ADS1255IDB Burr-Brown Corporation, ADS1255IDB Datasheet - Page 19

• Manufacturer Part Number: ADS1255IDB
• Description: Very Low Noise/ 24-Bit Analog-to-Digital Converter
• Manufacturer: Burr-Brown Corporation

FREQUENCY RESPONSE
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.
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.
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.
64
sin
www.ti.com
100
120
140
·
20
40
60
80
| H ( f )| + H
0
sin
Figure 14. Frequency Response for
0
256p
f
CLKIN
f
CLKIN
4p
·
f
15
·
f
Data Rate = 30kSPS
sinc 5
5
·
30
( f )
Num_Ave
45
Frequency (kHz)
sin
·
H
Averager
256p
60
·
f
·
CLKIN
Num_Ave f
( f ) +
sin
75
256p
f
CLKIN
f
DATA
90
·
f
= 30kSPS
105
120
(2)
The digital filter low-pass characteristic repeats at
multiples of the modulator rate of f
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,
NOTE: f CLKIN = 7.68MHz.
(1) Notch at 60Hz.
(2) Notch at 50Hz.
(3) Notch at 50Hz and 60Hz.
DATA RATE
12
18
24
30
36
42
48
54
60
30,000
15,000
(SPS)
0
2.5 (3)
6
60 (1)
50 (2)
30 (1)
25 (2)
15 (1)
10 (3)
7500
3750
2000
1000
5 (3)
Table 12. First Notch Frequency and
500
100
Figure 15. Frequency Response for
0
5
−3dB Filter Bandwidth
10
Data Rate = 2.5SPS
SBAS288D − JUNE 2003 − REVISED AUGUST 2004
15
FIRST NOTCH
20
Frequency (Hz)
30,000
15,000
7500
3750
2000
1000
(Hz)
500
100
25
2.5
60
50
30
25
15
10
5
30
35
CLKIN
40
−3dB BANDWIDTH
/4. Figure 16 and
f
DATA
45
ADS1255
ADS1256
= 2.5SPS
6106
4807
3003
1615
50
(Hz)
44.2
26.5
22.1
13.3
11.1
6.63
4.42
2.21
878
441
221
1.1
55
60
19