Biopotential AmplifiersBiopotential Amplifiers

ECGAmplifier

Basic RequirementsBasicRequirements

Essentialfunctionofabiopotential amplifieristotakeaweakelectricsignalsofbiologicaloriginandincreaseitsamplifier

Theymusthavehighinputimpedancesothattheyprovideminimalloadingtoavoiddistortionofthesignal.Typicalinputimpedancesare1M.

Inputcircuitmustprovideprotection.Nocurrentsmustappearattheinputterminals.

Outputcircuitisprimarilyusedtodrivetheamplifierloadoutputimpedanceshouldbelow.p p

Biopotential amplifiersmustbedesignedtobeoptimalinaparticularfrequencyrangeasneededbythesignaltoobtainoptimalsignaltonoiseratios.p g

ECGRecordingSystem

Thefirststageisatransducer(AgCl electrode),whichi l i l l h l i i hconvertECGintoelectricalvoltage. Thevoltageisinthe

rangeof1mV~5mV

The second stage is an instrumentation amplifier, whichThesecondstageisaninstrumentationamplifier,whichhasaveryhighCMRR(90dB)andhighgain(1000)

Optocouplertoisolatetheinputandoutputofamplifierbyconvertingtheelectricalsignaltolightandthenback

Bandpass filterof0.04Hzto150Hzfilter.Normallyimplemented by cascading a lowpass filter and a highimplementedbycascadingalow passfilterandahighpassfilter.

CardiacVector Heartgeneratesanelectricalsignal Electricalactivityoftheheartcanbemodeledasan

electricdipolelocatedinaconductingmediumwhereadipoleconsistsofpointsofequalpositiveandnegativechargeseparatedfromoneanotherandisdenotedbythedipolemoment

Thedipolemomentisavectorfromnegativechargetopositivechargehavingthemagnitudeproportionaltotheseparationofthesecharges.

Thisdipolemomentiscalledthecardiacvector,representedbyM

Itsmagnitudeanddirectionvaryduringthecardiaccycleasthedipolefieldvariesitself.

Thecardiacvectorindicatesthedirectionofthedepolarizationintime.

WewanttocapturethecardiacvectorMbylookingt t t

Va1=M.a1=|M|cosatvectorcomponents.

We can do that bya2

MWecandothatbyconnectingleadsonthesurfaceofthebodytod b l h

a1 detectbiopotentials,thenthevoltagedifferenceintroduced in the lead is the

Figure 6 2 Relationships between the two lead

a1+

introducedintheleadistheprojectionofthecardiacvector

Figure6.2Relationshipsbetweenthetwoleadvectorsa1 anda2 andthecardiacvectorM.ThecomponentofM inthedirectionofa1 isgivenbythedotproductofthesetwovectorsanddenoted

Aleadisdefinedasaconnection between 2

onthefigurebyval.Leadvectora2 isperpendiculartothecardiacvector,sonovoltagecomponentis

seeninthislead.

connectionbetween2electrodesplacedonthebody

ExampleofLeads Eindhovenstriangle

Connectionbetween2electrodesTh i l d Theprimaryleadsare LeadI:LAtoRALead II: LL to RA LeadII:LLtoRA

LeadIII:LLtoLA RL for groundRLforground

ForaleadIIsystemwhichisverycommon,LLandRAarey ,fedtotheinputsoftheinstrumentationdiff ti l lifi I+III=IIdifferentialamplifier I+III=II

ConceptofWilsonsCentralTerminal Wilsonetal.suggestedtheuse

ofthecentralterminal asareference for measuring thereferenceformeasuringtheelectrodepotentials

Thisreferencewasformedbyconnectinga5kWresistorfromthelimbelectrodetothecommon pointcommonpoint.

