Texas Instruments TPA3003D2 - Manual (Page 30)

TPA3003D2
SLOS406A − FEBRUARY 2003 − REVISED MARCH 2003
30 www.ti.com
APPLICATION INFORMATION
The transfer function for this circuit is shown in equation (10) where
ω
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 k Ω .
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
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 3 for the recommended filter component values.
Once f
C
is determined and R
FILT
is selected, the filter capacitance is calculated using equation (12). 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 (11).
C
FILT
+ 1
2
p
f
C
R
FILT
Table 3 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 3. Typical RC Measurement Filter Values
MEASUREMENT R
FILT
C
FILT
Efficiency 1 000 Ω 5 600 pF
All other measurements 100
Ω
56 000 pF
(10)
(11)
(12)