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C
IN
Audio Power
Amplifier
Generator
Low−Pass
RC Filter
C
IN
R
GEN
R
GEN
R
IN
R
IN
V
GEN
R
OUT
R
OUT
Analyzer
R
ANA
R
ANA
C
ANA
Low−Pass
RC Filter
R
L
C
ANA
Twisted-Pair Wire
Evaluation Module
Twisted-Pair Wire
CLASS-DRCLOW-PASSFILTER
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
Figure22.DifferentialInput,BTLOutputMeasurementCircuit
Thegeneratorshouldhavebalancedoutputs,andthesignalshouldbebalancedforbestresults.Anunbalanced
outputcanbeused,butitmaycreateagroundloopthataffectsthemeasurementaccuracy.Theanalyzermust
alsohavebalancedinputsforthesystemtobefullybalanced,therebycancellingoutanycommon-modenoisein
thecircuitandprovidingthemostaccuratemeasurement.
ThefollowinggeneralrulesshouldbefollowedwhenconnectingtoAPAswithdifferentialinputsandBTLoutputs:
•Useabalancedsourcetosupplytheinputsignal.
•Useananalyzerwithbalancedinputs.
•Usetwisted-pairwireforallconnections.
•Useshieldingwhenthesystemenvironmentisnoisy.
•EnsurethatthecablesfromthepowersupplytotheAPA,andfromtheAPAtotheload,canhandlethelarge
currents(seeTable2).
Table2showstherecommendedwiresizeforthepowersupplyandloadcablesoftheAPAsystem.Thereal
concernisthedcoracpowerlossthatoccursasthecurrentflowsthroughthecable.Theserecommendations
arebasedon12-inchlongwirewitha20-kHzsine-wavesignalat25°C.
Table2.RecommendedMinimumWireSizeforPowerCables
DCPOWERLOSSACPOWERLOSS
P
OUT
(W)R
L
(Ω)AWGSize
(MW)(MW)
104182216401842
2418223.283.78.5
182228282.18.1
<0.75822281.56.11.66.2
AnRCfilterisusedtoreducethesquare-waveoutputwhentheanalyzerinputscannotprocessthepulse-width
modulatedclass-Doutputwaveform.Thisfilterhaslittleeffectonthemeasurementaccuracybecausethecutoff
frequencyissetabovetheaudioband.Thehighfrequencyofthesquarewavehasnegligibleimpacton
measurementaccuracybecauseitiswellabovetheaudiblefrequencyrange,andthespeakerconecannot
respondatsuchafastrate.TheRCfilterisnotrequiredwhenanLClow-passfilterisused,suchaswiththe
class-DAPAsthatemploythetraditionalmodulationscheme(TPA032D0x,TPA005Dxx).
ThecomponentvaluesoftheRCfilterareselectedusingtheequivalentoutputcircuitasshowninFigure23.R
L
istheloadimpedancethattheAPAisdrivingforthetest.Theanalyzerinputimpedancespecificationsshouldbe
availableandsubstitutedforR
ANA
andC
ANA
.Thefiltercomponents,R
FILT
andC
FILT
,canthenbederivedforthe
system.ThefiltershouldbegroundedtotheAPAneartheoutputgroundpinsoratthepowersupplygroundpin
tominimizegroundloops.
22