The onboard Vref is good to +/- .2%, is it absolute accuracy that is your issue ? If thats
not good enough external Vref in order .....?
Your target accuracy, resolution ? Seems like ~ 10 bits
DelSig, if you bypass input buffer, the inputs can common mode 100 mV outside either
rail. So run the DelSig at 16 bits, and using precision divider on input force input into
the 100 mV outside rail limit ? Dividing input by 8 seems to do the trick, that means
effective res would be 16 bits - 3 bits or 13 bits.
Just a thought.
Alternatively offset the input signal into CM range for inputs, and use a cal routine to remove offsetting R's error.
Could even use onboard DAC to supply cal offset Vdd for the divider, as shown below. There is an ap note
on creating 10 bit DAC.