Measuring microvolt/nanovolt differences in circuit using Arduino ADC

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67

You're putting your 81 ( sorry) in backwards.

A 1mV drop across R1 becomes 1/81 mV difference, ignoring tolerances.at the bottom of your divider chains.

Suppose the voltage on one side of R1 is V1, and on the other side V2. Their difference is V1-V2

Then the voltages at the bottom of the divider chains are V1/81 and V2/81 , and their difference is

V1/81 - V2/81 = (V1-V2)/81

Allan