Sorry, but I'm getting a little overloaded with information. Let me see if I
can summarise the major things so far.

There are 2 things that are possibly going to throw my readings of:
1) Noise
2) Aliasing

The noise is something that will only really come out in practice but should
be planned for from the start. Noise will come from the following sources:
1) Clock
    This will be at whatever clock frequencies are present.  Robert said
"Don't forget that clock dividing is going on inside your chip. (E.g.,
divide by 4), so lower frequencies will be present." but I'm planning to use
a Scenix. I don't think these have a divided clock. They get 50MIPS at 50MHz
so they can't be dividing the clock can they? Must be something like 4
parallel paths running simultaneously.

2) Power frequency
    50\60Hz.

3) Any EMI my circuit chooses to recieve.

I can try and avoid noise but the only way to know how much of a problem it
will be is by trial and error. Can someone point me at a good source of
information. I have Art Of Electronics (lent out at the moment tho so can't
look), is it's section on noise any good? Can anyone recommend a good source
of information on topics related to analog data aquisition such as EMI,
grounding, filtering etc.


Aliasing is something I don't quite understand. I know vaguely how it works
in theory but am having trouble linkling the theory to practice. I know it
is related to frequencies in the signal and the sampling rate. I can see how
it works in a perfect audio system (i.e. the only frequencies are those of
the audio (no noise)) but have trouble linking that to a knob box.

What frequencies will the knobs generate? What if you tweak the knob as fast
as you can (say 5Hz (I don't think it'd be easy to change direction more
than 5 times a second so this seems pretty over the top)) between two points
on the knob. Let's say it's a 7-bit knob (obviously the knob has infinite
resolution but I'm sampling at 7-bit) and the two positions are say 60 and
70. So your tweaking it 5 times a second between 2 positions around the
middle of the knob. So if you sampled it you would get something like
55->60->70->60->70->60->80. Now, if I filtered that with say a 4Hz rolloff
LPF. What is the signal now? Would it completely remove the 5Hz 60-70
oscillation giving an output something like 55->0->0->0->0->0->80 or what?
Now if I took that signal and didn't LPF it and sampled at 8Hz say (less
than 2x5Hz) what would happen. According to my understanding of the Nyquist
theorem I will get aliasing on components of the signal with a frequency
above the Nyquist freq. (1\2 the sampling rate = 4Hz). Thus the 5Hz signal
will be aliased down into the 1-4Hz range. What will this do to my signal? I
don't care whether the 60->70 oscillation is at 4Hz or 80MHz I just care
about the amplitude of the signal at my sampling points. How will this be
effected?

Tom.