Wilsonsterminalisnotground buttheaverageofthelimbpotentialswiththetotalcurrentatthispointtobezero

There are other leadThereareotherleadconfigurationscalledAugmentedLeads

OtherLeads AugmentedForsignalaugmentation Disconnecttheunipolarelectrodeyouaremeasuringfromthewilsonsy gterminalandthenmeasure

ChestLeads

V1V6ChestleadsV3V4bestforseptal defects

ThemostcommonlyusedclinicalECGsystem,the12leadECGsystem,consistsofthe following 12 leads whichthefollowing12leads,whichare:

I,II,III

aVR,aVL,aVF

V1,V2,V3,V4,V5,V6

ECGWaveECGNominalData

wave Lead I Lead II Lead IIIwave LeadI LeadII LeadIIIP 0.015to0.12 0.000to0.19 0.073to0.13Q 0.0to0.16 0.0to0.18 0.0to0.28R 0 02 t 1 13 0 18 t 1 68 0 03 t 1 31R 0.02to1.13 0.18to1.68 0.03to1.31S 0.0to0.36 0.0to0.49 0.0to0.55T 0.06to0.42 0.06to0.55 0.06to0.30

DesignofanECGcircuit

Rightlegelectrode

Drivenrightlegcircuit

Sensingelectrodes

Leadfaildetect ADC Memory

Amplifierprotectioncircuit

Leadselector Preamplifier

Auto Baseline Isolated

Isolationcircuit

Driveramplifier

Recorderprinter

Autocalibration

Baselinerestoration powersupply

Parallelcircuitsforsimultaneousrecordingsfromdifferentleads

Controlprogram

MicrocomputerOperatordisplay

Ke board

Figure6.7Blockdiagramofanelectrocardiograph

ECGanalysisprogram

Keyboard

MainComponentsoftheECGCircuitPreamplifier

InitialAmplification

NeedsveryhighI/Pimpedance

HighCMRR

Typically,itisa3opamp

Isolationcircuitry

BlockstheECGfrompowerlinefrequencies

differentialamplifierwithagaincontrolswitch

eque c es

Drivenrightlegcircuit

Provides a reference point on theDriverAmplifier

AmplificationoftheECGsignalforappropriate recording

Providesareferencepointonthebodyinsteadofground

appropriaterecording

PreamplifierDesignDesignSpecificationsg p

AmplificationRange:202000FrequencyRange(0.05150Hz)HighInputImpedance2.5M

Hi h CMRR (E 60dB)HighCMRR(Ex60dB)

Step1:SingleOpampDifferentialAmplifier

Forthisdifferentialamplifier

For a CMRR>60dB or CMRR>1000

VOUT =(V1 V2)R4/R3

ForaCMRR>60dBorCMRR>1000Gd/Gc>1000Gd isgovernedbyR4/R3ifwechooseR4=47K and R3=10K Gd=4 7 andR4 47KandR3 10K,Gd 4.7andGc=0.0047whichisgoodCommonModerejection.WecanreplaceR4inthiscircuitbyapotentiometertoy padjusttoincreasecommonmoderejection.

PreamplifierDesignCont. Step2:Considerthe2opampstageanddesignitforhighgain

VOUT = (V1 V2)(1+2R2/R1) VOUTGain= 1+2R2/R1 If we choose R2=22K and R1=10K thenIf we choose R2 22K and R1 10K, then gain=(1+(2*22)/10))=5.4

PreamplifierDesignCont. Step3:Cascadethe2opampstagewiththedifferential amplifierdifferentialamplifier

TotalGainoftheinstrumentationamplifier=4.7*5.4~25

VOUT = (V1 V2)(1 + 2R2/R1)(R4/R3)

PreamplifierwithFilteringSTEP5

LowPassf=1/(2*pi*RC)~106HzTruncatesTruncatesfrequencies>106Hz

NoninvertingamplifierGain=(1+150K/4.7K)~32

STEP6

( / )TotalGain=25*32=800

HighPass=RC=3.3sf=1/(2*pi*RC)~0.05Hz

STEP4

Passesfrequencies>0.05Hz

Someadditionaldesignconsiderations

Highgainstagesearlyinthesignalpath.However,theHighPassFilterstageshouldbeplacedimmediatelyafterthed ff l l f h ff hdifferentialamplifiertochopofftheDCcomponentofitsoutput.Otherwise,thisDCcomponentwillbeamplifiedbythegainstageandmaysaturatetheg g yfollowingopamps

ItsgainisdeterminedbytheresistorRg.

