I asked about this some weeks ago but a second query may be useful.

I am trialling a Sigma Delta analog to digital converter. This SHOULD
allow low cost high accuracy A2D with modest conversion speed.

Results to data only get so good (about 1/2 percent accuracy -
slightly worse than 8 bits)(plus a few worse results here and there)
and then seem to offer rapidly diminishing returns. Using more
conversion cycles doesn't seem top help much. I'm using 2048 counts
at present and would EXPECT much better results.

I have used both a standard microprocessor input pin as the sense
element (PIC) and an internal  comparator (Z8) and an external op-amp
(LM324) as comparator. I am aware of the poor performance of the
LM324 as a comparator but am compelled to use it if at all possible
due to cost considerations. If it CANT be used then something else
will be used but at around $US0.16 in volume for a quad op-amp it is
very attractive. I also need to deal with a  0 to 0.2V input signal
so an amplifier is desirable.

Anybody had better than about 1/2% accuracy results from a Sigma
Delta built discretely?
Any thoughts on the process?
This is a stunningly cheap and simple method for A2D if it would only
work :-) - once sorted it would/will prove a boon to other PIC users.


regards


Russell McMahon