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In the simulation, the input is made from using two complementary signals.

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RI and RF of the difference amp are controlled via voltage controlled resistors; they are ideal resistors and perfectly matched for each case of 100, 500, and 1kOhm.

Here are the results.

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Why does the output signal's THD and Vos get worse when RI and RF resistance decrease?

I am not sure how to explain why changing the input & feedback resistance of a difference amp affected the THD and offset since the resistances are perfectly matched.

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Why does the output signal's THD and Vos get worse when RI and RF resistance decrease?

This happens with all op-amps. With lowering values you are demanding more current from the opamp output to flow through the feedback resistor and input resistor. If you read the data sheet you'll see distortion figues getting worse with higher load currents.

Don't expect perfection from opamps.

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  • \$\begingroup\$ Of course it happens to a different extent with different opamps... \$\endgroup\$ – Brian Drummond Jun 13 '18 at 15:13

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