Datasheet
0
-
-
-
-
-
-
-
20
40
60
80
100
120
140
Magnitude(dB)
0 10 20 30 40 50 60
Frequency(kHz)
0
-
-
-
-
-
-
-
20
40
60
80
100
120
140
Magnitude(dB)
0 10 20 30 40 50 60
Frequency(kHz)
sinc
1
sinc
2
0
10-
-
-
-
-
-
-
-
-
-
20
30
40
50
60
70
80
90
100
Magnitude(dB)
55
56
57 58 59 60 65
Frequency(Hz)
61 62 63 64
sinc
1
sinc
2
ADS1259
www.ti.com
SBAS424D –JUNE 2009– REVISED AUGUST 2011
The sinc
5
filter produces wide notches at f
CLK
/512 and
multiples thereof. At these frequencies the filter has
zero gain. Figure 40 shows the response data rate =
14.4kSPS.
With decimation of the second stage, the wide
notches produced by the sinc
5
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.
The second stage filter has notches (or zeroes) at the
data rate and multiples thereof. Figure 41 shows the
response of the second stage filter combined with the
sinc
5
stage. Decimation of 4 produces three
equally-spaced notches between each main notch of
Figure 40. Frequency Response
the sinc
5
filter. The frequency response of the other
for Data Rate = 14.4kSPS
data rates (higher decimation ratios) produces a
similar pattern, but with more equally-spaced notches
between the main sinc
5
notches. Table 4 lists the first
notch frequency and the –3dB bandwidth.
Figure 42 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 sinc
1
filter and 80dB by the
sinc
2
filter.
Table 4. First Notch Frequency and –3dB Filter
Bandwidth
(1)
–3dB BANDWIDTH (Hz)
DATA RATE FIRST NOTCH
(SPS) (Hz) sinc
1
sinc
2
10
(2)
10 4.3 3.1
16.6
(3)
16.6 7.3 5.2
50 50 22 16
Figure 41. Frequency Response
60 60 27 19
(Data Rate = 3600SPS, R = 4)
400 400 177 127
1200 1200 525 380
3600 3600 1440 1100
14400 14400 2930 See
(4)
(1) f
CLK
= 7.3728MHz.
(2) Notch at 50Hz and 60Hz.
(3) Notch at 50Hz.
(4) Same as sinc
1
.
Figure 42. Magnitude Response
for Data Rate = 60SPS
Copyright © 2009–2011, Texas Instruments Incorporated 19