I agree, Jason, but one interesting thing is that I've seen this same
problem (sample and hold cap charges directly from input) on dedicated ADC
ICs, too.

On Wed, May 4, 2016 at 10:32 AM, Jason White <
whitewaterssoftwareinfo@gmail.com> wrote:

> 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 AD=
C
> channels.
>
> *Why no buffer you ask? A buffer would introduce additional
> error/non-linearity to the ADC, something that engineers have to fight ve=
ry
> 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 buffe=
r
> > amplifier between the capacitor and the switch?
> >
> > --
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>
>
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> Jason White
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