Jon,

Can't answer all your questions but will clear up some things.

>   Or maybe someone could tell me what to look for in an A/D converter?
> I understand a couple of things regarding them; resolution in bits (I
> assume 8 bits would give me 0-255 (or 1-256?) as an output,

Eight bits will give you a reading of 0-255.  There are 256 possible
combinations of eight bits.  The first being all off (0) and
the last being all on (255).

The range of an a/d is dependent on the reference voltage hooked to
the chip.  This refference voltage may be a voltage developed
independly of the 5 volts supplying the ic's, or it may be the same
5v that powers the ic's.

Lets say that the a/d can convert voltages from 0v to 5v.  Then
a count of 0 is 0v and a count of 255 is 5v.  So there are
255 steps and 5v/255steps give us 19.6 mv per count.
Lets call it 20 mv.  And you wanted a resolution of 2-4 mv.

As we add a bit to the a/d, we double the resolution.  So a 10 bit
a/d give a 20mv/4 or 5mv resolution.  A 12 bit a/d would be
20mv/16 or 1.25mv.  And there is the fact that the accuracy may
be some part of the LSB or Least Significant bit (one count).

Hope that helps a little.

Bill C.   bill@cornutt.com