This may not apply directly to your situation. I tried rolling my own ÊV to
I converter to use as a single slope A/D converter with a PIC a year ago
(Similiar to one of their app notes). It worked ok but could not get good
stability and repeatability better than 0.5% to 1%. I needed 0.1% so was
forced to go with a commercial dual slope A/D. I would like to get a sigma
delta unit which will give me a good 14 bits. Understand Tel-Com is working
on one but not available yet. If you come up with something I would be very
interested.

@
Sent by: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
07/13/99 08:25 AM ZE10
Please respond to pic microcontroller discussion list

To: PICLIST@MITVMA.MIT.EDU
cc:
bcc:
Subject: Re: Sigma Delta Converter


At 01:13 13/07/99 +1200, you wrote:
>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 Êcomparator (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 Ê0 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
>
>


An LMC660 comes close to the bargin price, but the 324 should do. Some more
information is required, are you using a split supply for the OPAMP, if not
how close to 0V do you get. When you say 0.5% error at what voltage input
level is this? Is the error at the same input level or over the entire
range?
The 324 has a 2mV offset that's up to 1% error in 200mV! Also are you using
dual slope conversion or single slope?
The number of counts not altering the value indicates that this is what the
converter is reading, so I would dare say that the reference is also ok.
But a Z8! Bad boy!
Please tell me more

Dennis