Hello Matt,

At 10:41 25/06/2002 -0500, Matt Pobursky wrote:
>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?

Ops, what is AVSS? The PIC I'm using (16F872) doesn't seem to
have any pin labeled like that -- just one Vdd pin and two Vss
pins that as far as I can tell are both used for logic/digital
ground as well. Am I missing something?

>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.

Good hint, I'll look at that.


>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.

I'm using the internal Vdd as a reference, but can't avoid that
since I need:

- Three analog channels
- RA5 _must_ be a digital input (since it's used as the SPI slave
   select line for the PIC in slave mode)

So the only suitable combination is 0100 at PCFG3:0 which makes
RA0, RA1, RA3 analog inputs, with RA2 and RA5 digital and uses
Vdd as a reference.

However I'll also check if the Vdd in general is noisy.

Thanks!
G.


>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?
>
>--
>http://www.piclist.com hint: To leave the PICList
>mailto:piclist-unsubscribe-request@mitvma.mit.edu

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu