>Hi,

>Over the last week I've adjusted my programming and
>circuit to use an AC square wave of aprox 272Hz.
>This obviously works way better than a DC signal did.

>But I am still noticing a slight plating effect,
>so want to increase the frequency further, which is easy to
>do.  The reason I haven't increased it beyond 272Hz,
>is that this frequency still allows the cap to charge
>in one half cycle.  This allowed me to take the measurement
>as if I were using DC. Which I believe simplifies
>the hardware involved.  Increaing the frequency further,
>will cause the cap to start discharging before it
>finishes charging.  So, before the signal reaches the cap,
>I have to rectify it.

I am not sure, what your setup is. Is your conductivity probe still
connected one wire to the Vdd and the other to some PIC input and thru
capacitor to the ground ? In this way even if you charge and discharge the
capacitor often enough, there is still some DC voltage across the solution,
causing necessarily some plating effect (also called polarization).

>(btw, the solutions I'm measuring are in the ranges of
>400uS-3600uS, and will read 4-36)

For this range, 272Hz is OK.

>Please Input Comments :>

Keep in mind, that there is always some voltage limit, above which you will
make changes to the solution, possibly irreversible, and that will increase
you problems with polarization. That is why the peak (not average) voltage
across the probe should not exceed this limit. For most solutions, the limit
is 1V - 2V, so 5 V is for sure too much, several hundred mV is generally
considered safe. For your precission and resolution requirements, you can
certainly use square-wave instead of ideal sinus-wave, but the no DC rule is
still valid. I cannot come up with any simple way like the resistor +
capacitor for DC measurement. What I have in the simplest instrument is one
TL064 + one 4053 (+ ICL7136), no PICs. Maybe you could try RC oscilator and
convert R to frequency. I would opt for 2 OZ type with modification - two
antiparallel diodes will limit the voltage on the probe to about 650 mV:

      +-------------------------------------------------------------+
      |                    C                                        |
      |             +------||----+
+------/\/\/-----------+--|<|---->Vdd/2
      |        R    | |\         |           |       R2             |
      |      probe  | | \        |    R1     |   |\                 | 2x 1N4148
      +------/\/\/--+-|- \       |           |   | \                |
                      |   >------+---/\/\/---+---|+ \               |
             Vdd/2 <--|+ /                       |
>--+--/\/\/----+--|>|---->Vdd/2
                      | /               Vdd/2 <--|- /               |
                      |/                         | /                |
                                                 |/                 +---> to PIC


         GND <-----/\/\/-----+-----/\/\/------>Vdd
                             |
                             | |\
                             | | \
                             +-|+ \
                               |   >--+-->Vdd/2
                             +-|- /   |
                             | | /    |             3/4 LM324 (TL064 ...)
                             | |/     |
                             |        |
                             +--------+
R1 < R2

f = const. / RC


Regards,

Josef