Its interesting for me how long is averaging?
You use AD7730, then You dont need opamp, i am sure (i'm playing 5 years with ad7730).
On other hand for AD7730 there is one important thing - for better results You must use AC exitation (AD7730 has outputs to drive transistor bridge, see datasheet. For first tests You can try pnp/npn transistors instead MOS, but i've never tryed such configuration ). With DC excitation AD7730 has some unpredictable drift.
regards