Motorola DSP56156 Manual page 174

6.6.4.2 D/A Interpolation Filter
In addition to reconstruction, the D/A interpolation filter also needs to compensate the
D/A section frequency response. For a decimation ratio D=81 and a master clock
F=1.944 MHz, the frequency response of the D/A section is:
The transfer function of the interpolation filter will have to be shaped by 1/F(f) in order to
flatten the response of the D/A section.
An example of such a filter is given in Figure 6-37.
Receive Interpolation and Compensation Filter
10
5
L
0
o
-5
g
-10
M -15
a
-20
g
-25
n
-30
i
-35
t
-40
u
-45
d
e -50
-55
(
-60
d
-65
B
-70
)
-75
-80
0
Figure 6-37 Example of a Receive Reconstruction-interpolation IIR Filter
The IIR filter shown in Figure 6-37 is implemented using 4 biquadratic sections. The gain
and coefficients of the four section are listed in Table 6-22.
MOTOROLA
APPLICATION EXAMPLES
1
F f ( )
-------------------- -
=
×
81 128
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
×
2
 
 2π 81 
×
sin -------------------- - f
 
 
×
2 1944
 
------------------------------------------ -
 
 
2π
×
sin f
-------------------- -
 
 
×
2 1944
Frequency (kHz)
compda81.IIR.Filter
12
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