Sean Breheny wrote:

> I am seeing an ADC behavior as follows: if I apply a voltage of, say,
> 2.00 volts to the ADC, I might get a code of 102, as expected.
> HOWEVER, if I apply 4.00 volts, I might get 206 instead of 204.

  OK, I don't see the problem!  102 means 102 +/- 1 which means it could
easily be 103.  Twice 103 is 206, which is what you get.  No problem!

  If you want to make the FSD match, you need a tweakable reference.  If
you want zero to match, you need a tweakable ground.  If the results you
get are stable and repeatable, the ADC has fulfilled its mission.
Absolute accuracy is another thing again.

  I think you've got enough answers regarding the crystal.
--
  Cheers,
        Paul B.