Analog Front-EndDesign for ECG Systems Using Delta …

1 ApplicationReportSBAA160A March2009 RevisedApril2010 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsKarthikSoundar apandian, (ECG) ( 16-bit) (SequentialSampling)..44 -Based,Low-CostECGS ignalChain(SequentialSampling)..55 -Based,Low-CostECGS ignalChain(SimultaneousSampling).. (ECG)systemanalogfront-end(AFE)devicesar etypicallydesignedwithdiscreteoff-the-sh elfcomponentsfromvarioussemiconductorven dorsorcustom-designedasapplication-speci ficintegratedcircuits(ASICs).Thecostsofa nASIC designcouldrunintomillionsofdollarsbythe timethedevicereachesfullproduction, traditionaldiscreteECGAFE includeinstrumentationamplifiers,operati onalamplifiersthatimplementactivefilters ,andanalog-to-digitalconverters(ADCs). March2009 RevisedApril2010 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsSubmitDocument ationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporatedCommon- Mode50 Hz to 60 V0 VElectrodeOffset 300 mVECGS ignalECGS ignal Hz to 150 Hz Hz to 150 ADCswitha combinationofspeeds,resolution, ,thenecessityforlow-costandlow-powerECGm achineshasincreased, ( ) ADCsfromTexasInstrumentsascomponentsofan ECGAFE system ,anddiscusseshowtooptimizethecosto fanECGAFE froma anysystemdesignis tofullyunderstandthesignalthatis showsthewidelyaccepteddetailsoftheECGsig nalasit appearsattheinputoftheECGmeasurementsyst em(seeRef1).

2 It consistsofthreecomponents:theactual(diff erential)ECGsignal,thedifferentialelectr odeoffset, anyleadconfigurationis limitedto 5 mVin ,togetherwiththeresolutionrequiredfromth eECGsignal, ,dependingonthetypeofADCusedin thesystem:eithereliminateit completelyorpreserveit. ,thehumanbodycanpickuplargeinterferences ignalsfrompowerlines,fluorescentlights, software-implemented,50- ,ontheotherhand,is generallycounteredin oneofthreeways: Increasingtheisolationofthegroundofthefr ont-endelectronicsfromtheearthgroundasmu chaspossible; Increasingthecommon-moderejectionofthesi gnalprocessingcircuitry(ontheorderof100d B); Drivingthepatientbodywithanout-of-phasec ommon-modesignal(alsocalledastherightleg drive).Oneoftheprimaryspecificationsofan ECGfront-endis is typicallyspecifiedtobelessthan30mVRMS fortheentiresystemovera bandwidthof150Hz(IEC60601-2-51,27). March2009 RevisedApril2010 SubmitDocumentationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporatedAmplitu deSignal ChainAtelectrodeAtelectrodeHigh gainwithlow-noiseamplifiersAtADCAtADCN oise-freedynamic rangeNoise-freedynamic rangeAmpnoiseADCN oiseAmplitudeSignal ChainLow gain(a)(b) , ,eitherintheanalogdomain(beforetheADC)or in thedigitaldomain(aftertheADC).

3 Typically,ECGmachineshavea ,oneoftheadvantagesofusingsoftwarepacede tectionis thataspacemakerdevicesevolve,thedetectio nthresholdscanbedynamicallychangedviasof tware, ( 16-bit)ADCsvsHigh-Resolution(24-bit)ADCs BasedontheresolutionoftheADCusedin thesignalchain, touselow-noiseamplifiersandtogaintheinpu tsignalsignificantly(approximately500),a ndthususea low-resolution(approximately16-bit)ADC, , lowergain(approximately5)anda high-resolution(approximately24-bit)ADC, asshownin , thatthesystemnoisereferredtotheinputis thesamein , ,a decisionaboutusingoneoftheseapproachessi gnificantlyaffectsthespecificationsofthe individualcomponentsofthesystem,andconse quentlytheoverallcost,asexplainedin ,whichimpliesbotha tousea dedicatedADCforeachchannel, tomultiplextheleadsignalssothata ,asit sounds, becomesquiteobvious,though,thatthespeedo ftheADCin a sequentialsamplingarchitectureshouldbesi gnificantlyhigherthanin a ,in turn, result,it is frontoftheADCalsoplaysasignificantrolein determiningtherequiredspeedoftheADCsin ,however,thatwhenusinga sequentialsamplingapproach,thesamplesfro mdifferentchannelsareskewedin March2009 RevisedApril2010 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsSubmitDocument ationFeedbackCopyright 2009 2010, Hz150 HzDC BlockingHPFA dditionalGainHigh-order,AntialiasingFilt erINAINAADC16-bit,100 kspsRLPatientProtection,LeadSelectionEle c 9 Elec 8 Elec 2 Elec 1 ECGAFE withLow-Resolution( 16-bit)

