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BASICMEASUREMENTSYSTEM
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
Thisapplicationnotefocusesonmethodsthatusethebasicequipmentlistedbelow:
•Audioanalyzerorspectrumanalyzer
•Digitalmultimeter(DMM)
•Oscilloscope
•Twisted-pairwires
•Signalgenerator
•Powerresistor(s)
•Linearregulatedpowersupply
•Filtercomponents
•EVMorothercompleteaudiocircuit
Figure21showstheblockdiagramsofbasicmeasurementsystemsforclass-ABandclass-Damplifiers.Asine
waveisnormallyusedastheinputsignalbecauseitconsistsofthefundamentalfrequencyonly(noother
harmonicsarepresent).AnanalyzeristhenconnectedtotheAPAoutputtomeasurethevoltageoutput.The
analyzermustbecapableofmeasuringtheentireaudiobandwidth.Aregulateddcpowersupplyisusedto
reducethenoiseanddistortioninjectedintotheAPAthroughthepowerpins.ASystemTwoaudiomeasurement
system(AP-II)(Reference1)byAudioPrecisionincludesthesignalgeneratorandanalyzerinonepackage.
Thegeneratoroutputandamplifierinputmustbeac-coupled.However,theEVMsalreadyhavetheac-coupling
capacitors,(C
IN
),sonoadditionalcouplingisrequired.Thegeneratoroutputimpedanceshouldbelowtoavoid
attenuatingthetestsignal,andisimportantbecausetheinputresistanceofAPAsisnothigh.Conversely,the
analyzer-inputimpedanceshouldbehigh.Theoutputimpedance,R
OUT
,oftheAPAisnormallyinthehundreds
ofmilliohmsandcanbeignoredforallbutthepower-relatedcalculations.
Figure21(a)showsaclass-ABamplifiersystem.Ittakesananalogsignalinputandproducesananalogsignal
output.ThisamplifiercircuitcanbedirectlyconnectedtotheAP-IIorotheranalyzerinput.
Thisisnottrueoftheclass-DamplifiersystemshowninFigure21(b),whichrequireslow-passfiltersinmost
casesinordertomeasuretheaudiooutputwaveforms.Thisisbecauseittakesananaloginputsignaland
convertsitintoapulse-widthmodulated(PWM)outputsignalthatisnotaccuratelyprocessedbysome
analyzers.
20