuredwithaPGAgainof8 V/Vwitha thiscaseis ;therefore, showstheinput-referrednoisein March2013 ADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporated ADCADS1248 Digital FilterPGAAIN0 AIN1 IEXC1(or AINx)IDAC Current SourceIDAC = 500 ARTDREFP0 REFN0 RBIASAVSSAGND5 nF51 nF5 k k 5 k 5 k 5 nF51 nF5 nFIDAC Current SourceIDAC = 500 AIEXC2(or AINx)RBIAS should be a precision low drift resistor placed in close proximity to the IDACRLEAD represents the lead wire k , ,andtheexcitationcurrentsourcesareclosel ymatched,theerrorsduetothelineseriesresi stancecancel.

10 ,wherethesensorwiringis pronetonoiseinterference, ,whenaddingthefilterresistorsin theexcitationcurrentpath,theinputcommon- moderangeis toconnecttheIDAC excitationcurrentin theconfigurationasshownin sourcedfromanothersetofunusedinputchanne lsorin thecaseoftheADS1248, , Three-WireRatiometricConfigurationwithRC Filters11 SBAA201 March2013 RTDR atiometricMeasurementsandFilteringUsingt heADS1148andADS1248 FamilyofDevicesSubmitDocumentationFeedba ckCopyright 2013,TexasInstrumentsIncorporatedNoisePe rformanceUsingtheADS1248in ,makesuretoaccountforboththenoisesignals presentin theenvironment, ,thesignalproducedbytheRTDmaybetreatedas essentiallya , ,wheninitiallybiasingtheRTD, ,whenperforminga 20-bitresolutionmeasurement,afterinitial lybiasingthesensor,theusermustwaitupto14 RCfiltertimeconstantsforthemeasurementto settlewithin showstherequiredRCfiltertimeconstantstos ettleto LSBR esolutionResolution(Bits)