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I.IntroductionTheheartrateisakeyindicatorvaluereflectingthehealthofthebody.Simplyput,theheartratereferstothefrequencyofcardiovascularbeatswithin1minute.Thetestofheartratecanshowscientificevidenceinworksuchasdiseasediagnosis,patientcare,andathletetraining.Inrecentyears,manymedicalequipmentandfitnessequipmentdevelopedandmanufacturedbycountriesaroundtheworldhaveadoptedheartratetestpowercircuits.Thelowcostofproductdevelopmentandhighperformanceheartratetestpowercircuitshaveimportantapplicationvalues.ThearticleintroducesthiskindofheartbeatratedetectionsystembasedonAD620integratedICindetail.Usingtheexcellentlow-noisecharacteristicsofAD620integratedIC,pluseffectivefilteringandamplifyingcircuits,combinedwithmicroprocessorsolutions,ahigh-precisionheartratemonitoringsystemisobtained.Figure1AD620CatalogI.IntroductionII.AD620ChipIII.CircuitDesign3.1BlockDiagram3.2SignalExtractionCircuitBasedonAD6203.3FilterAmplifierCircuit3.4MicroprocessorCircuit3.5ExperimentalResultsandDiscussionIV.ConclusionFAQOrdering&QuantityII.AD620chipAD620isalow-cost,high-precisioninstrumentationamplifier.Itonlyneedsanexternalresistortosetthegain,andthegainrangeis1to10000.Inaddition,AD620adopts8-pinSOICandDIPpackage,thesizeissmallerthanthediscretecircuitdesign,andthepowerconsumptionislower,soitisverysuitableforbattery-poweredandportableapplications.Itscharacteristicsareasfollows:EASYTOUSEGainSetwithOneExternalResistor(GainRange1to10,000)WidePowerSupplyRange(2.3Vto18V)HigherPerformancethanThreeOpAmpIADesignsAvailablein8-LeadDIPandSOICPackagingLowPower,1.3mAmaxSupplyLOWNOISE9nV/Hz,@1kHz,InputVoltageNoise0.28Vp-pNoise(0.1Hzto10Hz)EXCELLENTDCPERFORMANCE(BGRADE)50Vmax,InputOffsetVoltage0.6V/Cmax,InputOffsetDrift1.0nAmax,InputBiasCurrent100dBminCommon-ModeRejectionRatio(G=10)EXCELLENTACSPECIFICATIONS120kHzBandwidth(G=100)15sSettlingTimeto0.01%III.CircuitDesign3.1BlockDiagramThesurfaceofthehumanskincontainshumanECG,EMG,andpowerfrequencysignals.Generally,thenoiseoftheECGsignalcontainingheartrateinformationismuchsmallerthanthatofthepowerfrequencysignal.InordertoextracttheweakECGsignal,alow-noiseoperationalamplifiermustbeusedandareasonablefilteramplifiercircuitmustbedesigned.Figure2isablockdiagramoftheheartratedetectionsystem.Thewholeheartratedetectionsystemconsistsoffourparts:thesensorheadincontactwiththehumanskinsurface,thesignalextractioncircuit,thefilteramplifiercircuitandthemicroprocessorcircuit.Thesensorheadisgenerallyametalthatiseasytoconductelectricity.Aftercontactingthesurfaceofthehumanskin,ithascomplexelectricalsignalssuchashumanECGsignals,electromyographicsignals,andpowerfrequencysignals.Weusethelow-noiseAD620operationalamplifierasthecorechipoftheheartbeatrateextractioncircuit.Inthefilteringandamplifyingcircuitpart,asimplelow-passfilteringcircuitisused.Theexperimentalresultsshowthatthisfilteringcircuitissufficienttoextracttheheartratesignal.Afterfilteringtheamplifiedsignal,anadjustablecomparatorcombinedwithatransistorcircuitisusedtoforma5voltTTLlevelsignal,andfinallyconnectedtothemicroprocessor,theheartbeatsignalisprocessedbythemicrocomputer,andtheheartbeatrateiscalculatedanddisplayed.Figure2Blockdiagramofheartratedetectionprinciple3.2SignalExtractionCircuitBasedonAD620AD620operationalamplifier,usuallyusedinhigh-precisiontestinstruments