Datasheet

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SBAS306F − NOVEMBER 2004 − REVISED OCTOBER 2007
www.ti.com
22
POWER-UP SEQUENCE
The analog and digital supplies should be applied before
any analog or digital input is driven. The power supplies
may be sequenced in any order. Once the supplies and the
voltage reference inputs have stabilized, data can be read
from the device.
FREQUENCY RESPONSE
The digital filter sets the overall frequency response. The filter
uses a multi-stage FIR topology to provide linear phase with
minimal passband ripple and high stopband attenuation. The
oversampling ratio of the digital filter (that is, the ratio of the
modulator sampling to the output data rate: f
MOD
/f
DATA
) is a
function of the selected mode, as shown in Table 7. f
MOD
is
CLK/2, CLK/4, or CLK/8, depending on the mode.
Table 7. Oversampling Ratio versus Mode
MODE OVERSAMPLING RATIO (f
MOD
/f
DATA
)
High-Speed 64
High-Resolution 128
Low-Power 64
High-Speed and Low-Power Modes
The digital filter configuration is the same in both
High-Speed and Low-Power modes with the oversampling
ratio set to 64. Figure 54 shows the frequency response in
High-Speed and Low-Power modes normalized to f
DATA
.
Figure 55 shows the passband ripple. The transition from
passband to stop band is illustrated in Figure 56. The
overall frequency response repeats at 64x multiples of the
modulator frequency f
MOD
, as shown in Figure 57. 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 ADS1271
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 ADS1271 inputs
is recommended to limit possible high-amplitude
out-of-band signals and noise.
0
−
20
−
40
−
60
−
80
−
100
−
120
−
140
0.4
Normalized Input Frequency (f
IN
/f
DATA
)
Amplitude (dB)
00.2 0.60.81.0
Figure 54. Frequency Response for High-Speed
and Low-Power Modes
0.02
0
−
0.02
−
0.04
−
0.06
−
0.08
−
0.10
0.2
Normalized Input Frequency (f
IN
/f
DATA
)
Amplitude (dB)
0 0.1 0.3 0.4 0.5 0.6
Figure 55. Passband Response for High-Speed
and Low-Power Modes