The LT2940 measures a high side current and a differential voltage, multiplies them and outputs a current proportional to instantaneous power. Bidirectional high side currents and bipolar voltage differences are correctly handled by the four-quadrant multiplier and push-pull output stage, which allows the LT2940 to indicate forward and reverse power flow.
4-quadrant operation allows the LT2940 to be used in AC applications. Note that the LT2940 outputs a current proportional to instantaneous power, which is different from RMS-to-DC power metering such as the LTC1968 provides. The LT2940 monitors the load power and current on a 12.6V secondary winding in the circuit above. A split supply is derived from the same winding so that bidirectional measurement of instantaneous real power and instantaneous current is possible. Note that averaging the power output results in average real power. The load can be any combination of resistance, reactance, or nonlinear devices including chopped or rectified circuits.