Cirrus Logic CS5508 - Manual (Page 19)

Digital Filter Characteristics
The digital filter in the CS5505/6/7/8 is the com-
bination of a comb filter and a low pass filter.
The comb filter has zeros in its transfer function
which are optimally placed to reject line interfer-
ence frequencies (50 and 60 Hz and their
multiples) when the CS5505/6/7/8 is clocked at
32.768 kHz. Figures 9, 10 and 11 illustrate the
magnitude and phase characteristics of the filter.
Figure 9 illustrates the filter attenuation from dc
to 260 Hz. At exactly 50, 60, 100, and 120 Hz
the filter provides over 120 dB of rejection. Ta-
ble 3 indicates the filter attenuation for each of
the potential line interference frequencies when
the converter is operating with a 32.768 kHz
clock. The converter yields excellent attenuation
of these interference frequencies even if the fun-
damental line frequency should vary
±1% from
its specified frequency. The -3 dB corner fre-
quency of the filter when operating from a
32.768 kHz clock is 17 Hz. Figure 11 illustrates
that the phase characteristics of the filter are pre-
cisely linear phase.
0 5 10 15 20 25 30 35 40 45 50
Frequency (Hz)
-180
-135
-90
-45
0
45
90
135
180
Phas
e (D
eg
rees
)
XIN = 32.768 kHz
Figure 11. Filter Phase Plot to 50 Hz
0 5 10 15 20 25 30 35 40 45 50
Frequency (Hz)
-140
-120
-100
-80
-60
-40
-20
0
A
tt
enua
ti
o
n
(
d
B)
Flatness
dB
-0.010
-0.041
-0.093
-0.166
-0.259
-0.510
-0.667
-0.846
-1.047
-3.093
1
2
3
4
5
6
7
8
9
10
17
XIN = 32.768 kHz
Frequency
-0.374
Figure 10. Filter Magnitude Plot to 50 Hz
Frequency
(Hz)
Notch
Depth
(dB)
Frequency
(Hz)
Minimum
Attenuation
(dB)
50 125.6 50 ±1% 55.5
60 126.7 60 ±1% 58.4
100 145.7 100 ±1% 62.2
120 136.0 120 ±1% 68.4
150 118.4 150 ±1% 74.9
180 132.9 180 ±1% 87.9
200 102.5 200 ±1% 94.0
240 108.4 240 ±1% 104.4
Table 3. Filter Notch Attenuation (XIN = 32.768 kHz)
0
0 40
198.97
80
397.95
120
596.92
160
795.10
200
993.87
240
1193.85
Frequency (Hz)
-160
-140
-120
-100
-80
-60
-40
-20
0
Atte
n
u
at
io
n (dB
)
XIN = 32.768 kHz
X1
X2
X1 = 32.768kHz
X2 = 163.00kHz
Figure 9. Filter Magnitude Plot to 260 Hz
CS5505/6/7/8
DS59F4 19
CS5505/6/7/8
DS59F7 19