This document discusses factors that need consideration when an I2C-bus is used to communicate between two points that do not share a common logic ground potential. I2C logic signals are referenced to a ground level and it is expected that connected devices share that reference. In practice, when an I2C-bus is extended to link devices that are separated by distances exceeding about 1 meter there can be many factors that cause differences in the local ground potential of those devices and that in turn affects the noise margins of the system and can lead to device ratings being exceeded. In other cases, where an I2C-bus could provide an attractive data link there is no possibility for a shared ground reference. Examples include the need to provide safety isolation between the I2C control signals and AC mains power, telephone lines, or patient monitoring medical equipment. In other cases, such as Power over Ethernet, standards require isolation of the grounds. Solutions are provided to deal with ground differences ranging from less than 1 V up to kilovolt levels by using Opto or transformer isolators in the signal path. Key to all solutions is the availability of I2C isolation devices.
