Datasheet

½H(f) =½
5
Nsin
p ´ f
f
MOD
p ´N f
f
MOD
sin
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
0 1 2
NormalizedFrequency(f /f )
IN DATA
0
-20
-40
-60
-80
-100
-120
-140
Gain(dB)
3 4 5
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
ADS1281
www.ti.com SBAS378D –AUGUST 2007–REVISED JUNE 2010
The frequency domain transfer function of the sinc
filter is shown in Equation 3.
(3)
where:
N = decimation ratio (see Table 5)
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. Figure 26 shows
Figure 27. Sinc Filter Roll-Off
the frequency response of the sinc filter and
Figure 27 shows the roll-off of the sinc filter.
FIR Stage
The second stage of the ADS1281 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 Figure 28. 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. Table 24
in the Appendix section at the end of this document
lists the FIR stage coefficients. Table 6 lists the data
rates and overall decimation ratio of the FIR stage.
Figure 26. Sinc Filter Frequency Response
Table 6. FIR Filter Data Rates
DR[1:0] PINS DR[2:0] REGISTER DECIMATION RATIO (N) FIR DATA RATE (SPS)
00 000 4096 250
01 001 2048 500
10 010 1024 1000
11 011 512 2000
— 100 256 4000
Figure 28. FIR Filter Sub-Stages
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