I need a help with lock in amplifier. I want to measure third-harmonic voltage from lock in amplifier. I have metal liner on a film on the substrate and I pass ac current through this metal liner which acts both as a heater and a thermometer. I want to calculate the third harmonic voltage from this. I have differential amplifier circuit where I eliminate the primary voltage component with the help of lock-in amp. I just dont know where to connect what. Should I connect the BNC connector to the input of lock in amp, that subtracts the two voltage. Should I have to connect the cable in osc out or ref in.