,themaximumnonlinearerrorof40ppm,themaximumvoltageoffsetof50uV,themaximumtemperaturedriftof0.6uV/℃,becauseofitslownoise,lowbiascurrent,lowpowerconsumptioncharacteristics,itItiswidelyusedinmedicalfieldssuchaselectrocardiogram(ECG)andbloodpressuremonitoring.Figure3isasignalextractioncircuitbasedonAD620,inwhichtheLEFT_ARM,RIGHT_ARM,LEGthreeleadsareconnectedtothealuminumsheet(iethesensorhead),whicharerespectivelyconnectedtotheleftandrighthandsandrightfeetofthehumanbody.OurexperimentalresearchresultsshowthattheR4gainofLEFT_ARMis1K,thecorrespondingAD620operationalamplifiergainis50.Toomuchgainwillweakenthefinalsignal-to-noiseratio,sotheR4resistancevalueshouldbesetreasonablyintheexperiment.The0.1uFcapacitancebetweentheLEFT_ARMandRIGHT_ARMleadsistoeffectivelyweakenthepowerfrequencynoise.TheLEGleadisconnectedtoAD620throughTL082A,whichprovidesthereferencepotentialofthehumanbodyforthedifferentialsignalofLEFT_ARMandRIGHT_ARM.Figure3SignalextractioncircuitbasedonAD6203.3FilterAmplifierCircuitFigure4isafilteramplifiercircuit.Threeoperationalamplifiersconstituteathree-stageamplification,eachamplifying100times.Duetocircuitloss,especiallythelossoftheisolationcapacitor,theactualsignalamplificationislessthan1milliontimes.TheratiooftheresistancevaluesofR6andR5,R9andR8,R11andR10inthecircuitdeterminesthemagnificationfactor,andtheseresistancevaluesshouldbeadjustedreasonablyinpracticalapplications.Alow-passfiltercircuitshouldbeusedwhileamplifyingthesignaltoachievetheeffectoffilteringpowerfrequencynoise.Sincethefrequencyofthepowerfrequencynoiseis50Hz,thedesignedfiltercircuithasapassbandbandwidthoflessthan50Hz,thatis,theRCtimeconstantofthecapacitorresistormustbeofthesameorderofmagnitudeasthepowerfrequencysignalperiod.R7andC10inthecircuitformalow-passfilter.Weuse1uFcapacitorisolationbetweenlevelsofamplification.Thesecapacitorswillattenuatethesignalatthesametime,andthethreeoperationalamplifiersareselectedforsignalamplification,sothesignal-to-noiseratioisimproved.Itshouldbepointedoutthatiftheisolationcapacitoristoolarge,itiseasytocausetheoutputelectricalsignaltodrift.Figure4FilteramplifiercircuitFigure5istheshapingcircuit.Theheartbeatratesignalandsignal-to-noiseratioofthe2ND_OUTleadarelargeenough(thepulserateoftheheartbeatrateis1~5V),afterthehalf-waveshapingofD1,thentheadjustablecomparator,andfinallythetransistorQ1isconvertedtothemicrocontrollerlevel.Figure5Shapingcircuit3.4MicroprocessorCircuitThefinalsignalisprocessedbyAT89C51,andtheheartrateisdisplayedbyLEDdigitaltube.ThemicroprocessorcircuitwithAT89C51asthecoreisverymature,soitsnoneedtorepeathere.3.5ExperimentalResultsandDiscussionFigure6showstheactualmeasurementresultsofthehumanheartrateusingtheabovesystem.Figure6(a)isthevoltagesignalafterthesignal2ND_OUTisshapedbyD1.ItcanbeseenfromthefigurethatthisisactuallyacompleteECGsignal.Inacycleofsignals,therearetwomoreobviouspulsesignals.Thispulsecharacteristicvariesfrompersontoperson.ItisfoundthatthereisatleastonepulsesignalthroughactualmeasurementoftheECGsignalsofdifferentpeople.Figure6(b)showstheelectricalsignalofHEART_PULSE,whichisobtainedafterthesignalofFigure6(a)passesthroughthecomparatorandthetransistorswitch.Thesignalcanbedirectlyinputtotheportofthemicroprocessor,andthemicroprocessorcalculatesandoutputstheheartrate.Figure6HeartratesignaldiagramWhendevelopingtheabove-mentionedheartratedetectionsystem,thereareseveralkeypointstopayspecialattentionto:(1)ConnectingcapacitorstoLEFT_ARMandRIGHT_ARMcangreatlyimprovethesignal-to-noiseratio;themainenergyoftheheartratesignalonLEFT_ARMandRIGHT_ARMisatafrequencyofabout1Hz,andthecapacitorisalow-passfilterthatcanfilterandsuppressnoise;Choosealargecapacitortoeliminatehigh-frequencynoise.Intheexperiment,a10uFnon-polarcapacitorisused,andtheeffectisverygood;(2)Thelow-passfiltercircuiteffectivelyweakensthepowerfrequencysignalandimprovesthesignal-to-noiseratio;itadoptsacombinationofactivefilteringandpassivefilteringtofilterwhileamplifying,whichhasabettereffectthanfilteringafteramplification;(3)Capacitorisolationattenuationandmulti-stageamplificationarebeneficialtoimprovethesignal-to-noiseratio;themainnoiseis50Hzpowerfrequencysignals.Althoughthemulti-stageamplificationandfilteringincreasethermalnoise,ithasgreatsuppressionofpowerfrequencynoise.Ofcourse,thenumberofamplificationstagescannotbeinfinite,andthebestnumberofstagesistheminimumsumofpowerfrequencynoiseandthermalnoise;(4)SmallcapacitorisolationshouldbeusedtosuppressDCsignaldrift.Thisisshownbytheexperimentalresults,andthereasonneedstobefurtherstudied.IV.ConclusionThisarticlediscussesaschemebasedonAD620chipheartratedetection,andintroducesthecircuitdesignofthesensorhead,signalextraction,filteramplificationandmicroprocessorthatmakeupthesystem.Givesthemethodtoimprovethesystemperformance.ExperimentsshowthatthesystemcanobtainbetterECGsignalsandaccurateheartrate.Theheartbeatratedetectionsysteminthisarticlehasstronganti-interferenceability,simplestructureandlowcost.FAQWhatisAD620?AD620isalow-cost,high-precisioninstrumentationamplifier.Itonlyrequiresanexternalresistortosetthegain.Thegainrangeis1to10,000.CanIchangeAD620toAD623whenmakingMCUproducts?BothAD620andAD623aresingleinstrumentationamplifiers,andthepinarrangementisexactlythesame.Themaindifferenceis:AD620mustusepositiveandnegativepowersupplies,AD623canbeapositiveandnegativepowersupplyorasinglepowersupply.IftheoriginalboardisAD620,youcanreplaceitwith623;iftheoriginalboardisAD623,youmaynotbeabletoreplaceitwith620(itdependsonwhetherthepowersupplyoftheoriginalboardcircuitisdualpowersupplyorsinglepowersupply).AfterreplacingAD620andAD623insingle-chipproducts,theprogramcanworknormallywithoutmodification.WhatisthedifferencebetweenAD620BRandAD620AN?Theirpackagesaredifferent.WhatistheoutputresistanceofAD620?Howtoadjustit?AD620isakindoflowpowerconsumptioninstrumentamplifier,itsoutputresistanceisabout10K,thisistheinherentcharacteristicofthischip,generallyitisdifficulttoadjust.Ifyouhaverequirementsforoutputresistance,youcangenerallyuseanexternalcircuittosolveit.IsAD620apositivephaseamplificationorareversephaseamplification?AD620isaninstrumentamplifier,theoutputvoltageis[(Vin+)-(Vin-)]*gain.Ifthedesiredsignalis(Vin+)-(Vin-),thegainispositive,whichisequivalenttopositiveamplification.Conversely,ifthedesiredsignalis(Vin-)-(Vin+),thegainisequivalenttonegative,whichisequivalenttoreverseamplification.Whatisaninstrumentationamplifier?Instrumentationamplifier,animprovementofthedifferentialamplifier,hasaninputbuffer,doesnotrequireinputimpedancematching,sothattheamplifierissuitableformeasurementandelectronicinstruments

STM32F205VGT7TR-100-LQFP-STMicroelectronics

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