Need ADC expertise in solving this problem

ericgibbs said:

hi,
The problem is that the Gain of U2 is not just simply set by R5/R6 , because R6 is connected to oV via R4 which is in parallel with the diode resistance in series with the output impedance of U1.
So as the voltage on U1 changes, so does the diode resistance, [ which is not a linear change] and therefore the Gain of U2 changes.
E

View attachment 162824

EDIT:
Try this asc file.

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