This circuit outputs the difference in voltage between two input signals. In this case, the first input is a 60 Hz signal, and the second input is a 60 Hz signal with 120 Hz square wave added in. (In this simulation, the two signals are added simply by connecting the two sources in series, which is convenient but not realistic.)

The second input signal is driving two resistors, which act as a voltage divider, and the + input of the op-amp is connected between them, where the voltage is equal to half the second input signal. The op-amp attempts to keep its – input at the same voltage as the + input. The two resistors on top act as a voltage divider, making the – input halfway between the first input signal and the op-amp output.

If V+ = 1/2 In2 = V– = 1/2 (In1 + output), then the output = In2 - In1, or the difference between the two inputs.

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