How is your analog ground at the PIC chip configured? Is it a
"star ground point" at the PIC AVSS pin with digital VSS ground?
Also, is the ground in your system (relatively) noise-free?
A small ground potential difference or ground noise can cause the
zero offsets you are seeing.a

Another thing -- what are you using for your Vref? If you are
using the internal VDD, is it noisy? If you are using an external
reference you should get much better noise performance (providing
it also isn't noisy). A noisy reference which bounces around a
few bits can also cause the effects you are seeing.

Just some thoughts...

Matt Pobursky
Maximum Performance Systems

On Tue, 25 Jun 2002 10:52:17 +0200, Guillermo Rodriguez Garcia
wrote:
>Hello all,
>
>An update of this.
>
.. SNIP ...
>I modified my code and made the acquisition time much larger.
>Microchip was recommending 20 us for a non-capacitive load,
>I made it 320 us just to be on the safe side. 320 us should
>be more than enough :)
>
>The result is that the error has been reduced as I'm now
>reading $002 instead of $004 or $008 in the 'unused' channels.
>This happens on all channels that are unused, not only in the
>one that is sampled after the 'active' 4V one (see the
>original msg below for the details), so I assume that the
>acquisition is no problem anymore and that what I'm now
>seeing is only noise or something else.
>
>Question is, is it normal to have so much noise so as to
>affect 2 bits of the converted value?
>
>Any hints on how could I reduce this effect?

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