I think you are using the ADCs out of specification.
There need to be 2V gap between the VREF+ and VREF-.

Datasheet of 16F87/88 DS30487B, Page 188
A21 VREF+: AVDD-2.5V  to AVDD + 0.3V 
A22 VREF-: AVSS-0.3V to	VREF+ - 2.0V
A25 VAIN: VSS-0.3V to VREF + 0.3V

So it may work but it is not recommended. And
even it seems to work, it is still not a very
good practice. Why not get the input signal right
and feed it to the ADC?

We have similar problem with comparators before.
The comparators of PIC are not rail to rail. So
we need to get the input signal right before
feeding it to the comparators.


Regards,
Xiaofan

-----Original Message-----
From: Wouter van Ooijen 
Sent: Tuesday, October 04, 2005 1:22 AM
To: 'Microcontroller discussion list - Public.'
Subject: RE: [PIC] Re: : stupid mistakes


> If you are not mistaken then is an interesting PIC AD behaviour.
> Probably something is changed since the first PICs on  16F series.What
> PIC are you using ?

16F88

> The standard 16F version neeed at least 2...2.2V margin between Vref-
> and Vref+ if supplied at 5V. At 1V margin as you have, resolution was
> very poor.

What was the behaviour you observed? Missing values?

Wouter van Ooijen
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