dclm,
Let's go back to the beginning. With perfect electronics, you still have errors from the load cell
and errors from the A/D module. The load cell data sheet is rather thin. Usually for a bridge,
there is a spec for zero offset and gain error. For the L6H5, the zero error is less than 2% of full scale.
The gain error (sensitivity) is 2mV +/-0.2 or a 10% error. The A/D module simply says 1% overall.
You are trying to create an absolute voltage output based on bridge drive that goes to an A/D wich
has a different reference. Usually weigh scales are done with a ratiometric design so the bridge
and the ADC have the same reference. We sell parts to a large number of customers that make
weigh scales, very similar to your application and have a fair number of parts and reference designs.
See:
http://instrumentation.analog.com/en/weigh-scales/segment/im.html
The OP177 is an older generation op amp. You may want to look at the ADA4077-1 or the ADA4638-1,
but I'm not hopeful that you can achieve the results you want with the current architecture.
Harry