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AnalogDialogue39-12,December(2005)1AReferenceDesignforHigh-Performance,Low-CostWeighScalesByColmSlattery[colm.slattery@analog.com]MariahNie[mariah.nie@analog.com]INTRODUCTIONThetrendinweighscalestowardshigheraccuracyandlowercosthasproducedanincreaseddemandforhigh-performanceanalogsignalprocessingatlowcost.Thescopeofthisrequirementisnotobvious;mostweighscalesoutputthefinalweightvalueataresolutionof1:3,000or1:10,000,whichiseasilymet(apparently)bya12-bitto14-bitADC(analog-to-digitalconverter).However,acloserexaminationofweighscalesshowsthatmeetingtheresolutionrequirementisnotthateasilyaccomplished;infact,theADCaccuracyneedstobecloserto20bits.Inthisarticle,wediscusssomeofthesystemspecificationsofweighscalesanddealwithconsiderationsfordesigningandbuildingaweigh-scalesystem.Themainareasconsideredarepeak-to-peak-noiseresolution,A/D-converterdynamicrange,gaindrift,andfiltering.Wecomparemeasureddatafromarealloadcelltoinputsfromastablevoltagereference,usingaweigh-scalereferencedesignasanevaluationboard.Load-CellSensorThemostcommonweigh-scaleimplementationistouseabridge-typeload-cellsensor,withvoltageoutputdirectlyproportionaltotheweightplacedonit.Atypicalload-cellbridgeisillustratedinFigure1;itisa4-resistorbridgecircuitwithatleasttwovariablearms,wheretheresistancechangewithweightappliedcreatesadifferentialvoltageatacommon-modelevelof2.5V(one-halfthesupplyvoltage).Atypicalbridgewillhaveresistorsoftheorderof300V.+6mV5V–Figure1.Basiccircuitofloadcell.Theloadcellisinherentlymonotonic.Themainparametersoftheloadcellaresensitivity,totalerror,anddrift.SensitivityAtypicalloadcell’selectricalsensitivity,definedastheratioofthefull-loadoutputtotheexcitationvoltage,is2mV/V.With2-mV/Vsensitivityand5-Vexcitation,thefull-scaleoutputvoltageis10mV.Often,inordertousethemostlinearportionoftheloadcell’sspan,onlyabouttwo-thirdsofthisrangewouldbeused.Thefullscaleoutputvoltagewouldthusbeabout6mV.Thechallengethusposedistomeasuresmallsignalchangeswithinthis6-mVfull-scalerangeinsuchawayastogetthehighestachievableperformance—notaneasytaskintheindustrialenvironmentswhereweighscaleswouldtypicallybeused.TotalErrorThetotalerroristheratiooftheoutputerrortotheratedoutput.Atypicalweighscalehasatotalerrorspecificationofabout0.02%.Itisaveryimportantspecification,becauseitlimitstheaccuracythatcouldbereachedwithanidealsignalconditioningcircuit.ItthusdeterminesthechoiceofA/D-converterresolution,aswellasthedesignoftheamplificationcircuitandfilter.(numberofbitschange,measuredusinga24-bitADC),showsatotaldriftof125LSBs,orabout7.5ppm.500–50–100051015202530TIME(HOURS)LOADCELLOUTPUTCHANGE(LSBs)Figure2.Long-termload-cellstability—24-hourplot.Weigh-ScaleSystemThemostimportantparameterstoconsiderwhendesigningaweigh-scalesystemareinternalcount,ADCdynamicrange,noise-freeresolution,updaterate,systemgain,andgain-errordrift.Thesystemmustbedesignedtoberatiometric,henceindependentofsupplyvoltage—thiswillbediscussedlater.InternalCountAsmentioned,theresolutionsoftypicalweigh-scalesystems,asseenbytheuser,rangefromacountof1:3,000atthelowendupto1:10,000forhigh-endsolutions.Forexample,aweighscalethatcanmeasureupto5kilogramswithacountof1:10,000hasaweightresolutionof0.5grams.Thisresolution,asseenontheLCDdisplay,isgenerallyreferredtoastheexternalcount.Inordertoguaranteethatthisresolutionismetaccurately,theinternalresolutionofthesystemmustbebetterbyatleastanorderofmagnitude.Infact,somestandardsdictatethattheinternalcountofthesystembeafactorof20timesbetterthanthatoftheexternalcount.Fortheexampleabove,theinternalcountwouldneedtobe1:200,000.5VIN-AMP6mVGAIN=128LPFADC24-BITHOSTSYSTEMDIGITALREFFigure3.Typicalweigh-scalesystem.ADCDynamicRangeInweigh-scaleapplicationsusingstandardhigh-resolutionA/Dconverters,theentirefull-scalerangeoftheADCisunlikelytobeused.IntheexampleofFigure1,theloadcellhasa5-Vsupplyandafull-scaleoutputof10mV.Thelinearrangeis6mV.Usingagain-of-128stageonthefrontend,theADCinputwillseeabout768mVfull-scale.Ifastandard2.5-Vreferenceisused,only30%oftheADC’sdynamicrangeisused.2AnalogDialogue39-12,December(2005)Iftheinternalcountneedstobe1:200,000accurateforthefull-scalerangeof770mV,theADCthereforeneedstobeoftheorderof33to43betterinordertomeettheperformancerequirements.Inthiscase,foracountof1:800,000,theADCwouldrequire19bitsto20bitsofaccuracy.Thepracticalchallengeposedbythesignal-processingrequirementcannowbeunderstood.GainandOffsetDriftIndustrialweigh-scalesystemstypicallyoperateovera50-degree(Celsius)temperaturerange.Designersmustconsidertheaccuracyofthesystemattemperaturesbeyondroomtemperature,sincegaindriftwithtemperaturecanbeadominantsourceoferror.Forexample,a20-bitstablesystemwitha1-ppm/8Cgain-errordriftwillhave50LSBsoferrorovera50-degreerange.Eventhoughthesystemmaybe1-LSBstableat258C,itisineffectonly50-LSBsaccurateoverthef