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6.6.2.2
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=125 and a master clock F=2 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 6-25.
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 6-25 Example of a Receive Reconstruction-interpolation IIR Filter
The IIR filter shown in Figure 6-25 is implemented using 5 biquadratic sections. The gain
and coefficients of the four section are listed in Table 6-16.
MOTOROLA
APPLICATION EXAMPLES
1
F f ( )
----------------------- -
=
×
125 128
1
1.5
2
2.5
3
DSP56156 ON-CHIP SIGMA/DELTA CODEC
×
2
2π 125
×
sin
f
---------------------
×
2 2000
------------------------------------------- -
2π
×
sin
-------------------- -
f
×
2 2000
3.5
4
4.5
5
5.5
Frequency (kHz)
compda125-3.IIR.Filter
6
6.5
7
8
6 - 43
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