IIRC one of the techniques to use is to have a capacitor on the ADC input p=
in that is large enough to swamp the ADC input capacitance, but with a low =
enough source impedance to be an LP at the highest frequency of interest, w=
ithout going beyond the source impedance restriction of the ADC.



> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Van Horn, David
> Sent: 05 May 2016 14:32
> To: Microcontroller discussion list - Public.
> Subject: RE: [PIC] Glitches on analog input
>=20
> Well, I was sort of hoping that part of the "simplification" would be glo=
ssing
> over that buffer.
>=20
> I'll have a look at it.  Parking the ADC on the ground input would probab=
ly be
> a good technique to make whatever effect this has on the reading more
> consistent.
>=20
> Thanks.
>=20
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of alan.b.pearce@stfc.ac.uk
> Sent: Thursday, May 5, 2016 2:44 AM
> To: piclist@mit.edu
> Subject: RE: [PIC] Glitches on analog input
>=20
> > > 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?
>=20
> Nope, not when you look at the simplified schematics they put in the data
> sheet and family manuals. That is one reason why there is a significant
> restriction on signal source impedance to maintain the ADC accuracy.
>=20
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>=20
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