Motorola DSP56156 Manual page 171

6.6.4.1 A/D Decimation DSP Filter
In addition to antialiasing, the decimation filter also needs to compensate the A/D comb
filter response. For a decimation ratio D=81 and a master clock F=1.944 MHz, the fre-
quency response of the A/D comb filter is:
The frequency response of the decimation filter will have to be shaped by 1/F(f) in order to
flatten the response of the A/D section. An example of such a filter is given in Figure 6-35.
Transmit Decimation and Compensation Filter
5
0
L
-5
o
-10
g
-15
M
-20
a
-25
g
-30
n
-35
i
t
-40
u
-45
d
-50
e
-55
( -60
d
-65
B
-70
)
-75
-80
0
Figure 6-35 Example of a Transmit Antialiasing-decimation IIR Filter
The IIR filter shown in Figure 6-35 is implemented using 5 biquadratic sections. The gain
and coefficients of the four section are listed in Table 6-21.
6 - 56
APPLICATION EXAMPLES
4
F f ( )
------------ -
=
( )
3
81
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 1010.5 11
DSP56156 ON-CHIP SIGMA/DELTA CODEC
×
3
 
 2π 81 
×
sin -------------------- - f
 
 
×
2 1944
 
------------------------------------------ -
 
 
2π
×
sin -------------------- - f
 
 
×
2 1944
Frequency (kHz)
compad81.IIR.Filter
12
MOTOROLA