ADS1274
ADS1278
www.ti.com
SBAS367–JUNE 2007
High-Speed, Low-Power, and Low-Speed Modes
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
The digital filter configuration is the same in
High-Speed, Low-Power, and Low-Speed modes with
the oversampling ratio set to 64. Figure 58 shows the
frequency response in High-Speed, Low-Power, and
Low-Speed modes normalized to fDATA. Figure 59
shows the passband ripple. The transition from
passband to stop band is shown in Figure 60. The
overall frequency response repeats at 64x multiples
of the modulator frequency fMOD, as shown in
Figure 61.
0
-20
0.45
0.47
0.49
0.51
0.53
0.55
Normalized Input Frequency (fIN/fDATA
)
Figure 60. Transition Band Response for
High-Speed, Low-Power, and Low-Speed Modes
-40
-60
-80
20
0
-100
-120
-140
-20
-40
-60
0
0.2
0.4
0.6
0.8
1.0
Normalized Input Frequency (fIN/fDATA
)
-80
-100
-120
-140
-160
Figure 58. Frequency Response for High-Speed,
Low-Power, and Low-Speed Modes
0.02
0
0
16
32
48
64
Input Frequency (fIN/fDATA
)
Figure 61. Frequency Response Out to fMOD for
High-Speed, Low-Power, and Low-Speed Modes
-0.02
-0.04
-0.06
-0.08
-0.10
These image frequencies, if present in the signal and
not externally filtered, will fold back (or alias) into the
passband, causing errors. The stop band of the
ADS1274/78 provides 100dB attenuation of
frequencies that begin just beyond the passband and
continue out to fMOD. Placing an antialiasing, low-pass
filter in front of the ADS1274/78 inputs is
recommended to limit possible high-amplitude,
out-of-band signals and noise. Often, a simple RC
filter is sufficient. Table 3 lists the image rejection
versus external filter order.
0
0.1
0.2
0.3
0.4
0.5
0.6
Normalized Input Frequency (fIN/fDATA
)
Figure 59. Passband Response for High-Speed,
Low-Power, and Low-Speed Modes
Table 3. Antialiasing Filter Order Image Rejection
IMAGE REJECTION (dB)
(f–3dB at fDATA
)
ANTIALIASING
FILTER ORDER
HS, LP, LS
HR
45
1
2
3
39
75
87
111
129
22
Copyright © 2007, Texas Instruments Incorporated
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