On 10/4/05, Chen Xiao Fan <xiaofan@sg.pepperl-fuchs.com> wrote:
> 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?

 Right. So I'm not wrong. There are more than 6-7 bits ?

 Chen, 2V margin is only on datasheet. On crude world must be *more* than 2V.

cheers,
Vasile

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