Datasheet

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
V
OUT
w
V
IN
w
O
R
ANA
+ R
FILT
R
ANA
1+j
(
(
(
)
)
)
=
f =
c
Ö2 x f
max
C =
FILT
1
2 xf xRp
c FILT
TPA3101D2
www.ti.com SLOS473C –DECEMBER 2005–REVISED SEPTEMBER 2007
Figure 37. Measurement Low-Pass Filter Derivation Circuit-Class-D APAs
The transfer function for this circuit is shown in Equation 5 where w
O
= R
EQ
C
EQ
, R
EQ
= R
FILT
|| R
ANA
and
C
EQ
= (C
FILT
+ C
ANA
). The filter frequency should be set above f
MAX
, the highest frequency of the measurement
bandwidth, to avoid attenuating the audio signal. Equation 6 provides this cutoff frequency, f
C
. The value of R
FILT
must be chosen large enough to minimize current that is shunted from the load, yet small enough to minimize the
attenuation of the analyzer-input voltage through the voltage divider formed by R
FILT
and R
ANA
. A general rule is
that R
FILT
should be small (~100 Ω) for most measurements. This reduces the measurement error to less than
1% for R
ANA
≥ 10 kΩ.
(5)
(6)
An exception occurs with the efficiency measurements, where R
FILT
must be increased by a factor of ten to
reduce the current shunted through the filter. C
FILT
must be decreased by a factor of ten to maintain the same
cutoff frequency. See Table 4 for the recommended filter component values.
Once f
C
is determined and R
FILT
is selected, the filter capacitance is calculated using . When the calculated value
is not available, it is better to choose a smaller capacitance value to keep f
C
above the minimum desired value
calculated in Equation 7.
(7)
Table 4 shows recommended values of R
FILT
and C
FILT
based on common component values. The value of f
C
was originally calculated to be 28 kHz for an f
MAX
of 20 kHz. C
FILT
, however, was calculated to be 57,000 pF, but
the nearest values of 56,000 pF and 51,000 pF were not available. A 47,000-pF capacitor was used instead, and
f
C
is 34 kHz, which is above the desired value of 28 kHz.
Table 4. Typical RC Measurement Filter Values
MEASUREMENT R
FILT
C
FILT
Efficiency 1000 Ω 5,600 pF
All other measurements 100 Ω 56,000 pF
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