4 ,theanalogfront-endhardwareforanECGsyste mcouldbeminimizedif therewasanADCwitha veryhighresolution(approximately24bits)a nda high-speed(approximately100ksps).Fortuna tely,sucha solutionis feasibleusingthelatesthigh-speed, hardwarebyusinga 24-bit, ( 16-bit)ADCsFigure3 showsa typicalECGAFE withsequentialsamplingusinga (SequentialSampling)Thefirstblockis intendedforpatientprotectionanddefibrill ationpulseclamping, (seeRef1 formoreinformation).TheECGelectrodesareh igh-impedancesignalsources;therefore,the yarefedintotheinstrumentationamplifiers, whichhavea veryhighCMRR(greaterthan100dB)anda highinputimpedance(greaterthan10M ). BeforetheECGsignalis passedtotheADC,itmustbeamplifiedsothatth eentiredynamicrangeoftheADCis used,asillustratedin typicalADCfull-scalevoltageis ,whichimpliesa gainof500(assuminga 5-mVinputsignal).Thetotalgainis distributedbetweentheinstrumentationampl ifier(INA) addedtotheINAin sucha V,themaximumINAgaincanbein therangeof5 , ,a high-passfilter(HPF)witha removed,thesignalis shouldbenotedthattheamplifiersusedforthe segainstagesmustbeverylownoise, ,theseamplifiersmustbelowpower(forbatter y-poweredsystems).

5 (SAR)ADCsmusthavea ,a fourthorderorhigheractivelow-passfilter( LPF)is followedbya multiplexerblock(mux) canbeseenin thistypeofsystemthatthereis a significantamountofanalogsignalprocessin gthatoccursbeforethesignalis digitized, ,signalprocessingin ,thegain,bandwidth,anddctracking(thatis, baselinewandering)arerequiredtobeoptimiz ed,andarebetterservedin greatdealofflexibility,it is March2009 RevisedApril2010 SubmitDocumentationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporatedMUXDOUT x5 INAINAADC24-bit,100 kspsRLPatientProtection,LeadSelectionEle c 9 Elec 8 Elec 2 Elec 1 ADS1258150 HzSimpleRC showsthesameECGfront-endwitha (greaterthan20bits)usingoversamplingandn oise-shapingprinciples(seeRef2). Traditionally, ,withsamplingratesuptohundredsofkilohert z(seeRef3, forexample).Figure4. -Based,Low-CostECGS ignalChain(SequentialSampling)Figure4 showssucha Delta -sigmaADC(theADS1258) anindustry-leading,24-bitconverterfromTe xasInstrumentsthatoffersanimpressivecomb inationoflowlatency,highspeed, ,makingit anidealfit andFigure4, it canbeseenthatthereis a significantreductionin hardware, (includingthehigh-passfilter,dcblockingf ilter,gainstage,anda steep,activelow-passfilter) , , ,whichcouldrequireseveralamplifierstoimp lement,canbereplacedbya simple, eliminatedaswell,becausetheinherentnoise oftheADCis significantlylowerthantheprevioussolutio n(Figure2b).

