Filter
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Analyzer
20 Hz − 20 kHz
(a) Basic Class−AB
APA
Signal
Generator
Power Supply
Analyzer
20 Hz − 20 kHz
R
L
(b) Filter-Free and Traditional Class-D
Class-D APA
Signal
Generator
Power Supply
R
L
Low-Pass RC
Filter
Low-Pass RC
Filter
(A)
(A)
For efficiency measurements with filter-free class-D, R
L
should be an inductive load like a speaker.
DIFFERENTIALINPUTANDBTLOUTPUT
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
Figure21.AudioMeasurementSystems
TheTPA3008D2usesamodulationschemethatdoesnotrequireanoutputfilterforoperation,buttheydo
sometimesrequireanRClow-passfilterwhenmakingmeasurements.Thisisbecausesomeanalyzerinputs
cannotaccuratelyprocesstherapidlychangingsquare-waveoutputandthereforerecordanextremelyhighlevel
ofdistortion.TheRClow-passmeasurementfilterisusedtoremovethemodulatedwaveformssotheanalyzer
canmeasuretheoutputsinewave.
Alloftheclass-DAPAsandmanyclass-ABAPAshavedifferentialinputsandbridge-tiedload(BTL)outputs.
Differentialinputshavetwoinputpinsperchannelandamplifythedifferenceinvoltagebetweenthepins.
Differentialinputsreducethecommon-modenoiseanddistortionoftheinputcircuit.BTLisatermcommonly
usedinaudiotodescribedifferentialoutputs.BTLoutputshavetwooutputpinsprovidingvoltagesthatare180
degreesoutofphase.Theloadisconnectedbetweenthesepins.Thishastheaddedbenefitsofquadruplingthe
outputpowertotheloadandeliminatingadcblockingcapacitor.
AblockdiagramofthemeasurementcircuitisshowninFigure22.Thedifferentialinputisabalancedinput,
meaningthepositive(+)andnegative(-)pinshavethesameimpedancetoground.Similarly,theBTLoutput
equatestoabalancedoutput.
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