Texas Instruments ADS1274, Manual

Page: 22 / 51

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- 1
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Normalized Input Frequency (f /f
IN T DA A
)
Amplitude(dB)
0.45 0.47 0.49 0.51 0.53 0.55 0
- 20
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- 60
- 80
- 100
- 120
- 140
0.4
Normalized Input Frequency (f
IN
/f
DATA
)
Amplitude(dB)
0 0.2 0.6 0.8 1.0
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0
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40
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Input Frequency (f /f
IN DATA
)
Gain(dB)
0 16 32 48 64 0.02
0
- 0.02
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0.04
- 0.06
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0.08
- 0.10
0.2
Normalized Input Frequency (f /f
IN DATA
)
Amplitude(dB)
0 0.1 0.3 0.4 0.5 0.6
ADS1274
ADS1278
SBAS367F – JUNE 2007 – REVISED FEBRUARY 2011 www.ti.com
High-Speed, Low-Power, and Low-Speed Modes
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 f
DATA
. 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 f
MOD
, as shown in
Figure 61 .
Figure 60. Transition Band Response for
High-Speed, Low-Power, and Low-Speed Modes
Figure 58. Frequency Response for High-Speed,
Low-Power, and Low-Speed Modes
Figure 61. Frequency Response Out to f
MOD
for
High-Speed, Low-Power, and Low-Speed Modes
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 f
MOD
. 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 4 lists the image rejection
Figure 59. Passband Response for High-Speed,
versus external filter order.
Low-Power, and Low-Speed Modes
Table 4. Antialiasing Filter Order Image Rejection
IMAGE REJECTION (dB)
(f
– 3dB
at f
DATA
)
ANTIALIASING
FILTER ORDER HS, LP, LS HR
1 39 45
2 75 87
3 111 129
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