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R
FILT
R
L
R
FILT
C
FILT
V
L
= V
IN
V
OUT
R
ANA
C
ANA
R
ANA
C
ANA
C
FILT
To APA
GND
AP Analyzer Input
RC Low-Pass Filters
Load
ǒ
V
OUT
V
IN
Ǔ
+
ǒ
R
ANA
R
ANA
)R
FILT
Ǔ
1 ) j
ǒ
w
w
O
Ǔ
(8)
f
C
+ 2
Ǹ
f
MAX
(9)
C
FILT
+
1
2p f
C
R
FILT
(10)
TPA3008D2
SLOS435A–MAY2004–REVISEDJULY2004
Figure23.MeasurementLow-PassFilterDerivationCircuit-Class-DAPAs
ThetransferfunctionforthiscircuitisshowninEquation8whereω
O
=R
EQ
C
EQ
,R
EQ
=R
FILT
||R
ANA
and
C
EQ
=(C
FILT
+C
ANA
).Thefilterfrequencyshouldbesetabovef
MAX
,thehighestfrequencyofthemeasurement
bandwidth,toavoidattenuatingtheaudiosignal.Equation9providesthiscutofffrequency,f
C
.ThevalueofR
FILT
mustbechosenlargeenoughtominimizecurrentthatisshuntedfromtheload,yetsmallenoughtominimizethe
attenuationoftheanalyzer-inputvoltagethroughthevoltagedividerformedbyR
FILT
andR
ANA
.Aruleofthumbis
thatR
FILT
shouldbesmall(~100Ω)formostmeasurements.Thisreducesthemeasurementerrortolessthan
1%forR
ANA
≥10kΩ.
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
MAX
of20kHz.C
FILT
,however,wascalculatedtobe57,000pF,but
thenearestvaluesof56,000pFand51,000pFwerenotavailable.A47,000-pFcapacitorwasusedinstead,and
f
C
is34kHz,whichisabovethedesiredvalueof28kHz.
Table3.TypicalRCMeasurementFilterValues
MEASUREMENTR
FILT
C
FILT
Efficiency1000Ω5,600pF
Allothermeasurements100Ω56,000pF
23