
R
FILT
R
L
R
FILT
C
FILT
V
L= VIN
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
V
IN
+
R
ANA
R
ANA)
R
FILT
1 ) j
w
O
f
C +
2
f
MAX
C
FILT +
1
2p
f
C
R
FILT
www.ti.com
SLOS435C – MAY 2004 – REVISED AUGUST 2010
Figure 23. Measurement Low-Pass Filter Derivation Circuit-Class-D APAs
The transfer function for this circuit is shown in Equation 8 where wO = REQCEQ, REQ = RFILT || RANA and CEQ = (CFILT + CANA). The filter frequency should be set above fMAX, the highest frequency of the measurement
bandwidth, to avoid attenuating the audio signal. Equation 9 provides this cutoff frequency, fC. The value of RFILT 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 RFILT and RANA. A rule of thumb is
that RFILT should be small (~100 ) for most measurements. This reduces the measurement error to less than
1% for RANA ≥ 10 k.
(8)
(9)
An exception occurs with the efficiency measurements, where RFILT must be increased by a factor of ten to
reduce the current shunted through the filter. CFILT must be decreased by a factor of ten to maintain the same
cutoff frequency. See
Table 4 for the recommended filter component values.
Once fC is determined and RFILT is selected, the filter capacitance is calculated using Equation 9. When the calculated value is not available, it is better to choose a smaller capacitance value to keep fC above the minimum
(10)
Table 4 shows recommended values of RFILT and CFILT based on common component values. The value of fC was originally calculated to be 28 kHz for an fMAX of 20 kHz. CFILT, 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
fC is 34 kHz, which is above the desired value of 28 kHz.
Table 4. Typical RC Measurement Filter Values
MEASUREMENT
RFILT
CFILT
Efficiency
1000
5,600 pF
All other measurements
100
56,000 pF
Copyright 2004–2010, Texas Instruments Incorporated
23