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MAY 2026 I I-CONNECT007 MAGAZINE 101 Because an external source affects different sig- nals 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. For example, if the voltage on node A (V A ) is 15 Vdc and the voltage on node B (V B ) is 10 Vdc, and you are measuring V A with respect to V B , then the difference (V AB ) is V A – V B , or 15 Vdc – 10 Vdc for a result of V AB = 5 Vdc. Since common-mode noise is usually generated by external sources, the primary method to mitigate it is shielding, and for wires in a cable, twisted pairs can be helpful. For differential-mode noise, differential can help mitigate it. Three common routing techniques used in PCB design: • Single-ended mode is the most common style. It uses a single wire or trace from the driver to the receiver. The signal propagates from the driver to the receiver on this con- ductor and returns to the driver using the GND/RTN system of the design. • Differential-mode, more properly called odd- mode, is the next most common style. This uses a pair of wires or traces from the driver to the receiver. One trace carries the posi- tive signal and the other carries the negative signal. The negative is equal in magnitude and opposite polarity to the positive signal. Since signals are equal and opposite, no cur- rent flows in the GND/RTN system, creating an ideal case. • Common-mode is more properly called even- mode, and is the least used style. It still uses two wires/traces, but current flows in the same direction on both conductors. This is typically created by unwanted noise or other variations caused by unintended or unex- pected conditions. When considering differential routing as a noise- reduction solution, there are pros and cons to consider. One major con is that differential routing requires two traces, in addition to the RTN, to route the signal from point A to point B. This means the board area required for differential routing is twice that for single-ended routing. The pros significantly outweigh the cons: • In low-voltage/signal-level designs, you ef- fectively get double the signal level. If you have the same voltage (V), one positive (V+), and one negative (V-), then the difference be- tween them is (V+) – (V-) = 2V since V- = -V+. This, in turn, effectively doubles the signal voltage, giving you better immunity to low- voltage noise. • There is no RTN current (in an ideal system). Since differential signals are by definition equal and opposite, there is no return current through any path other than the two signals of the pair. • Differential receivers are typically designed to be sensitive to the differential mode but are designed to ignore the common mode. • Switching timing is usually more precise. This is because the signals are referenced to each other, and the crossover point is tightly con- trolled. On the other hand, single-ended sig- nals reference some other signal in the design (CLK, EN, etc.) and therefore their crossover/ transition points are subject to noise, thresh- old level shifts, threshold detection, etc. Since devices and ICs can convert single-ended signals to differential signals, using differential routing instead of traditional single-ended routing is an excellent way to mitigate noise in our system. This is especially true if you need to route your traces a long distance on your PCB. With common- mode noise, converting the two signals to differ- ential to route over any significant distance, then converting back to single-ended at the destination, may help mitigate that noise as well. Kristin Moyer is an instructor at Sacramento State University and for the Global Electronics Association.