Answer 3—In the classic op-amp difference amplifier shown below, V_REF is normally connected to ground. This defines the output voltage when the two inputs are at the same voltage.

There’s no reason why this point cannot set to some other (positive) voltage in order to keep the output swing within the range of a singled-ended power supply for the op-amp.

In the case of the selectable-polarity power supply, you have the additional constraint that one input terminal will always be more positive than the other. For example, suppose you want the output of the difference amp to be in the range of 0 to 5 V, with a single 5-V supply for the op-amp. Suppose the power supply output is in the range 0 to 20 V. If V_REF is set at 0.5 V and R_X is 5× the value of R_Y, then the output of the difference amplifier falls neatly into the range 0.5 to 4.5 V, sidestepping issues related to rail-to-rail swings on either the inputs or the output. If an inverting arrangement is desired, simply swap the input connections to the op-amp and set V_REF to 4.5 V instead.

Contributor: David Tweed