AD7708: Update rate

Document created by analog-archivist Employee on Feb 23, 2016
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In this case,  I have to use ad7708 confidured as 315Hz to achive 20Hz reading
for each channel in chop disable mode. But I have a doubt that, I'll confugure
it as 315Hz o/p rate but will read it as 100Hz. If so how peak-to-peak
resolution will change. It will be seen according to 315Hz o/p rate or 100Hz?
Smilarly, which one of these data output rate defines digital filter response,
315Hz or 100Hz? And how can I calculate resolution bit according to data output
rate and PGA(programmable gain amplifier). Is there any formula to find it?

 

When you change channels the first o/p is automatically delayed….the 2nd and
subsequent o/p’s will be at the programmed rate, provided you do not change
channels.
The pp resolution will be as defined in the programmed o/p rate..in this case
315Hz.

CHOP at 105Hz
To re-cap, when you program the device for chop mode, with SF = 13, the digital
filter needs 3 samples taking a time of 3.1msec for each filter o/p. In chop
mode, you take 2 results from the digital filter to give one o/p.  This gives a
total of 19msec for the first result and 9.3msec for each subsequent result.. (
2nd sample uses the 2nd o/p of the digital filter as it’s first o/p). 
Resulting in an o/p rate of  52.5 for the first sample and 105Hz for every
subsequent sample.

NON-Chop at 315Hz
In non-chop mode, with SF= 13, again the digital filter needs 3 samples to
process the first o/p , i.e 9.3mec, however each subsequent o/p is available
3.1msec later as the digital filter uses 2 of the three previous samples to
process the 2nd result.
Giving an update rate of 105Hz for the first result and 315 for every
subsequent result, again provided you do not change channels. If you change
channels continuously, then the update rate is 105Hz.

The accuracy will of couse be for the programmed update rate, i.e non-chop at
315Hz, o/p reduced to 105Hz when switching each sample, o/p accuracy conforms
to 315Hz.

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