ADS1291
ADS1292
ADS1292R
SBAS502A –DECEMBER 2011–REVISED MARCH 2012
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The sinc filter has notches (or zeroes) that occur at the output data rate and multiples thereof. At these
frequencies, the filter has infinite attenuation. Figure 26 shows the sinc filter frequency response and Figure 27
shows the sinc filter roll-off. With a step change at input, the filter takes 3 tDR to settle. After a START signal
rising edge, the filter takes tSETTLE time to give the first data output. The filter settling times at various data rates
are discussed in the START subsection of the SPI Interface section. Figure 28 and Figure 29 show the filter
transfer function until fMOD/2 and fMOD/16, respectively, at different data rates. Figure 30 shows the transfer
function extended until 4 fMOD. It can be seen that the ADS1291, ADS1292, and ADS1292R passband repeats
itself at every fMOD. The input R-C anti-aliasing filters in the system should be chosen such that any interference
in frequencies around multiples of fMOD are attenuated sufficiently.
0
0
-0.5
-1
-20
-40
-60
-1.5
-2
-80
-100
-120
-140
-2.5
-3
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Normalized Frequency (fIN/fDR
)
Normalized Frequency (fIN/fDR
)
Figure 26. Sinc Filter Frequency Response
Figure 27. Sinc Filter Roll-Off
0
0
DR[2:0] = 000
DR[2:0] = 000
-20
-20
-40
DR[2:0] = 110
DR[2:0] = 110
-40
-60
-60
-80
-80
-100
-120
-140
-100
-120
-140
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Normalized Frequency (fIN/fMOD
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
)
Normalized Frequency (fIN/fMOD
)
Figure 28. Transfer Function of On-Chip
Decimation Filters Until fMOD/2
10
Figure 29. Transfer Function of On-Chip
Decimation Filters Until fMOD/16
DR[2:0] = 110
DR[2:0] = 000
-10
-30
-50
-70
-90
-110
-130
0
0.5
1
1.5
2
2.5
3
3.5
4
Normalized Frequency (fIN/fMOD
)
Figure 30. Transfer Function of On-Chip Decimation Filters
Until 4fMOD for DR[2:0] = 000 and DR[2:0] = 110
24
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