ADS1259
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SBAS424C –JUNE 2009–REVISED MARCH 2010
The sinc5 filter produces wide notches at fCLK/512 and
multiples thereof. At these frequencies the filter has
zero gain. Figure 39 shows the response data rate =
14.4kSPS.
0
-20
-40
With decimation of the second stage, the wide
notches produced by the sinc5 filter remain, but a
number of narrow notches are superimposed in the
response. The first of the notches occur at the data
rate. The number of superimposed notches is
determined by the decimation ratio, minus 1.
-60
-80
-100
-120
-140
The second stage filter has notches (or zeroes) at the
data rate and multiples thereof. Figure 40 shows the
response of the second stage filter combined with the
0
10
20
30
40
50
60
Frequency (kHz)
sinc5 stage. Decimation of
4
produces three
equally-spaced notches between each main notch of
the sinc5 filter. The frequency response of the other
data rates (higher decimation ratios) produces a
similar pattern, but with more equally-spaced notches
between the main sinc5 notches. Table 4 lists the first
notch frequency and the –3dB bandwidth.
Figure 39. Frequency Response
for Data Rate = 14.4kSPS
0
-20
sinc2
sinc1
Figure 41 illustrates the detail of the magnitude
response with data rate = 60SPS. Note that input
frequencies within the ±1% 60Hz bandwidth are
attenuated 40dB by the sinc1 filter and 80dB by the
sinc2 filter.
-40
-60
-80
Table 4. First Notch Frequency and –3dB Filter
Bandwidth(1)
-100
-120
-140
–3dB BANDWIDTH (Hz)
DATA RATE FIRST NOTCH
(SPS)
10(2)
16.6(3)
50
(Hz)
sinc1
sinc2
0
10
20
30
40
50
60
10
4.3
3.1
Frequency (kHz)
16.6
50
7.3
5.2
22
16
Figure 40. Frequency Response
(Data Rate = 3600SPS, R = 4)
60
60
27
19
400
400
177
525
1440
2930
127
380
1100
See (4)
1200
3600
14400
1200
3600
14400
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
(1) fCLK = 7.3728MHz.
(2) Notch at 50Hz and 60Hz.
(3) Notch at 50Hz.
sinc1
sinc2
(4) Same as sinc1.
55
56
57
58
59
60
61
62
63
64 65
Frequency (Hz)
Figure 41. Magnitude Response
for Data Rate = 60SPS
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