RE: [avr-gcc-list] ADC anomaly

[Loveny Design]

There is only one internal reference, a 1.2V bandgap, the 2.56V reference is
derived from that. The accuracy specs are given for the 2.56V reference from
which you can deduce (or guess) the specs for the bandgap. Either way, the
reference isn't very accurate and nor would I expect it to be when placed on
the same silicon as a processor!

Regards,
Jon


> -----Original Message-----
> From: Douglas Dotson [mailto:address@hidden
> Sent: Monday, 16 February 2004 11:54 AM
> To: Loveny Design; address@hidden
> Subject: RE: [avr-gcc-list] ADC anomaly
>
>
> Do you mean the internal reference or the reference
> that is selected with the MUX4:0 bits set to 11110?
> I don't see any specs on the accuracy of that voltage.
> I'm not using the internal 2.56v reference due to the
> inaccuracy. I'm using a precision 2.5v external
> reference that appears to be stable to within a couple
> of millivolts.
>
> I understand what you are saying about thermal
> equilibrium. But in this case just tapping the reset
> button wouldn't be cause for the thermal equalibrium
> to
> be upset. Also, the beast has been setting here for
> several hours and hasn't drifted a bit from the
> expected value. Go figure :)
>
> Doug
>
> --- Loveny Design <address@hidden> wrote:
> > The value 477 is approx 5% low, which is within spec
> > for the internal
> > reference. I think your expectations for the ADC are
> > an order of magnitude
> > too high ;-)
> >
> > As for your other problem, it is usual for analogue
> > circuits to be given a
> > few seconds (or even minutes) to reach thermal
> > equilibrium... which is
> > possibly the cause of that observation.
> >
> > Regards,
> > Jon
> >
> >
> > > -----Original Message-----
> > > From: address@hidden
> > > [mailto:address@hidden Behalf Of
> > Douglas Dotson
> > > Sent: Monday, 16 February 2004 8:03 AM
> > > To: Douglas Dotson; Richard Urwin;
> > address@hidden
> > > Subject: Re: [avr-gcc-list] ADC anomaly
> > >
> > >
> > > Things just got even stranger. I measured the
> > > 1.22v internal reference on channel 31. Measures
> > > low by 22 counts. Reads 477, should be 499. I'm
> > > starting to question the usefullness of this ADC
> > > as 10 bits.
> > >
> > > Doug
> > >
> >
> >
> > _______________________________________________
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> > http://www.avr1.org/mailman/listinfo/avr-gcc-list
>
>