Datasheet

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DIGITAL FILTER
Adjustable Digital Filter
Data Out
Modulator
Output
Fast Settling
Sinc
2
Sinc
3
FILTER SETTLING TIME
NOTE: (1) With Synchronized Channel Changes.
FILTER
SETTLING TIME
(Conversion Cycles)
Sinc
3
Sinc
2
Fast
3
(1)
2
(1)
1
(1)
AUTO MODE FILTER SELECTION
1 2 3 4
CONVERSION CYCLE
Discard
Sinc
2
Sinc
3
Fast
ADS1218
SBAS187C – SEPTEMBER 2001 – REVISED SEPTEMBER 2005
Calibration must be performed after power on, a
change in decimation ratio, or a change of the PGA.
For operation with a reference voltage greater than
(AV
DD
– 1.5V), the buffer must also be turned off
during calibration.
At the completion of calibration, the DRDY signal
goes low, which indicates the calibration is finished
and valid data is available. See Application Report
Calibration Routine and Register Value Generation
for the ADS121x Series (SBAA099 ) for more
information.
The Digital Filter can use either the fast settling,
sinc
2
, or sinc
3
filter, as shown in Figure 27 . In
addition, the Auto mode changes the sinc filter after
the input channel or PGA is changed. When
switching to a new channel, it will use the fast settling
filter for the next two conversions, the first of which
should be discarded. It will then use the sinc
2
followed by the sinc
3
filter. This combines the
low-noise advantage of the sinc
3
filter with the quick
response of the fast settling time filter. See Figure 28
for the frequency response of each filter.
When using the fast setting filter, select a decimation
value set by the DEC0 and M/DEC1 registers that is
Figure 27. Filter Step Responses
evenly divisible by four for the best gain accuracy.
For example, choose 260 rather than 261.
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