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I.IntroductionTheamplificationofweaksignalshashighrequirementsandhighdifficulty.Thesignalamplificationisrelatedtotherequirementsofstabilityandaccuracyofsignalamplification.Differentialamplificationtechnologyhasthecharacteristicsofsuppressingcommonmodesignalsandonlyamplifyingdifferentialmodesignalswithhighgain,soitisappliedtosmallsignalamplificationtechnology.ThesystemdesignadoptstheAD620chipwithdifferentialamplificationfunctiontoamplifytheweakvoltagesignalofthestrainsensortoachievethehighprecisionrequirementsofthesystem.Thisarticleusesvirtualinstrumenttechnologytocollectandanalyzetheamplifiedsignal,andwritethecorrespondingdisplayinterface.Themeasurementdataisanalyzedbythesecond-orderinterpolationmethodtoverifytheaccuracyofthecircuit.AD620CatalogI.IntroductionII.SystemDesignIII.SystemHardwareCircuitDesign3.1PressureMeasurementCircuit3.2VoltageSignalAmplifierCircuit3.3ReferenceVoltageSourceCircuitandVoltageZeroingCircuit3.4Voltage-currentConversionCircuitIV.TheOverallSoftwareDesignoftheSystemV.QuantitativeTestingandResultAnalysis5.1DataProcessingMethod5.2DataProcessingResults5.3ErrorAnalysisVI.ConclusionFAQOrdering&QuantityII.SystemDesignThesystemisprovidedwithtwovoltagesof12Vand5VfromaDCstabilizedsource.Whensetting12Vpowersupply,thesystemvoltageoutputfullrangeis5V,andthesensorwithstandsstaticpressurefullrangeis19.6N.Whenmeasuringwithinthefull-scalerange,themaximumabsoluteerrorofthestaticpressuresignalis9.810-3N,andtherelativeerroris0.02%.Theoutputsignaloftheloadcellprovidestwooutputmodes:voltageoutputandcurrentoutputaftertheamplifiercircuit.III.SystemHardwareCircuitDesignTheoveralldesignprocessofthesystemisshowninFigure1.ThesystemhardwarecircuitismainlycomposedofLC7012loadcell,AD620instrumentationamplifier,referencevoltagesource,voltagezeroingcircuit,signalfilteringandshapingcircuitandvoltage-currentconversioncircuit.Figure1Systemhardwarecircuitoveralldesignprocess3.1PressureMeasurementCircuitPressuremeasurementadoptsLC7012loadcell,withfullbridgemeasurementcircuit.LC7012loadcellhasthefollowingtwocharacteristicswhensubjectedtopressure:(1)Underthesamepressure,thestrainofthesensorstraingaugeandtheoutputvoltageofthebridgeareconstantandhavenothingtodowiththeprecisepositionofthepressureactingontheloadendofthesensor.(2)Theoutputvoltageandpressureofthefullbridgecircuitcomposedofstraingaugesarebasicallylinear.The4piecesofresistancestraingaugesintheLC7012loadcellareattachedtothestrainzoneofthedouble-holebeam.Whenthereisstaticpressure,thedouble-holebeamproducesquadrilateraldeformationundertheactionofthepressureandthesupportingforceofthesystemchassisonthedouble-holebeam.Thefourstraingaugesareconnectedtoafullbridgecircuitinafullbridgemode.Undertheexcitationofthebridgevoltage,differentweakvoltagesignalsareoutputwithdifferentpressures,andtheamplifiercircuitamplifiestheweakvoltagesignalssentbythebridge.Thefull-bridgeequal-armbridgehassimplestructure,strongsymmetry,highsensitivity,andgoodconsistencyoftheparametersofeacharm.Theeffectsofvariousinterferencescancanceleachother,forexample,itcansuppresstheeffectsoftemperaturechangesandsuppresstheinterferenceoflateralforces.Itiseasiertosolvetheproblemofcompensationoftheloadcell.Thefull-bridgemeasurementcircuitenablestheoutputoftheweakvoltagesignaltoeliminateerrorscausedbythecircuititselfasmuchaspossible,andprovidestheinitialguaranteefortheoverallaccuracyofthesystem.3.2VoltageSignalAmplifierCircuitInordertoimprovetheamplificationaccuracyoftheweakvoltagesignaloutputbythebridge,thesignalamplifyingcircuitselectstheAD620chipproducedbyADIasthecoreelement,anddesignsaspecialadjustablereferencevoltagesourceforittomeetthereferencevoltagerequirementsofdifferentvoltagesources.Andtheneedtoaccuratelyamplifyweaksignals.Figure2AD620PinoutAD620isalow-cost,high-precisioninstrumentationamplifier.Itonlyneedsanexternalresistortosetthegain.Thegainrangeis1to10000dB.AndAD620powerconsumptionislow,themaximumoperatingcurrentis1.3mA.AD620hasthecharacteristicsofhighprecision(maximumlinearity4010-6),lowoffsetvoltage(maximum50V)andlowoffsetdrift(maximum0.6V/℃),makingitanidealchoiceforprecisiondataacquisitionsystemssuchassensorinterfaces.Figure2showsitspinarrangement.AD620monolithicstructureandlasercrystaladjustmentallowcircuitcomponentstobecloselymatchedandtracked,thusensuringtheinherenthighperformanceofthecircuit.AD620isathree-op-ampintegratedinstrumentationamplifierstructure,inordertoprotectthehighprecisionofgaincontrol,theinputtransistorprovidesdifferentialbipolarinput,andusesprocesstoobtainlowerinputbiascurrent,throughthefeedbackoftheinputstageinternalop-amp,Keepthecollectorcurrentoftheinputtransistorconstant,andaddtheinputvoltagetotheexternalgaincontrolresistorRG.AD620internalgainresistanceisadjustedtoanabsolutevalueof24.7k,soanexternalresistancecanbeusedtoachievepreciseprogrammingofthegain.ThegainformulaisThevoltagesignalamplifiedbytheAD620canpassthroughafilteringandshapingcircuitandbedisplayedindigitalformwithadigitaltubethroughtheanalog-to-digitalconvertermodule.Inordertofullyutilizeanddemonstratethefunctionsofvirtualinstruments,thesystemusesLabVIEWtodesignthecorrespondingsignalacquisitionandprocessingprogramanddisplayinterface.3.3ReferenceVoltageSourceCircuitandVoltageZeroingCircuitThereferencevoltagesourcecircuitismainlycomposedofaZenerdiodeLM285,alow-powerdualoperationalamplifierchipLM258,avariableresistorandanumberoffixedresistanceresistors,asshowninthelowerleftpartofFigure3.ThisreferencevoltagesourcecircuitcanprovideAD620with1.25Vor2.5Vaccuratereferencevoltage.Figure3VoltagesignalamplifiercircuitThevoltagestabilizingdiodeLM285providestheprimarystablevoltage,butthetemperaturedriftofthediodeislarge,andthevoltagestabilizationvalueofdifferentdiodesinthesamebatchisnotthesame,sothecorrespondingauxiliaryvoltagestabilizingcircuitmustbedesignedforit.TheoperationalamplifierLM258U1AamplifiesthevoltagefromtheZenerdiodeandfeedsbacktheoutputvoltagethroughthefeedbackresistorR2,makingtheoutputvoltagemorestable.ResistorR5andpotentiometerW1dividetheoutputvoltageoftheZenerdiode.PotentiometerW1hastwofunctions:(1)AdjustingW1canmakethevoltagefollowercomposedofoperationalamplifierLM258U1Bhavedifferentoutputvoltages,andthenprovidedifferentstablereferencevoltagestoAD620.