Gregg Kricorissian wrote:
>
> 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."
> >
> <snip>
> >
> "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

One way to get past these first and second order problems, is to
use one of the outputs as a reference. Measure the time-to-trip one
the (known) output and use this to calibrate the timer. This way you
can even compensate for temperature effects.

--
Friendly Regards

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