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 InputRC 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
f
C
+ 2
Ǹ
f
MAX
C
FILT
+
1
2p f
C
R
FILT
TPA3004D2
SLOS407E –FEBRUARY 2003–REVISED JANUARY 2011
www.ti.com
Figure 51. Measurement Low-Pass Filter Derivation Circuit—Class-D APAs
The transfer function for this circuit is shown in Equation 10 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 11 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
rule of thumb 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.
(10)
(11)
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 2 for the recommended filter component values.
Once f
C
is determined and R
FILT
is selected, the filter capacitance is calculated using Equation 11. 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 12.
(12)
Table 5 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 57000 pF, but
the nearest values of 56000 pF and 51000 pF were not available. A 47000 pF capacitor was used instead, and f
C
is 34 kHz, which is above the desired value of 28 kHz.
Table 5. Typical RC Measurement Filter Values
MEASUREMENT R
FILT
C
FILT
Efficiency 1000 Ω 5600 pF
All other measurements 100 Ω 56000 pF
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