The diagram on this slide shows the typical application using the Si8900. In this case, the system master controller communicates with the Si8900 via a UART interface. A connection is made to the AC mains line through an inexpensive interface circuit. This circuit is made up of a dual packaged op-amp and several resistors. R1 acts as the sense resistor for current reading. The power supply for the high-voltage side of the device is derived from the mains line as well. U2 is a 3.0 V LDO that regulates power to the device and provides a bias for the interface circuitry.