I don't have an F877 datasheet handy (the 18c452 datasheet I DO have
handy says 2.5k max for its 10 bit A/D). But assuming it is indeed a 10k
max, that'd allow a 40k pot. The maximum Thevinin resistance is Rpot/4.
If you have a 40k pot, the Thevinin resistance is 20k//20k or 10k.
        For more on Thevenin Equivalents, see
http://www.hallikainen.org/rw/theory/theory5.html

Harold

On Tue, 11 Jun 2002 11:42:29 -0500 Dale Botkin <dale@BOTKIN.ORG> writes:
> On Tue, 11 Jun 2002, Lyle Killough wrote:
>
> >  . . . Disclaimer . . .  I have never even read the F877 data
> sheet
>
> I have...
>
> > Are you maybe having trouble charging the input capacitance of the
> ADC
> > through 50k?  I kind of remember a 5k recommended maximum on the
> data
> > sheets for some of the other PICs with analog inputs.
>
> 10K.  That's the spec maximum input impedance, meaning you can use a
> 20K
> pot at most (right??).
>
> Dale
>
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