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ON-CHIP CODEC FREQUENCY RESPONSE AND GAIN ANALYSIS
6.5.2.2
D/A Analog Comb Decimating Filter
The analog comb filter is a third-order decimate-by-four low-pass filter implemented in ful-
ly differential switched capacitor circuitry. The 1-bit digital input stream coming from the
digital modulator is converted to an analog signal by controlling the switching of input ca-
pacitors between VREF and VGNDA. The purpose of the filter is to prevent aliasing of out-
of-band noise when the clock rate is reduced by a factor of four. Eqn. 6-3 gives the transfer
function and the frequency response of the filter:
Eqn. 6-3 D/A Analog Comb Filter Transfer Function
Figure 6-9 and Figure 6-10 show an example of the D/A analog comb filter log magnitude
response using a 2.048 MHz master clock. The figures show the comb filter frequency re-
sponse in the 0-1024 KHz, 0-256 KHz, and 0-8 KHz bands. It can be noticed that the re-
sponse in the band 0-8 KHz is flat.
MOTOROLA
H z ( )
=
1
F f ( )
---- -
=
3
4
F: comb filter integrator clock
DSP56156 ON-CHIP SIGMA/DELTA CODEC
3
1
4 –
---- - 1 z
–
----------------
3
4
1 –
1 z
–
×
3
2π 4
×
sin
---------------
f
2F
------------------------------------- -
2π
×
sin
f
------ -
2F
6 - 21
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