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BANDPASSSELECTIONFORBIOPOTENTIALAMPLIFIERS第1页FrequencyRangesofVariousBiopotentialSignals第2页CommonbiopotentialsignalsAsshowninTable2.1,commonbiopotentialsignalsspantherangedcto10kHz.Underidealconditions,abiopotentialamplifierwithwidebandresponsewouldservemostapplications.However,thepresenceofcommon-modepotentials,electrodepolarization,andotherinterferingsignalsoftenobscurethebiopotentialsignalunderinvestigation.Assuch,thefrequencyresponseofabiopotentialamplifiershouldbetunedtothespecificspectralcontentexpectedfromtheapplicationathand.第3页FrequencyRangesofVariousBiopotentialSignals第4页FrequencyRangesofVariousBiopotentialSignals第5页WIDEBANDBIOPOTENTIALAMPLIFIERThebiopotentialamplifiercircuitdescribedbytheschematicdiagramsofFigures2.2and2.3coversthecompletefrequencyrangeofcommonlyrecordedbiopotentialswithhighCMR.第6页-3dBbandpassSpectralanalysisisthemostcommonwayofdeterminingthebandwidthrequiredtoprocessphysiologicalsignals.Forafirstestimate,however,therigorsofspectralanalysiscanbeavoidedsimplybyevaluatingthedurationsofhigh-andlow-frequencycomponentsofthesignal.Koide[1996]proposedamethodforestimatingthe-3dBbandpassbasedonacceptabledistortion.第7页AstereotypicalintracellularsignalThedurationofthehighest-frequencycomponent,tHF,isestimatedfromastereotypicalsignaltobetheminimumriseorfalltimeofasignalvariation.Thedurationofthelowestfrequencycomponent,tLF,ontheotherhand,ismeasuredfromthetiltofthebaselineorofthelowest-frequencycomponentofinterest.Koideillustratedthiswithanexample.Figure2.1showsastereotypicalintracellularpotentialmeasuredfromthepacemakercellsinamammalianheartSAnode.Inthisexample,tHF=75msandtLF=610ms.UsingtheformulasofTable2.2,theamplificationsystemmusthavea3-dBbandpassof0.0026to41.3Hztoreproducethesignalwithnegligibledistortion(1%).Acceptabledistortion,usuallyconsideredtobe5%orlessforphysiologicalsignals,wouldrequireanarrower-3dBbandpass,of0.013to18.7Hz.第8页Figure2.1AstereotypicalintracellularpotentialmeasuredfromthepacemakercellsinamammalianheartSAnodehasaminimumrisetimeoftHF=75msandatiltoftLF=610ms.The-3dBbandpassneededtoreproducethissignalwith1%distortionisof0.0026to41.3Hz.第9页Approximate-3dBFrequenciesRequired第10页Widebanddc-coupledbiopotentialamplifier
Figure2.2第11页Figure2.2Thiswidebanddc-coupledbiopotentialamplifierfrontendcoversthecompletefrequencyrangeofcommonlyrecordedbiopotentials.ABurr-BrownINA110AGICIAisdc-coupledtotheelectrodesviacurrent-limitingresistorsR22andR23andIS-1-3.3DPfaultcurrentlimiters.Capacitorsanddiodesareusedtoprotecttheamplifierfromhigh-frequencycurrents,suchasthoseusedinelectrosurgeryandablationproceduresaswellasfromhigh-voltagetransientssuchasthosethatmaybeexpectedfromdefibrillationandelectrostaticdischarge.第12页第13页Figure2.3TheoutputoftheISO107isolationamplifierisfedtoIC2B,whichhasitsgainselectablethroughswitchSW3.ThecircuitbuiltaroundIC2AnullsdcoffsetsautomaticallywhenSW1isclosed.Thefeaturesofthisbiopotentialamplifiermakeitanidealchoiceforrecordingcardiacmonophasicactionpotentials(MAPs)usingelectrodesindirectcontactwiththeheart.第14页AC-COUPLEDINSTRUMENTATIONBIOPOTENTIAL
AMPLIFIERFRONTENDThecircuitofFigure2.5embodiestheclassicimplementationofamedium-impedance(10-MΩ)instrumentationbiopotentialamplifierbasedonthepopularAD521ICIAbyAnalogDevices.Thegainofthiscircuitisadjustablebetween10and1000andmaintainsaCMRofatleast110dB.第15页第16页Figure2.4Dcandverylowfrequencypotentialsarepreventedfrompropagatingbeyondthefront-endamplifierthroughatechniquecommonlyreferredtoasdcrejection.Here,IC4C,togetherwithR11andC17,areusedtooffsetIC1’sreferencetosuppressabaselinecomposedofcomponentsintherangedcto0.48Hz.第17页AC-COUPLEDINSTRUMENTATIONBIOPOTENTIAL
