Scott Stephens asked:
"I need an A/D converter as cheap & fast as possible. I like AN513; it uses a
counter synchronized to a charging cap, which the input voltage is compared
to; A single slope converter."
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"My first attempt at this has produced disapointing results- critical
adjustments of the RC time constants of an exponential ramp generator,...."


Scott,
You've pointed out the main problems with the simple RC design ... the
exponential ramp, and the inconsistencies in the threshold voltage of the
PIC's input mean that the AN513 A to D can never be very linear, stable, or
precise.  A possible solution may be to use a dual opamp, with one section
used to create a constant current source to linearize the charging of the
integrating capacitor, while the other section is used as a comparator.

Not as simple as a single PIC, but then it should be a lot more accurate.

Also, as in all single-slope converters, any drift in the clock will also
cause measurement errors in the AN513 A to D: that's why a better approach
is to use a dual slope converter.

Hope this helps,
.. Gregg