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RFILT
RL
RFILT
CFILT
VL= VIN VOUT
RANA
CANA
RANA
CANA
CFILT
To APA
GND
AP Analyzer Input
RC Low-Pass Filters
Load
VOUT
VIN RANA
RANARFILT
1j
O
(8)
fC2
fMAX
(9)
CFILT 1
2fCRFILT
(10)
TPA3008D2
SLOS435A – MAY 2004 – REVISED JULY 2004
Figure 23. Measurement Low-Pass Filter Derivation Circuit-Class-D APAs
The transfer function for this circuit is shown in Equation 8 where ω
O
= R
EQ
C
EQ
, R
EQ
= R
FILT
|| R
ANA
andC
EQ
= (C
FILT
+ C
ANA
). The filter frequency should be set above f
MAX
, the highest frequency of the measurementbandwidth, to avoid attenuating the audio signal. Equation 9 provides this cutoff frequency, f
C
. The value of R
FILTmust be chosen large enough to minimize current that is shunted from the load, yet small enough to minimize theattenuation of the analyzer-input voltage through the voltage divider formed by R
FILT
and R
ANA
. A rule of thumb isthat R
FILT
should be small (~100 Ω) for most measurements. This reduces the measurement error to less than1% for R
ANA
≥10 kΩ.
An exception occurs with the efficiency measurements, where R
FILT
must be increased by a factor of ten toreduce the current shunted through the filter. C
FILT
must be decreased by a factor of ten to maintain the samecutoff frequency. See Table 3 for the recommended filter component values.
Once f
C
is determined and R
FILT
is selected, the filter capacitance is calculated using Equation 9 . When thecalculated value is not available, it is better to choose a smaller capacitance value to keep f
C
above the minimumdesired value calculated in Equation 10 .
Table 3 shows recommended values of R
FILT
and C
FILT
based on common component values. The value of f
Cwas originally calculated to be 28 kHz for an f
MAX
of 20 kHz. C
FILT
, however, was calculated to be 57,000 pF, butthe nearest values of 56,000 pF and 51,000 pF were not available. A 47,000-pF capacitor was used instead, andf
C
is 34 kHz, which is above the desired value of 28 kHz.
Table 3. Typical RC Measurement Filter Values
MEASUREMENT R
FILT
C
FILT
Efficiency 1000 Ω5,600 pF
All other measurements 100 Ω56,000 pF
23