Common vs. Differential Mode in Routing and Signal Integrity

Estimated reading time: 1 minute

In high-speed designs, the distinction between common-mode and differential-mode is becoming increasingly critical in PCB routing. While most designers are familiar with differential routing as a technique for certain signals on the PCB, many may not realize its underlying purpose. Differential routing is used to mitigate differential mode noise in the system. To understand this, we need a bit of electrical engineering background on signal and current flow in a system.

The current that flows from the source (driver) down the positive trace to the load (receiver) flows in a loop, and within that loop, the current is constant. This current must return (RTN) from the load (receiver) back to the source (driver). Because the current flows in one direction from the driver to the receiver and then in the opposite direction from the receiver back to the driver, these two currents are thought to be equal in magnitude and opposite in polarity. This means any noise from the sources is moving in different directions, creating differential noise.

Common-mode noise, on the other hand, occurs when noise (current) from an external source, such as EMI, appears on both the signal line and the return line traveling in the same (common) direction. Because an external source affects different signals by slightly different amounts, common-mode noise usually manifests as an unwanted voltage in the signal, since the voltage seen at any node with respect to a different node is simply the difference between the two voltages.

To continue reading this article, which appeared in the May 2026 I-Connect007 Magazine, click here.