Though I haven't used that chip (my experience is with the 16c74), you
should be able to get a maximum of 1 lsb jitter (maybe 240 to 241) when
the voltage your feeding in is "close to the edge" between values. It
should be stable when you are not close to the edge.
        In my view, instability is due to either noise on the input or noise on
the reference. There could also be noise due to a high source impedance,
though I think this would be a slower variation in level due to input
capacity charge time and variation in input bias current with
temperature.
        My trick to getting stable A/D readings is to use a ground plane on the
board (I do a copper fill of both sides) and use bypass capacitors right
at the pin that needs bypassing (to the ground plane). I've got a product
that has 24 slide pots driving 4051 multiplexors driving analog inputs of
the 16c74. No extra filtering, just pot driving 4051 driving analog
input. You can set the pot to a position and have an absolutely stable
A/D reading. If you move it a bit you'll get to a point where it switches
back and forth between the current number and the next number. The amount
of noise determines how wide this indeterminate area is. Ideally the A/D
would settle on one or the other.
        So, again, I haven't used the chip you are, but if the A/D is the same
as that in the 16c74, it should be possible to make it stable.

Harold


On Fri, 11 Feb 2000 17:08:36 -0200 Edson Brusque <brusque@FLYNET.COM.BR>
writes:
> Hello,
>
>     I'm making a voltage reader with the internal A/D from a
> PIC16F877.
> While my voltmeter shows that my input have steady 1.18V, PIC's A/D
> gives me
> random values from 240 to 247. Is this normal? Is there a way to
> make the
> A/D shows a steady reading?
>
>     Best regards,
>
>     Brusque
>
> +------------------------+
> |     Edson Brusque      |
> | Research & Development |
> |  www.citronics.com.br  |
> +------------------------+

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