A FET input voltage follower would sure be nice! Could also do an op-amp
current to voltage converter off the sensor. But, what would happen if
you went above Microchip's suggested 10k max source resistance?
        I haven't checked it out fully, but a quick look at the A/D section of
the 16c716 datasheet shows a little about the A/D. One concern of
increased source resistance is the amount of time it takes to charge
Chold. It seems that this can be dealt with by either slowing down the
A/D (allowing more time for Chold to charge), or by adding a capacitor
between the analog input and ground (letting Chold quickly charge from
THAT capacitor instead of from your sensor). In either case, speed of
response slows down, but this may not be a major concern.
        Second, there's leakage or bias current to consider. The drawing in the
spec sheet shows +/- 500nA. If your sensor is indeed 2M, this gives an
error of +/- 1 VOLT!...
        OK, I guess a buffer is needed....  I was thinking the error could be
ignored, but a 1V error with a 5V reference would be pretty bad!
        Another fun thing to consider with high source resistances is the analog
mux. If the off resistance is near the source resistance, it makes little
difference whether the channel is selected or not... I ran into this on
another product years ago...
        Well, back on the original problem... A fet op amp voltage follower
would sure be the simple one part solution. With somewhat less accuracy,
it'd be interesting to try a source follower (one fet plus one resistor).
If you could find a fet with a "turn on" voltage of zero along with a
very high transconductance, it seems like the source voltage should
precisely follow the gate...

Good luck!

Harold


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