AMPLIFIERFRONTENDTheheartofthecircuitisIC1,themonolithicICinstrumentationamplifier.Biopotentialsareac-coupledtotheamplifier’sinputsthroughC1andC2.Althoughinstrumentationamplifiershavedifferentialinputs,biascurrentswouldchargestraycapacitancesattheamplifier’sinput.Assuch,resistorsR1andR2arerequiredtoprovideadcpathtogroundfortheamplifier’sinputbiascurrents.Theseresistorslimittheimpedanceofeachinputto10MΩreferredtoground.Thehigh-passfilter,formedbytheac-couplingcapacitorandthebiasshuntresistoroneachoftheICIA’sinputs,hasa3-dBcutofffrequencyof0.12Hz.第18页AC-COUPLEDINSTRUMENTATIONBIOPOTENTIAL
AMPLIFIERFRONTENDThegainofIC1isdeterminedbytheratiobetweenR3andR4.Usinga20-kΩmultiturnpotentiometer,andgiventhatthevalueoftherange-settingresistorR3is100kΩ,thedifferentialgainoftheamplifiercanbetrimmedbetween5and1000.TheoutputoffsetoftheamplifiercanbetrimmedthroughR5,which,atanygivengain,introducesanoutputoffsetequalandoppositetotheinputoffsetvoltagemultipliedbythegain.Thus,thetotaloutputoffsetcanbereducedtozerobyadjustingthispotentiometer.Theinstrumentationamplifierprovidesalow-impedanceoutput(0.1Ω)withapermissibleswingof10Vandcansourceorsinkupto10mA.第19页Figure2.5第20页Figure2.5Thisisaclassicmedium-impedance(10-MΩ)instrumentationbiopotentialamplifierbasedonthepopularAnalogDevicesAD521ICIA.Thegainisadjustablebetween10and1000andmaintainsaCMRofatleast110dB.The40-kHzsignalbandwidthmakesthisfrontendsuitableforrecordingEMGorECGsignalsorasageneral-purposehigh-impedanceac-coupledtransduceramplifier.第21页BOOTSTRAPPEDAC-COUPLEDBIOPOTENTIALAMPLIFIERDirectaccouplingoftheinstrumentationamplifier’sinputsbywayofRChigh-passfiltersacrosstheinputsdegradestheperformanceoftheamplifier.Thispracticeloadstheinputoftheamplifier,whichsubstantiallylowersinputimpedanceanddegradestheCMRRofthedifferentialamplifier.Althoughunity-gaininputbufferscanbeusedtopresentahighinputimpedancetothebiopotentialsource,anyimpedancemismatchintheaccouplingofthesetoaninstrumentationamplifierstagedegradestheCMRperformanceofthebiopotentialamplifier.第22页Figure2.6第23页Figure2.6Thisbootstrappeddesignyieldsanac-coupleddifferentialamplifierthatretainsallofthesuperiorperformanceinherentindccoupledinstrumentationamplifiers.AcvoltagesfromtheoutputsoftheICIA’sdifferentialinputstagearefedtotheinvertinginputsoftheirrespectiveamplifiersviaC3andC4.ThiscausestheacvoltagedropacrossR1andR4tobevirtuallyzero.AccurrentflowthroughresistorsR1andR4ispracticallyzero,whiledcbiascurrentscanflowfreelytoground.第24页PASSIVEFILTERSThesimplestfiltersarethosethatcompriseonlypassivecomponents.Thesefilterscontainsomecombinationofresistive(R),capacitive(C),andinductive(L)elements.Theinductiveand/orcapacitivecomponentsarerequiredbecausetheseelementspresentvaryingimpedancetoaccurrentsatdifferentfrequencies.第25页Figure2.7第26页Figure2.7Inthisbiopotentialamplifier,biopotentialsareamplifiedbyaBurr-BrownINA128Uinstrumentationamplifier.R5,R6,R9,R10,andC22implementalow-passfilterwithacutoffofapproximately3.6kHz.Thebiopotentialamplifier’smainlow-passfiltersareimplementedbytwocascadedRCpassivefilterswithselectablecutofffrequency.IC4buffersthesignalsbetweenthecascadedsections.ThetwoRCsectionsareidentical,thereforesettingapoleatthesamefrequency.However,theeffectofthesecondRCcanbesuppressedbydisconnectingitscapacitorthroughswitchSW2.第27页Figure2.8第28页Figure2.8Thehigh-passfiltersfortheamplifierofFigure2.7areimplementedinessentiallythesamewayasthelow-passsections.Eachhigh-passsectionhascapacitors(C50andC53)whichopposecurrentflowwithanimpedance
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