Or is it something much more nasty, namely repeatability error, means hysteresis?
as I mention in http://www.8052.com/faqs.phtml?FAQ=128848 "An experienced design team (decades of esperience) went through 28 layouts till they got a layout satisfactory for an economy scale, just visuailze the effort that goes into a precision scale."

If you look at scales for sale and ignore the totally irrelevant resolution, that so many love to publish, you will find that to obtain a precision like the one you aim for you need to fork out $10k -up. That should say something about the design effort required.

I am not expressing doubts that you can do it (no evidence either way exist) but your statement "the scale is already built using the AD7730" does seem to indicate that you have no idea whatsoever of the layout effort it takes to obtain such precision. Also, using a very sensitive A/D may work, but maybe you lose the ability to amplify at the loadcell which very well may lead to longer wires carrying microvolt signals. The road to a precicon scale is full of potholes and I doubt you have found even a tenth of them. The last5 set of scales I participated in developing was a 20 man year effort, well we sold 820,000 of them.

Good luck,

Erik