Op-amp comparator and differential signals
I hope this circuit configuration helps. The first two opamp acts as a voltage buffer so that extra current doesn't flow through 120 Resistor, which may alter the output. After the first stage, both inputs are passed to the differential amplifier configuration of OpAmp. The resistors R2, R3, R4, R5 are chosen according to the gain required. After altering a little bit with OpAmp IC and other values, you may adapt to your need. I think buffer stages are required to properly measure the voltage.