Datasheet

ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
19
Figure 14 and Figure 15 show the responses at the data
rate extremes of 30kSPS and 2.5SPS respectively.
Table 12 summarizes the first-notch frequency and −3dB
bandwidth for the different data rate settings.
0
−
20
−
40
−
60
−
80
−
100
−
120
−
140
Gain (dB)
0 153045607590105120
Frequency (kHz)
f
DATA
=30kSPS
Figure 14. Frequency Response for
Data Rate = 30kSPS
0
−
6
−
12
−
18
−
24
−
30
−
36
−
42
−
48
−
54
−
60
Gain (dB)
01520105 253035404550 6055
Frequency (Hz)
f
DATA
=2.5SPS
Figure 15. Frequency Response for
Data Rate = 2.5SPS
Table 12. First Notch Frequency and
−3dB Filter Bandwidth
DATA RATE
(SPS)
FIRST NOTCH
(Hz)
−3dB BANDWIDTH
(Hz)
30,000 30,000 6106
15,000 15,000 4807
7500 7500 3003
3750 3750 1615
2000 2000 878
1000 1000 441
500 500 221
100 100 44.2
60
(1)
60 26.5
50
(2)
50 22.1
30
(1)
30 13.3
25
(2)
25 11.1
15
(1)
15 6.63
10
(3)
10 4.42
5
(3)
5 2.21
2.5
(3)
2.5 1.1
NOTE: f
CLKIN
= 7.68MHz.
(1)
Notch at 60Hz.
(2)
Notch at 50Hz.
(3)
Notch at 50Hz and 60Hz.
The digital filter low-pass characteristic repeats at
multiples of the modulator rate of f
CLKIN
/4. Figure 16 and
Figure 17 show the responses plotted out to 7.68MHz at
the data rate extremes of 30kSPS and 2.5SPS. Notice
how the responses near DC, 1.92MHz, 3.84MHz,
5.76MHz, 7.68MHz, are the same. The digital filter will
attenuate high-frequency noise on the ADS1255/6 inputs
up to the frequency where the response repeats. If
significant noise on the inputs is present above this
frequency, make sure to remove with external filtering.
Fortunately, this can be done on the ADS1255/6 with a
simple RC filter, as shown in the Applications Section (see
Figure 25).