6 Inthisway,thedcinformationis notlost, ,whenreferredtotheinputofthesystem,gives 1mVRMSto3mVRMS dependingontheoutputdatarate, March2009 RevisedApril2010 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsSubmitDocument ationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporatedINAADC1 50 HzSimple RC FilterINAADC24-bit, 10 ksps24-bit, 10 kspsDOUT1 DOUT8 ADS1278 PatientProtectionLeadSelectionElec 1 Elec 8 Elec 2 Elec 9x sequentialsolution,theADS1258-basedAFEha sa skewof42ms ;however,forcertainapplicationssuchasele ctroencephalography(EEG)andvectorimaging Systems ,therequiredskewis lessthan25ms. Intheseapplications,a simultaneoussampling,high-resolutionADCa pproachis a a uniquedevicein TexasInstruments' portfolio,offeringa integrateseightdedicated24-bitdelta-sigm aADCsandeightdigitaldecimationfiltersin a single,64-pin, -Based,Low-CostECGS ignalChain(SimultaneousSampling)6 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsSBAA160A March2009 RevisedApril2010 SubmitDocumentationFeedbackCopyright 2009 2010, Reference15 FmV4V5V110 kW100 kW100 kWVREFBW: 150 HzRALAD ynatech NevadaModel ECG-100 RLG = 4 Note: If input voltage exceeds supply voltage,use external Schottky diode clamps (BAT549WT1)+9 V+9 V+ V+ V-9 V-9 testconfigurationasshownin Figure6 is (1)NoiseEffectiveDataRate(mVRMS, inputDRATE[1:0]DataRate(SPS)(16-ChannelS PS)lead-referred) (1)ADS1258parameters.

7 FCLKIN= ,DLY= 010,CHOP= March2009 RevisedApril2010 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsSubmitDocument ationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporated25 k23 k21 k19 k17 k15 kOutput ( V) (sec)CH0 ( V) ,a input(safety) theADS1258muxoutputloopprovidesslightlyb etternoiseperformanceandbetteroffsetmatc hbetweenchannelscomparedtoabypassconnect ion(nobuffer).Thenoisedatawereobtainedwi thshortedinputsoftheINA121withthe100-k seriesresistorsin March2009 RevisedApril2010 SubmitDocumentationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporated+5 V+ V+ nF (x8) FmAINP1 AVDDIOVDDDVDDAINP2 AINP3 AINP4 AINP5 AINP6 AINP7 AINP8 AINN1 AINN2 AINN3 AINN4 AINN5 AINN6 AINN7 AINN8 VREFPVREFNGNDADS1278 SPII nterface1 kW100 kW100 kWAgilent33120 AINA121 RALA+9 V-9 Figure8 input(safety) + , theLow-SpeedMode(thatis,witha datarateequalto10kSPS).Thismodedissipate sonly7 (referredtotheADCinputs). ,increasingtheADCoutputnoisefrom8mV (ADCoutput-referrednoise).

8 9 SBAA160A March2009 RevisedApril2010 AnalogFront-EndDesignforECGS ystemsUsingDelta-SigmaADCsSubmitDocument ationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporated10,0008 ,0006,0004,0002,00002,0004,000--ADC Output ( V)m012345 Time (sec) (withthe100-k resistorsin place).Table2 showstheinput-referrednoiseoftheADS1278s amplingat10kSPS,andalsoshowstheinput-ref errednoisewiththeADCdatafilteredwitha thiswayisstraightforwardtoimplement,andr educesthenoiseapproximatelybythesquarero otoftheaveragingfactorwhilereducingtheda taratebythesamefactorsof4 and8 (decimationin time). (ShortedInputs)Input-ReferredNoiseLeadIn put-ReferredDataRate(mVRMS)Noise(mVPP) lessthan10mVPP. Thisamountofnoiseis showstheADS1278outputdatawitha , (1998).Medicalinstrumentation:Applicatio nanddesign(3rdedition). , ,Schreier, ,andTemes, (1997). Delta -sigmadataconverters:Theory, :16-Channel, :Octal,128-kHz, March2009 RevisedApril2010 SubmitDocumentationFeedbackCopyright 2009 2010,TexasInstrumentsIncorporatedIMPORTA NT NOTICET exas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,and other changes to its products and services at any time and to discontinue any product or service without notice.

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