Exactly, but it's consistent.  A constant offset, I can deal with. =20

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 Sean Breheny
Sent: Thursday, May 5, 2016 12:25 PM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] Glitches on analog input

Note that going to GND in-between channel switching will not get rid of spi=
kes, it will just make them negative and independent of the values on other=
 channels. Instead of crosstalk you will have a gain error (albeit a small =
one)

On Thu, May 5, 2016 at 10:44 AM, Van Horn, David < david.vanhorn@backcountr=
yaccess.com> wrote:

> Yup.  Just applied that to all analog inputs.    I'm also going to
> recommend that SW park the ADC on ground when not in use, and that=20
> they look hard at the settling time again.
>
> All gremlins must die.
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On=20
> Behalf Of alan.b.pearce@stfc.ac.uk
> Sent: Thursday, May 5, 2016 8:40 AM
> To: piclist@mit.edu
> Subject: RE: [PIC] Glitches on analog input
>
> IIRC one of the techniques to use is to have a capacitor on the ADC=20
> input pin that is large enough to swamp the ADC input capacitance, but=20
> with a low enough source impedance to be an LP at the highest=20
> frequency of interest, without going beyond the source impedance restrict=
ion of the ADC.
>
>
>
> > -----Original Message-----
> > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On=20
> > Behalf Of Van Horn, David
> > Sent: 05 May 2016 14:32
> > To: Microcontroller discussion list - Public.
> > Subject: RE: [PIC] Glitches on analog input
> >
> > Well, I was sort of hoping that part of the "simplification" would=20
> > be glossing over that buffer.
> >
> > I'll have a look at it.  Parking the ADC on the ground input would=20
> > probably be a good technique to make whatever effect this has on the=20
> > reading more consistent.
> >
> > Thanks.
> >
> > -----Original Message-----
> > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On=20
> > 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
> >
> > > > A possible cause is that you are seeing the charge from the=20
> > > > ADC's internal sampling capacitor. Another possible cause is=20
> > > > leakage caused by an analog input clamping a voltage above the supp=
ly rail.
> > >
> > > I would hope that wasn't the case.  Shouldn't there be at least a=20
> > > buffer amplifier between the capacitor and the switch?
> >
> > Nope, not when you look at the simplified schematics they put in the=20
> > data sheet and family manuals. That is one reason why there is a=20
> > significant restriction on signal source impedance to maintain the=20
> > ADC
> accuracy.
> >
> > --
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