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LT2940 Fully Isolated AC Power and Current Monitor

Fully Isolated AC Power and Current Monitor

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.

This circuit highlights the use of the LT2940 in power and current monitoring in an isolated AC application. The LT2940 excels in this type of applications since it can monitor where both the current and the voltage may be variable due to supply voltage uncertainty, component parametric changes, transient conditions, time-varying signals, and so forth. Unlike traditional power monitors that rely on data converters and multiplying registers to calculate power, the LT2940’s four-quadrant multiplier calculates instantaneous power from its voltage sense and current sense inputs resulting in a 5% power measurement accuracy and a 3% current measurement accuracy. Its output driver sources and sinks current proportional to power (magnitude and direction), which provides a flexible voltage scaling, simple filtering and, into a reference, bipolar signals as shown. In addition, the LT2940 provides a proportional-to-current output that allows for equally straightforward scaling, filtering and monitoring of the sensed current. The LT2940 is suitable for use in line cards and servers, power-sense circuit breakers, power control loops and a wide variety of metering applications.