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.
The note discusses the challenges of using the I2C‑bus when devices do not share a common ground, such as noise margins, voltage rating violations, and safety requirements for isolation from mains, telephone lines, or PoE. It presents several isolation techniques, including increased signal amplitudes, conversion to differential signalling (CAN or RS‑485), and true galvanic isolation using opto‑couplers.
Practical circuit examples are provided for speeds ranging from a few kilohertz up to 1 MHz. The document shows how to split SDA/SCL into uni‑directional streams, use differential drivers, and select fast opto‑couplers (e.g., 6N137) to achieve higher bus speeds while tolerating ground‑potential differences from less than 1 V to kilovolt levels.
| Maximum I2C bus speed with opto‑coupling | 1 MHz |
|---|---|
| Supported ground‑potential difference | up to kilovolt levels |
With fast opto‑couplers and proper timing, the bus can operate at a real 1 MHz speed.
The presented solutions cover differences from less than 1 V up to kilovolt levels.
Couplers such as 6N137 with Schottky‑clamped output transistors provide the needed speed for 100 kHz.
Yes, the note shows how CAN or RS‑485 drivers can carry SDA and SCL as differential signals, providing common‑mode immunity of around +7 V.