Datasheet

0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
NormalizedInputFrequency(f /f
IN TDA A
)
Amplitude(dB)
0.45 0.47 0.49 0.51 0.53 0.55
0
-20
-40
-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
20
0
-20
-40
-60
-80
-100
-120
-140
-160
InputFrequency(f /f
IN DATA
)
Gain(dB)
0 16 32 48 64
0.02
0
-0.02
-0.04
-0.06
-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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