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Laboratory1Differentialamplifiers;measuringtheperformanceofadifferentialamplifier;
Twostagedifferentialamplifier;ACcoupleddifferentialamplifierAlexPatriciumodifiedbyHubertdeBruinandLauraPravato
Scopeofthelab:
⎯ Introducethefundamentalpropertiesofdifferentialamplifiers⎯ Presentthemeasurementofcommonmodegain;CMRR;CMR⎯ IllustratetheeffectofDCbiasonamplifiedsignal⎯ Analyzethepropertiesofamulti-stageinstrumentationamplifier⎯ BecomefamiliarwithMacECEBiomedLabplatform.
Beforethelab:
1. ReviewtheschematicsofathreeOP-AMPdifferentialamplifier.ReviewthecomputationofCMMR.2. DownloadandreadthedocumentationfortheinstrumentationamplifierLT1920andforthe
operationalamplifierTLC2274.3. Reviewthepassivefilterstheory;reviewtheactivefilterstheory.
Objectivesduringthelab:
A. IdentifythedifferentcomponentsoftheMacECEBiomedLabplatform1. Identifythedifferentmodulesontheboard(instrumentationstage,HPfilterstage,notchfilterstage
etc.)2. Identifythebreadboardmodule3. Identifytheisolationamplifier
B. Measuretheamplifiergainandcompareitwiththetheoreticalone.
1. Usinga5.5kΩresistor,buildtheinstrumentationamplifieronstage1oftheboard.Thiswillamplifytheinputsignalbyagainof10(GDifferential=10).
2. Connecttheinputoftheinstrumentationamplifiertothesignalgeneratorandtheoutputtotheoscilloscope.UsethesettingspresentedinFigure1.
3. ComputethetheoreticalgainofthesignalandverifythatthesignaldisplayedontheoscilloscopehastheamplitudeGDifferential*inputsignalamplitude.
4. WithoutchangingtheinstrumentsettingsconnectthecircuitasinFigure2.5. Whatistheamplitudeofthesignalnow?Whataboutitsphase?6. Repeatthemeasurementsfor100Hz,1kHz,10kHz,100kHz,500kHz,600kHz,700kHz.7. Plotthemeasuredgainasafunctionoffrequency.
Figure1:Schematicformeasuringnon-invertinggain.
Figure2:Schematicformeasuringinvertinggain.
C. MeasuretheCMRR1. ConnecttheequipmentasshowninFigure3.2. SettheinputasshowninFigure3.(100Hz,3Vpp).Thiswillbeyourcommonmodevoltage.3. Measurethecommonmodeoutputvoltageandexplainwhatishappening.4. Calculatethecommonmodegain:GCM=VoutCM/VinCM.5. CalculatethecommonmoderejectionratioCMRR=GDifferential/GCMandCMR(dB)=20log10(CMRR)6. Repeatthemeasurementsfor100Hz,1kHz,10kHz,100kHz,500kHz,600kHz,700kHz.7. PlotthemeasuredCMMRasafunctionoffrequency.
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Figure3:Schematicformeasuringcommonmodegain
D. Addthesecondamplificationstage
1. Addasecondamplificationstagewithagainof11tothedifferentialamplifierasshowninFigure4.UsetheTLC2274.Whatisthenewgainofthecircuit?
2. Generateasinusoidalsignalwith>20mVp-pamplitudeand100Hzfrequency.Usethenon-invertinginput(Figure2)andverifythecorrectgain.
3. Repeatthemeasurements100Hz,1kHz,10kHz,100kHz,500kHz,600kHz,700kHz.4. Plotthegainversusfrequency.Explainwhatishappening.
Figure4:Instrumentationamplifierwithsecondamplificationstage.
E. StudytheeffectofaDCoffsetontheoutput.
1. Setthesignalgeneratortoaddanoffsetof20mVtothesinusoidalsignal.2. Whatdoestheoutputsignallooklike?Explainwhatisgoingon.3. DCoffsetsmaybepresentinbiomedicalsignals.Whatcanyoudotoremovethem?
F. BiomedicalAmplifierwithfirst-orderHPandLPfilters1. InsertapassiveHPandanactiveLPfilterinthebiomedicalamplifierasshowninFigure5.Calculate
thecomponentvaluessothatyouachieveaband-passfilterwithcutoff-frequenciesof1.5Hzand400Hz.
2. Generateasinusoidalsignalwith10mVp-pamplitudeand100Hzfrequencyand20mVDCoffset.Connectthebiomedicalamplifiertothesignalgeneratorandtotheoscilloscope.
3. Whatdoesyouroutputlooklike?4. Verifythatthehigh-passfiltereffectivelyremovestheDCcomponentfromtheinputsignal.
Figure5:InstrumentationamplifierwithHPFandLPF.
LabReport
– Dueoneweekafteryoudoyourlab.– Prepareaformallabreportwiththequestions/calculations/plotsrequiredbyquestions:
o B3.B5.B7.o C3.C4.C5.C7.o D1.D4.o E2.E3.o F1.F3.
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