Gerhard Fiedler wrote:
>
> >Why not charge a cap and measure time-to-trip on an input? You only
> >need a cap and two resistors.
>
> I've never tried this approach, but I saw a Philips app note doing some
> analysis of a similar method to measure resistance, and they came up with
> around 5 bit for the most simple version (one port, one C, and one R),
> about 8 bit for a version using two ports and two HC buffers, and around 14
> bit using two comparators.
>
> Since what you suggest seems to be similar, I think the results might be
> similar, too. Do you have any experience with the precision and
> repeatability (also over temp range) with this simple circuit?

Your resolution depends on the time you have for charging the cap. If
you use
a 16 bit counter, that will be you resolution. On the repeatability
side, you
can improve it by using one of the output as a reference to calibrate
your
reading.

I've tried it once, and it worked so well that decided to never use a
PIC
with the A/D onboard again if I can help it.

There is (of course) a catch. In time critical software you'd have a
hard
time to fit it into your timing constraints. If you go the whole hog and
imlement a 16 bit AD with calibration, you'll need some 16 bit maths
too.

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za
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