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R
FILT
R
L
R
FILT
C
FILT
V
L
= v
在
V
输出
R
ANA
C
ANA
R
ANA
C
ANA
C
FILT
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V
输出
V
在
R
ANA
R
ANA
R
FILT
1
j
O
(8)
f
C
2
f
最大值
(9)
C
FILT
1
2
f
C
R
FILT
(10)
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
figure23.measurementlow-passfilterderivationcircuit-类-dapas
ThetransferfunctionforthiscircuitisshowninEquation8where
ω
O
=R
EQ
C
EQ
,r
EQ
=R
FILT
||R
ANA
和
C
EQ
=(c
FILT
+C
ANA
).thefilterfrequencyshouldbesetabovef
最大值
,thehighestfrequencyofthemeasurement
带宽,toavoidattenuatingtheaudiosignal.equation9providesthiscutofffrequency,f
C
.thevalueofr
FILT
mustbechosenlargeenoughtominimizecurrentthatisshuntedfromtheload,yetsmallenoughtominimizethe
attenuationoftheanalyzer-inputvoltagethroughthevoltagedividerformedbyr
FILT
andR
ANA
.aruleofthumbis
thatR
FILT
shouldbesmall(~100
Ω
)formostmeasurements.thisreducesthemeasurementerrortolessthan
1%forR
ANA
≥
10k
Ω
.
anexceptionoccurswiththeefficiencymeasurements,wherer
FILT
mustbeincreasedbyafactoroftento
reducethecurrentshuntedthroughthefilter.c
FILT
mustbedecreasedbyafactoroftentomaintainthesame
cutofffrequency.seetable3fortherecommendedfiltercomponentvalues.
Oncef
C
isdeterminedandR
FILT
isselected,thefiltercapacitanceiscalculatedusingequation9.whenthe
calculatedvalueisnotavailable,itisbettertochooseasmallercapacitancevaluetokeepf
C
abovetheminimum
desiredvaluecalculatedinequation10.
Table3showsrecommendedvaluesofR
FILT
andC
FILT
basedoncommoncomponentvalues.thevalueoff
C
wasoriginallycalculatedtobe28kHzforanf
最大值
of20khz.c
FILT
,不管怎样,wascalculatedtobe57,000pf,但是
thenearestvaluesof56,000pfand51,000pfwerenotavailable.a47,000-pfcapacitorwasusedinstead,和
f
C
is34khz,whichisabovethedesiredvalueof28khz.
table3.typicalrcmeasurementfiltervalues
MEASUREMENTR
FILT
C
FILT
Efficiency1000
Ω
5,600pf
Allothermeasurements100
Ω
56,000pf
23
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