Assuring Data Integrity in an Optically Isolated 3.3 V I²C Bus

The inter-IC bus (I²C bus) is being used in an increasing number of applications, including consumer appliances, communications equipment and industrial equipment. In practically all cases, low voltage optocouplers are used to provide galvanic isolation in the 3.3V I²C bus interface. In a minimum configuration, three high speed optocouplers are used to isolate the CLK, SDA_in and SDA_out bus signals.

This paper discusses how the parameters related to optocoupler propagation delay can affect data integrity and system reliability for applications requiring isolation of the I²C bus interface, including IEEE 802.3af-compliant power-over-Ethernet switches and the analog-to-digital conversion interface to microcontrollers.