> Following this thread's indicated that the max. impedence for reliable
> operation should NOT be more than 20K. Are there any suggestions?

I'd consider signal conditioning circuitry.
Whenever sampling data it's a good idea to condition the signal before =
entering the ADC. Use it to amplify, clean up (filter), scale and =
impendace match the signal to the ADC inputs.

The cheapest and simplest item is an Op-Amp (High input impedance (so =
won't effect the sensor being observed) and low output impedance (so =
ideal for the ADC).=20

I've used op-amps to amplify the uV signals from a K-Type thermocouple =
probe before entering the ADC of a PIC. With correct gain setting of the =
ADC I was able to set it up so that the 10-bit number read from the ADC =
was the exact temperature. So no software scaling or look up tables =
required.

If you're interested in dynamic changes of the signal then you really =
should think about Nyquist theory. Limit the frequency bandwidth of the =
signal to 1/2 the sampling frequency, otherwise you'll get aliasing and =
a host of unusual effects. (Or rather... sample at twice the maximum =
frequency bandwidth of the signal of interest)

Simple R/C filters will do the trick for many systems, you can set gain =
and filtering with various op-amp circuits. Anything more and you should =
really be using a DSP as the PIC will begin to struggle.

Regards
Pete

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.