I'm using a 16c74a A/D to read 64 channels (8 4051 muxes in front
of PIC).  I'm driving the circuit with a 10K pot on each channel (one end
grounded, the other end to +5V).  It is quite stable.  If I adjust a pot
right to a threshold of the LSB, there is some jitter in the last digit,
but that's all.  I DID have some instability when the 7805 regulator was
oscillating.
        The board I'm using has a ground plane on both sides and uses 50
mil traces to carry the +5V around.  There's a 100nF bypass at each chip.
        If you think the hardware is ok (good solid grounds, etc.), try
checking the A/D settling time.  After selecting a channel, you have to
let the mux settle before starting the A/D.  Details are in the Microchip
Embedded Systems Handbook.

Good luck!

Harold


On Tue, 11 Nov 1997 07:33:04 +0200 Gordon Zerf
<kerzer@HARARE.IAFRICA.COM> writes:
>I am using a 16C71 as a 4 channel voltmeter, but find that the last
>digit
>fluctuates so much that it is a problem. I am using internal referance
>and
>have measured the supply etc. and everything is stable. How do I solve
>this
>problem ?
>
>Gordon
>