David, while I am not privy to the implementation details of the ADC, I
would suspect there is no such amplifier*. However, what you can do is
perform a simple test to find out. Sample ground before each measurement,
this will discharge the capacitor. If the pulse goes away then you know
what the culprit is.

Note: most PIC's let you internally sample Vdd and GND using dedicated ADC
channels.

*Why no buffer you ask? A buffer would introduce additional
error/non-linearity to the ADC, something that engineers have to fight very
hard to get rid off when they design the ADC. Just look at the built-in
opamps and comparators on most PICs, their specs are terrible! Those
represent the absolute best (analog-wise) that Microchip can achieve with
this particular semiconductor process. If the ADC can do without a buffer,
they almost certainly will omit it in order to improve the ADC's
performance.

On Wed, May 4, 2016 at 9:23 AM, Van Horn, David <
david.vanhorn@backcountryaccess.com> wrote:

> I am certain that no input to the processor is exceeding the rails, or
> even coming within half a volt of them.
>
> > A possible cause is that you are seeing the charge from the ADC's
> > internal sampling capacitor. Another possible cause is leakage caused
> > by an analog input clamping a voltage above the supply rail.
>
> I would hope that wasn't the case.  Shouldn't there be at least a buffer
> amplifier between the capacitor and the switch?
>
> --
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Jason White
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