However, when set to bipolar there is a reference
voltage
> applied at the input and if your input isn't strong enough to
drive it
> to 0V then there may be a slightly higher voltage still on the pin
> (the 0.6V that I mentioned or the 123 numbers in the cat /dev/adc3
> output).
>
> If this is the case, you will need to provide buffering for all
your
> inputs, because you would find that the ADC loading should be
> affecting all the inputs, it is just not as obvious in its effects.
>
Thanks, it was that obviously wrong voltage that made me doubt
anything was right in the default settings.
I'd already seen from the 197 spec that the input impedance was
pretty minimal so I have op-amp voltage followers on all signals. My
primalry interest in 0-5V range anyway.
You nicely anticipated my next question on changing device numbers.
That trivial edit did the job perfectly and I no longer have to
double and triple check each time I compare adc_logger to the /dev
output.
It would be good to have a /proc entry to set the range but for now I
have rebuilt as 5V unipolar and that will probably do for current
project.
I have not yet answered my initial question of max sample rate but I
now have the tools to experiment a bit. Looks like it lies between
20k and 60k. If I can get near 40k without too much jitter it should
do the job.
Thanks again for providing the source and the module. It's a text
book example of clear , well structured , maintainable code. Kudos.
regards,
/js
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