2nd orderfilterSalleyKeyhighpassfilter

Power line 120 V

InterferencefromElectricDevices Powerlineinterference

ProblemswithECG.Powerline 120V

C3C1C2

Thereiselectricfieldcouplingbetweenthepowerlineandtheleadwires and/or ECG amplifier. This

interference

A

B

Z1

Z2

Id1

Id2

wiresand/orECGamplifier.Thiscouplingismodeledasacapacitor.Itcausesacurrenttoflowfromthepowerlinethroughtheskinelectrode

Electrocardiograph

G

impedancethroughthebodytoground.Bodyimpedanceislow~500.HencethevoltageVA VB =

ZG Id1+Id2

Id1*Z1Id2*Z2.Iftheelectrodesareplacedclosetogetherthecurrentsareapproximatelythesame.VA VB =Id *(Z Z ) ~120V if Id is in nA and Figure6.10 Amechanismofelectric

fieldpickupofanelectrocardiographresultingfromthepowerline.Couplingcapacitancebetweenthehotsideof

Id1*(Z1Z2)~120VifId1isinnA anddifferenceofZ1Z2isinK.Thisisquitehigh.This can be minimized by shielding thepowerlineandleadwirescauses

currenttoflowthroughskinelectrodeimpedancesonitswaytoground.

ThiscanbeminimizedbyshieldingtheleadsandgroundingeachshieldattheECGunit.Alsoloweringskinelectrodeimpedancesmayhelp.

Powerline 120V Thereisalsoapossibilityofcurrentfromthepowerline

ProblemswithECG.Cbidb

toflowthroughthebodyasshowncausingacommonmodelvoltagetoappearinthesignal.

The magnitude of this signal is V =i *Z Typical valuesElectrocardiograph

A

Zin

Z1cm

B

cm

ThemagnitudeofthissignalisVcm=idb ZG.Typicalvaluesare10mVforidb=0.2AandZG=50K.

ForaperfectamplifierthisisnoproblemastheZ2 B

G

Zin

cm

differentialamplifierwithrejectthecommonmodesignal.Butforrealamplifierswithfiniteinputimpedance,thereissomeVcm thatappearsintheoutput

ZGidb

output.

VAVB=Vcm ((Z2Z1)/Zin)ifZ1 andZ2 are

ProblemswithECGCont. Othersourcesofinterference interference

Magneticfieldpickup

EMG i t fEMGinterference

Figure6.12Magneticfieldpickupbytheelctrocardiograph (a)LeadwiresforleadImakeaclosedloop(shadedarea)whenpatientandelectrocardiograph

Figure6.9 (a)60Hzpowerlineinterference (b) Electrom ographic

areconsideredinthecircuit.Thechangeinmagneticfieldpassing

throughthisareainducesacurrentintheloop.(b)Thiseffectcanbe

interference.(b)ElectromyographicinterferenceontheECG.

minimizedbytwistingtheleadwirestogetherandkeepingthemclosetothe

bodyinordertosubtendamuchsmallerarea.

ProblemswithTransients ToprotecttheECGcircuitagainsthighvoltagesweneedvoltage

limitingcircuitry.

These occur for example in the operating room when the ECG isTheseoccurforexampleintheoperatingroomwhentheECGiscombinedwiththeuseofanelectrosurgicalunitthatwillinducehightransientvoltagesintothepatient.

Voltage limiting devices such as diodes are used for protecting the VoltagelimitingdevicessuchasdiodesareusedforprotectingtheECGcircuitryandareconnectedbetweentheleadandRLground.

Figure 6 14 Voltagelimiting devices (a) CurrentFigure6.14Voltage limitingdevices(a)Currentvoltagecharacteristicsofavoltagelimiting

device.(b)Parallelsilicondiodevoltagelimitingcircuit.(c)BacktobacksiliconZenerdiode

Figure6.13Avoltageprotectionschemeattheinputofanelectrocardiograph

OtherProblemsfrequentlyencountered with the ECGencounteredwiththeECG

FrequencyDistortion:Highfrequencydistortion RoundingofftheQRSwaveform and diminishing itswaveformanddiminishingitsamplitude.Lowfrequencydistortionbaselineisnolongerhorizontalafteranevent.

