Motorola DSP56156 Manual page 154

Table 6-14 describes the DC response of the codec section when the ratio 125 is selected.
Table 6-14 Codec DC Constant for 125 Decimation/interpolation Ratio
CRS1CRS0
0 0
In order to compensate this DC response to a unity response, the gain of the A/D transmit
decimation filter will have to be divided by 0.9313 and the gain of the receive D/A interpo-
lation filter will have to be multiplied by 1/1.7366.
The DSP decimation and interpolation filter may also need to fulfill some predefined fre-
quency response constraints. Figure 6-22 shows the characteristics of the transmit and
receive filters that we consider for this example.
10
5
L
0
o
-5
g
-10
M -15
a
-20
g
-25
n
-30
i
-35
t
-40
u
-45
d
-50
e
-55
(
-60
d
-65
B
-70
)
-75
-80
0
Figure 6-22 Example of Transmit and Receive Filter Performance Constraints
MOTOROLA
APPLICATION EXAMPLES
Decimation Output level and attenuation
Ratio Rate factor of the A/D for a maximum
125 -0.618 dB
±0.5dB
1 1.5 2 2.5 3
DSP56156 ON-CHIP SIGMA/DELTA CODEC
input voltage
0.9313
3.5 4 4.5 5 5.5
Frequency (kHz)
Gain of the D/A
Section
(VC=0dB)
4.794 dB 1.7366
6 6.5 7 8
6 - 39