> Microchip specifies that your source impedance should be no higher
> than 10K - you have a worst case source impedance of 25K (when the pot is
> in the exact middle).  The cure is simple - add a 0.1 - 1 uF cap from the
> analog input to gnd.  The cap supplies the charge required when the input
> is sampled and restores full accuracy to the a/d input.

Not so fast.  Yes, a large cap will hold the pin voltage constant during the
brief acquisition interval, but accuracy may still be off and the "cross
talk" between channels may become worse.  If the A/D input is constantly
cycled between channels, then a current path is effectively created from
each channel to the previous channel.  The magnitude of this current is
proportional to the capacitor in the PIC and the full input channels cycle
frequency.  Given such a steady charge pump pin current, the external
capacitor will average to an offset error of the current times the source
impedence.  Since the large external capacitor is holding the offset error,
it will not have time to decay away during a normal acquisition time.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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