(2)ThepotentiometerW1alsoplaysaroleofzeroadjustmentontheamplifyingcircuitcomposedofAD620.Thevoltagefollowerisusedbecausethevoltagefollowercanincreasetheinputimpedanceandreducetheoutputimpedance,andtherequirementofthepowersupplyisthatthecircuithasasmalleroutputresistance.AD620itselfhasaninternalzeroadjustmentfunction,butaccordingtoactualmeasurement,itisfoundthatwhenthedifferentialinputiszero,theoutputisnotzero,butaboutafewtenthsofmV.Therefore,inordertoimprovetheaccuracyoftheoutput,itisnecessarytoperformtheAD620Externalzeroadjustment,byprovidingdifferentreferencevoltagestotheAD620referencevoltagepins,theoutputvoltageoftheinstrumentationamplifierAD620canbezerowhenthedifferentialinputiszero.ThecircuitjustadjustsW1tomaketheoutputterminalofthevoltagefollowerhavedifferentvoltageoutput,adjuststhereferencevoltageofAD620,thusplaystheroleofzeroadjustmenttoAD620.TheinstabilityofthereferencevoltagewilldirectlyaffectthestabilityoftheamplifiercircuitcomposedofAD620,andleadtoinaccuracyofthefinaloutputresult.Therefore,thesystemdoesnotdirectlyusetherelativelystable-12Vor-5VprovidedbytheDCstabilizedsourceasthereferencevoltage.3.4Voltage-currentConversionCircuitThevoltage-currentconversioncircuitenablesthesystemtooutputintheformofcurrent.TheAD620iscombinedwithanAD705operationalamplifierandtworesistors(asshowninFigure4)toformaquietcurrentsource.AD705providesabufferforthereferencepin.Ensuregoodcommonmoderejection(CMR)performance.TheoutputvoltageofAD620appearsontheresistanceRL,thelatterconvertsitintoelectriccurrentoutput.Figure4Schematicdiagramofvoltage-currentconversioncircuitAD705isalow-power,bipolaroperationalamplifierwithabipolarfieldeffecttransistorinputstage.Therefore,ithasthecharacteristicsofhighinputimpedance,lowinputoffsetvoltage,smallinputbiascurrent,andsmallinputoffsetvoltagedrift.TheinputbiascurrenthasreachedthepAlevel.Itnotonlyhasmanyadvantagesofbipolarfieldeffecttransistorsandbipolaroperationalamplifiers,butalsoovercomesthedefectoflargebiascurrentdriftinthefulltemperaturerange.Inthefulltemperaturerange,thetypicalvalueofthebiascurrentofAD705onlyincreasesby5times,andthebiascurrentofthegeneralbipolarfieldeffecttransistoroperationalamplifierincreasesby1,000times.ComparedwithOP07,thetemperaturedriftvalueis1/2ofOP07,themaximuminputbiascurrentisonly1/5ofOP07,andtheinputoffsetvoltageisonly1/20ofOP07.Becauseitisabipolarfieldeffecttransistorinputpole,thesignalsourceimpedanceismuchhigherthanOP07,whileitsDCaccuracyremainsunchanged.IV.TheOverallSoftwareDesignoftheSystemThesystemsoftwareiswritteninLabVIEW.LabVIEWisagraphicalprogramminglanguage,whichiswidelyusedinvariousfieldsasastandardfordataacquisitionandinstrumentcontrolsoftware.LabVIEWisapowerfulandflexiblesoftware.Useittoeasilybuildyourownvirtualinstrument.Inthecaseofonepieceofhardware,differentfunctionsofdifferentinstrumentscanberealizedbychangingthesoftwareprogramming,whichisconvenientandfast.Combinedwiththenewdevelopmentdirectionofthecurrenttestingfieldinstruments,thefinaloutputanalogvoltagesignaliscollectedbyAdvantechsUSB4716universaldataacquisitionmoduleandtransmittedtothecomputer.UseNIvirtualinstrument(LabVIEW)todesignvoltagesignalacquisitioncontrolprogramandvoltagedatareal-timedisplayinterface.UseLabVIEWsoftwareplatformtoanalyzeandprocessthedigitalvoltagesignalfromUSB4716.ThepartprogramofLabVIEWvoltagesignalacquisitioncontrolanddisplayisshowninFigure5.Figure5VoltagesignalacquisitionprogramV.QuantitativeTestingandResultAnalysis5.1DataProcessingMethodSecond-orderinterpolation(parabolicinterpolation):select(x0,y0),(x1,y1),(x2,y2)correspondinginterpolationequationsfromasetofdata.5.2DataProcessingResultsInordertoobtainanaccuratecorrespondencebetweenpressureandvoltageandfacilitatesubsequentanalysisofabsoluteandrelativeerrors,theexperimentusesstaticmeasurementmethodstomeasureaseriesofstaticpressurevalues,andquantitativelyanalyzetheexperimentalresultstodeterminetheaccuracyofthecircuit.Commonlyusedwaveformtimedomainandfrequencydomainanalysismethods.Table1Brightness/ContrastComparisonPressure/N0...2.94...8.829.8...13.72...19.6219.6Voltage/V0...0.75...2.2472.498...3.498...4.755.001Measure20staticpressurevaluesfromsmalltolargewithinthefullscalerange,andmakethepressureincrement△thesame.Let△=0.98N,andusethesecond-orderinterpolationmethodtoanalyzetherelationshipbetweenvoltageandpressure.SelectthreerepresentativepointsfromTable1:(x0,y0)=(0,0);(x1,y1)=(2.498V,9.8N);(x2,y2):(5.001V,19.6N).Bringinsecond-orderinterpolationTherelationshipcurvebetweenthepressureonthesensorandthesystemoutputvoltageisY=(-1.56810-3)x2+3.927x(3)5.3ErrorAnalysisTheabsoluteerrorreflectsthedeviationofthemeasuredvaluefromthetruevalue,thatis,theabsolutevalueofthedifferencebetweenthemeasuredvalueandthetruevalue.Theabsoluteerrorcanbedefinedas:=|X-L|(4)Intheformula,istheabsoluteerror;Xisthemeasuredvalue;Listhetruevalue.Relativeerroristheratioofabsoluteerrortothemeasuredvalueortheaveragevalueofmultiplemeasurements,andtheresultisusuallyexpressedasapercentage,soitisalsocalledpercentageerror.Absoluteerrorcanindicatethereliabilityofameasurementresult,whilerelativeerrorcancomparethereliabilityofdifferentmeasurementresults.Whenmeasuringwiththesametool,thelargerthemeasuredvalue,thesmallertherelativeerrorofthemeasurementresult.TheabsoluteerrorandrelativeerrorofthestraingaugepressuresensortestsystemareshowninFigure6andFigure7.Thetwofiguresrespectivelyshowtheabsoluteerrorandrelativeerrorcurvesoftwootherdataprocessingmethods:linearinterpolationandaverageselectionmethod.ItcanbeseenfromFigure6andFigure7thatthecalculationaccuracyofthesecond-orderinterpolationmethodishigherthantheothertwomethods,whichalsoprovesthatthechoiceofthedataprocessingmethodiscorrect.Figure6AbsoluteerrorcurveFigure7RelativeerrorcurveVI.ConclusionKnownfromtherelativeerrorandabsoluteerrorgraphthat,themeasurementresulterrorofthecircuitintherangeof0~4.9Nisrelativelylarge,butitstillmeetsthesystemdesignrequirements.Afteranalyzingthesensorandtheexperimentalmeasurementcircuit,itisbelievedthatthereasonforthelargererrorcomesfromtherigidityofthecantileverbeammaterialofthesensorandtheflexibleinfluenceoftheviscousmaterialthatfixesthestraingauge.Becausetheaccuracyoftheweakvoltagesignaloutputbythebridgeisaffected,theerrorisalsoamplifiedafterpassingthroughtheamplifyingcircuit,resultinginalargererrorintheexperimentalresultwhenthemeasuredvalueissmall.Insummary,thepressuresignalamplificationsystemsatisfiesthedesignrequirementsofabsolutefull-scaleerror9.810-3Nandrelativeerror.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

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