This circuit is designed to measure RMS current from the mains, using a split-core current transformer.

The LTC1966 (U2) is an RMS-to-DC converter, using a single capacitor to produce a voltage that accurately represents the average RMS current measured by the external split-core transformer.

Range is selected by choosing an appropriate split-core transformer - lower range giving higher accuracy.

The circuit is primarily designed for the Magnelab SCT-0400 series transformers, which produce 333mV at their rated top current.

Therefore, the output from the RMS-to-DC converter is amplified using an op-amp (U3) by a factor of 3 to produce a voltage in the range of roughly 0-1V.

This range was selected to be used with the internal 1.1V reference of the microcontroller.