Thanks for the info, I think a lot of my problems are due to this being
on a prototyping board, with unknown quality of connections between
parts, I've just tried liberally spreading capacitors around the power
rails, and its cleaned the signal up somewhat, I can see this being
something I will have to tune out once I've got a proto pcb in front of
me.

Is the general rule of thumb to use wide earth traces, and caps where
are all the power hungry/sensitive components are?

Thanks for your help.

Ed

On 22 Feb 2004, at 15:16, Dave Dilatush wrote:

> Ed wrote...
>
>> Improper biasing, I'm not sure I completely understand, but here is my
>> setup incase you can help?
>
> Not biasing, bypassing.  It's very important that the PIC's
> inputs- VDD, VREF, and the analog input- be very, VERY free of
> noise and spikes.  You should be making liberal use of bypassing
> (aka decoupling) capacitors on all chip power and reference
> connections.  This is even more of an issue than it otherwise
> would be, because you are (I assume) switching some pretty large
> currents in your circuit.
>
> Also, when you're switching large currents on and off, ground
> topology is important- in other words, the order in which various
> circuit ground connections are made, and the actual paths through
> which current is made to flow.  Current flowing through wires or
> circuit board traces causes resistive voltage drops; and rapid
> changes in current cause inductive voltage spikes as well.  Where
> these fluctuations occur, will influence what effect they have on
> your readings.
>
> [rest snipped...]
>
> It's hard to do anything more than make wild guesses, without
> seeing an actual schematic diagram of your circuit; even a far
> more detailed verbal description than the one you provided, only
> gives a vague idea of what could be going on.  The devil is in
> the details, so to speak.
>
> The more information you can provide, the more help we can give
> you.
>
> Dave D.
>
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