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.

In my limited experience, the easiest (and only) way,
I can think to do this is to use a diode to chop off
the negative cycle. However diodes seem to be
rather noisy. I'm worried this may affect my reading,
so am looking for a better way.


Please Input Comments :>
Thanks for all your help so far!


Feed your signal into two op amps in series with eachother
at unity gain.  Look first at one output, then at the other, and
do it at the rate you are chopping the signal into the salt
water probes.  The result is a DC level (with maybe a small
glitch at the switch points.)  This is called synchronous detection,
which is a lot better than the diode idea.

John Shreffler
New Directions