Motorola DSP56156 Manual page 129

ON-CHIP CODEC FREQUENCY RESPONSE AND GAIN ANALYSIS
0
-0.5
-1
-1.5
-2
-2.5
-3
0.001 0.5
Figure 6-5 Log Magnitude Frequency Response of the
A/D Comb Filter in the 0-4 KHz Band for F=2.048 MHz and D=128
The 3 dB drop-off in the band 0-4 KHz shown in Figure 6-5 can be compensated by the
decimation filter inside the DSP core. In the present example, with D=128 and F=2.048
MHz, the frequency response of the comb looks like
The frequency response of the compensating DSP filter can be shaped by 1/F(f) in order
to flatten the response of the A/D section.
Table 6-8 gives an example of a 4 biquad IIR low pass filter whose transfer function has
been shaped accordingly.
Figure 6-6 shows the filter frequency response and the effects on the overall D/A section
response.
6 - 14
1 1.5
Frequency (KHz)
1
F f ( )
--------------- -
=
( )
3
128
DSP56156 ON-CHIP SIGMA/DELTA CODEC
2 2.5 3
:
×
3
 
2π 128
 
×
sin f
---------------------
 
 × 
2 2048
 
------------------------------------------- -
 
 
2π
×
sin -------------------- - f
 
 
×
2 2048
Cascade of:
sum3-128.Filter
diff3-128.Filter
3.5 4
MOTOROLA