>If you're using the LM324 as a buffer amplifier (which I think is
>necessary for almost every application [the amp not the 324 per se])
then
>why not do something like so:
>
>  |       /|
>  |      /+|----------<< Vin
>  |     /  |
>  +----<   |
>  |     \  |
>P |      \-|--+
>I |       \|  |
>C |           |
>  |           |
>  +--/\/\/\---+
>  |           |
>  |          ===
>  |           |
>  |          ---
>  |          ///
>
>The 324 is being configured as a comparator. If Vin is greater than
the DC
>voltage on the capacitor, then the 324's output is high. The only
problem
>I see with this approach is that the voltage on the capacitor is a
>function of the PIC's supply voltage - which may be too noisy or too
>inaccurate for a 12-bit A/D converter. (This problem exists with the
1 and
>2 I/O line a/d converters too, btw.) The programmable current source
that
>you show above addresses this issue if it's powered from a voltage
>reference or perhaps a well filtered version of the power supply.
It's
>possible to also design a precise current source using a LM324.


I previously tried this but using the Z8's internal comparator as a
comparator (seemed like a good idea :-)).
Results were not as good as the prior version

Using an LM324 here is not practical at "real"speeds although some
other amps or comparators would be OK.  The LM324 has a woefully low
slew rate and gain-bandwidth and the output does not like being below
0.6 volts from lower rail and the input common mode voltage and
output max are Vcc-1.5 volts. All told this adds up to a lot of
limitations. Unfortunately its 5 times cheaper than anything else
(someone please point out something far superior and about as cheap
and I'll be very happy). LM324 is about $US0.16 in 10,000 volume.
Dual version is about the same price.

RM