ꢐ ꢕꢙ ꢚꢋ ꢛꢛ
ꢐ ꢕꢙ ꢚꢋ ꢛꢜ
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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
FREQUENCY RESPONSE
0
6
The low-pass digital filter sets the overall frequency
response for the ADS1255/6. The filter response is the
product of the responses of the fixed and programmable
filter sections and is given by Equation 2.
−
fDATA = 2.5SPS
−
−
−
−
−
−
−
−
−
12
18
24
30
36
42
48
54
60
Ť
( )Ť
Ť
f
Ť
( )
| ( )|
H f + Hsinc
·
HAverager f +
5
5
256p · Num_Ave f
256p · f
sinǒ Ǔ
sinǒ
Ǔ
ȧ
ȧ
ȧ
ȧ
ȧ ȧ
ȧ ȧ
(2)
f
f
CLKIN
CLKIN
ȧ
·
4p · f ȧ ȧ
256p · f ȧ
64 · sinǒ Ǔ Num_Ave · sinǒ Ǔ
f
f
ȧ
ȧ ȧ
ȧ
CLKIN
CLKIN
0
5
10 15 20 25 30 35 40 45 50 55 60
Frequency (Hz)
The digital filter attenuates noise on the modulator output,
including noise from within the ADS1255/6 and external
noise present on the ADS1255/6 input signal. Adjusting
the filtering by changing the number of averages used in
the programmable filter changes the filter bandwidth. With
a higher number of averages, bandwidth is reduced and
more noise is attenuated.
Figure 15. Frequency Response for
Data Rate = 2.5SPS
Table 12. First Notch Frequency and
−3dB Filter Bandwidth
The low-pass filter has notches (or zeros) at the data
output rate and multiples thereof. At these frequencies, the
filter has zero gain. This feature can be useful when trying
to eliminate a particular interference signal. For example,
to eliminate 60Hz (and the harmonics) pickup, set the data
rate equal to 2.5SPS, 5SPS, 10SPS, 15SPS, 30SPS, or
60SPS. To help illustrate the filter characteristics,
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.
DATA RATE
(SPS)
FIRST NOTCH
(Hz)
−3dB BANDWIDTH
(Hz)
30,000
15,000
7500
3750
2000
1000
500
30,000
15,000
7500
3750
2000
1000
500
100
60
6106
4807
3003
1615
878
441
221
100
44.2
26.5
22.1
13.3
11.1
6.63
4.42
2.21
1.1
(1)
60
(2)
50
(1)
30
(2)
25
(1)
15
(3)
10
50
0
30
fDATA = 30kSPS
25
−
−
−
−
20
40
60
80
15
10
(3)
5
5
(3)
2.5
2.5
NOTE: f
(1)
= 7.68MHz.
Notch at 60Hz.
Notch at 50Hz.
CLKIN
−
−
−
100
120
140
(2)
(3)
Notch at 50Hz and 60Hz.
The digital filter low-pass characteristic repeats at
multiples of the modulator rate of fCLKIN/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,
0
15
30
45
60
75
90
105
120
Frequency (kHz)
Figure 14. Frequency Response for
Data Rate = 30kSPS
19
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