CS5505/6/7/8
CS5505/6/7/8
Digital Filter Characteristics
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.
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.
0
X1 = 32.768kHz
X2 = 163.00kHz
-20
-40
Frequency
(Hz)
Notch
Depth
(dB)
Frequency Minimum
-60
(Hz)
Attenuation
(dB)
-80
50
125.6
55.5
50±1%
60±1%
60
126.7
145.7
136.0
118.4
132.9
102.5
108.4
58.4
62.2
68.4
74.9
87.9
94.0
104.4
-100
-120
-140
-160
100
120
150
180
200
240
100±1%
120±1%
150±1%
180±1%
200±1%
240±1%
XIN = 32.768 kHz
120 160 200
X1
X2
0
0
40
80
240
198.97 397.95 596.92 795.10 993.87 1193.85
Frequency (Hz)
Figure 9. Filter Magnitude Plot to 260 Hz
Table 3. Filter Notch Attenuation (XIN = 32.768 kHz)
180
0
135
90
45
0
Flatness
Frequency dB
-20
-40
1
2
-0.010
-0.041
-0.093
-0.166
-0.259
-0.374
-0.510
-0.667
-0.846
-1.047
-3.093
3
4
-60
5
6
-80
7
-45
8
9
-100
-120
-140
-90
10
17
XIN = 32.768 kHz
XIN = 32.768 kHz
-135
-180
0
5
10 15 20 25 30 35 40 45 50
0
5
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Frequency (Hz)
Figure 10. Filter Magnitude Plot to 50 Hz
Figure 11. Filter Phase Plot to 50 Hz
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