Output
FIR Stage 2
Decimate by 2
FIR Stage 1
Decimate by 2
Sinc
Filter
FIR Stage 4
Decimate by 2
FIR Stage 3
Decimate by 4
Linear
Minimum
PHASE Select
Coefficients
0
1
2
Normalized Frequency (f /f
)
IN DATA
0
-
20
-
40
-
60
-
80
-
100
-
120
-
140
Gain(dB)
3
4
5
Gain(dB)
Normalized Frequency (f /f
)
IN DATA
0
-
0.5
-
1.0
-
1.5
-
2.0
-
2.5
-
3.0
0
0.05
0.10
0.20
0.15
½
½
H(f) =
5
sin
p ´
N
f
f
MOD
N sin
p ´
f
f
MOD
SBAS418I – SEPTEMBER 2007 – REVISED MARCH 2015
shows the frequency domain transfer
function of the sinc filter.
(8)
where:
N = decimation ratio (see
)
The sinc filter has notches (or zeroes) that occur at
the output data rate and multiples thereof. At these
frequencies, the filter has zero gain.
shows
Figure 39. Sinc Filter Roll-Off
the
frequency
response
of
the
sinc
filter
and
shows the roll-off of the sinc filter.
9.14.2 FIR Stage
The second stage of the ADS1282 digital filter is an
FIR low-pass filter. Data are supplied to this stage
from the sinc filter. The FIR stage is segmented into
four sub-stages, as shown in
. The first two
sub-stages are half-band filters with decimation ratios
of 2. The third sub-stage decimates by 4 and the
fourth
sub-stage
decimates
by
2.
The
overall
decimation of the FIR stage is 32. Note that two
coefficient sets are used for the third and fourth
sections, depending on the phase selection.
(in the
section at the end of this document)
lists the FIR stage coefficients.
lists the data
rates and overall decimation ratio of the FIR stage.
Table 7. FIR Filter Data Rates
Figure 38. Sinc Filter Frequency Response
DECIMATION
FIR DATA RATE
DR[2:0] REGISTER
RATIO (N)
(SPS)
(N = 32)
000
4096
250
001
2048
500
010
1024
1000
011
512
2000
100
256
4000
Figure 40. FIR Filter Sub-Stages
Copyright © 2007–2015, Texas Instruments Incorporated
19
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