Answer 6—The single-ended impedance at the V_P pin is R_X + R_Y, while the impedance at the V_N pin is just R_X. The inverting input of the op-amp must be treated as a voltage source, because the feedback action of the op-amp acts to hold this pin at the same voltage as the noninverting input.

The differential input impedance between the two input pins is just 2 × R_X, again because of the feedback action of the op-amp. Furthermore, the common-mode voltage at both inputs of the op-amp is not midway between the two input voltages, so unequal currents flow in the two resistors R_X.

Contributor: David Tweed