Warning: this is a hardware idea from a mostly software guy....


If you don't have to sample very fast you could do the following:

1. Place a capacitor across the (high-impedance) voltage to be measured to
ground.

2. Tri-state the analog input PIC pin for a while.

3. Let the capacitor charge up to the voltage to be measured. Note that the
time constant will be based on the size of the cap and the impedence of the
source.

4. Turn the pin back into an analog input.

5. Measure your voltage. The capacitor will serve to reduce the effective
impedence of the source.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


----- Original Message -----
From: "Harold M Hallikainen" <haroldhallikainen@JUNO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, August 24, 2001 1:30 PM
Subject: Re: [PIC]: High impedance inputr to PIC A/D


>         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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