Saturationorcutoffdistortion HighoffsetvoltagesandimproperlyadjustedamplifierscanproducesaturatedECGs.Peaks of the QRS are cutoff

Figure6.8EffectofavoltagetransientonanECGrecordedonanelectrocardiographinwhichthetransientcausestheamplifierto

saturate,andafiniteperiodoftimeisPeaksoftheQRSarecutoff GroundLoops If1groundof1device

ishigherthantheECGground,acurrentwillflowthroughthepatient

requiredforthechargetobleedoffenoughtobringtheECGbackintotheamplifiers

activeregionofoperation.Thisisfollowedbyafirstorderrecoveryofthesystem.

g ppresentingasafetyproblemaswellaselevatingthepatientsbodypotentialprojectingerroneousvoltagesintheECG

ArtifactsfromLargeTransientsCausealargeabruptdeflectionintheECG,takelongtimeforrecoverydue

ECG tothelargechargebuiltupinthecapacitors.

Commonmodereductioncircuits Commonmodesignalfromthebodyor

powerlineisaproblem.Eventhoughtheamplifierwillhelpineliminating

id

R+

3

thesebecauseofthehighCMRR,wecantrytoeliminatethecommonmodelsignalatthesource.Forinstancel d f ld k

Ra

Ra

+

ElectricandMagneticfieldpickupcanbeminimizedbyelectrostaticshieldingandtwistingofleadwires.

h l i i h i i h R

Rf

RoAuxiliaryopamp +

+

RL

4cm

AnothersolutionistheDrivenRightLegSystemwheretheRLelectrodeisconnectedtotheO/Pofanauxiliaryopamp The common mode signal

RRLp p

opamp.Thecommonmodesignalsensedbythevoltagefollowersisamplifiedandfedbacktothebodyraising the RL potential This negativeraisingtheRLpotential.Thisnegativefeedbackcausestheoutputcommonmodesignaltobelow.

Designconsiderations withotheramplifiersp

Figure6.16Voltageandfrequencyrangesofsomecommonbiopotentialsignals;dcpotentialsincludeintracellularvoltagesaswellasvoltagesmeasuredfromseveral

pointsonthebody.EOGistheelectrooculogram,EEGistheelctroencephalogram,ECGistheelectrocardiogram,EMGistheelectromyogram,andAAPistheaxonaction

potential.

EMGAmplifier BasicsandDesign EMG stands for electromyogramEMGstandsforelectromyogram Itismeasurementofelectricalpotentialscreatedbythecontractionof

muscles. Musclesgeneratevoltagesaround100mVwhentheycontract.These

lt tl tt t d b i t l ti d th ki d thvoltagesaregreatlyattenuatedbyinternaltissueandtheskin,andtheyareweakbutmeasurableatthesurfaceoftheskin.

TypicalsurfaceEMGsignalsforlargemuscles,suchasthebicep,arearound12mVinamplitude.

EMGsignalscontainfrequenciesrangingfrom10Hzorlowerupto1kHzorhigher.

ToobserveanEMGsignal,weneedtobuildadifferentialamplifierwithhigh commonmode rejectionhighcommonmoderejection

Thedominantcommonmodevoltagesignalsonourbodiesisusuallya60Hzsinewavethatiscapacitively coupledtousfromthe120VACwiringinthewalls.

Werejectthissignalbylookingatthedifferenceinvoltagebetweentwonearbypointsontheskinoverthemuscleofinterest.

Wewillalsowanttouseacircuitthedrawsnearlyzerocurrentfromtheinputleads,sincedccurrentpassedthroughEMGelectrodescanleadtop , p glargedcoffsetsanddegradethelongtermusefulnessoftheelectrodes.

WecanbuildanEMGcircuitusinganinstrumentationamplifierwithopamps suchasLM741andLM324(BJTdevicesinputcurrentsof100500nA)orTL084devicewithJFETsinputcurrents