Using the phase sensitive detector circuit below the magnitude of sinusoidal signal can be measured. Beside that, this circuit also can be used to measures 180°-out-of-phase inputs with proper polarity. But, it depends on the relationship to the reference with less than 1% error. This circuit uses a AD741J op amp. Here is the schematic diagram of the circuit and the pin configuration of AD532:

Phase sensitive detector for sinusoidal signals circuit schematic