On Fri, 11 Feb 2000 18:54:04 -0200 Edson Brusque <brusque@FLYNET.COM.BR>
writes:
> Hello Harold,
>
> >         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.
>
>     Yes, I agree with it.
>
> >         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.
>
>     As I'm making tests on a proto-board, I think the problem is  noise
> *everywhere*. I hope the final product (on PCB) will be more stable.
>
> >         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).
>
>     Do you mean a copper fill *around* the copper tracks? With  bypass
> capacitors, do you mean a cap between the PIC A/D input and ground?
> What cap  value?
>

        Yes, it's a copper fill that connects to all component pins (and traces)
that are grounded, but goes around all other pins and traces. The bypass
capacitor is a power supply bypass capacitor, right from the power pin to
the ground plane. I do not add any capacity at the A/D input, as that
just slows down the settling time. My power supply bypass capacitors are
typically 220nF.

> > 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.
>
>     Is it a DMX lighting console? I'll start to develop one soon and
also
> pretend to make this way.
>

        Yep! It's a DMX controller. I'm right now moving it from the 16c74 to
the 18c452.


Harold



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