Return to Electromagnetic Compatibility Home Page

Printed Circuit Board Design and Layout Questions

3. How "balanced" does a differential trace pair on a printed circuit board have to be? (i.e. If the two trace lengths are within 1 mm of each other, is that good enough?

A circuit board trace that carries one-half of a differential signal must have the same length as the trace that carries the other half. Otherwise, the difference in the arrival times of the two signal halves introduces a common-mode component to the signal at the receiver. This common-mode signal voltage and the corresponding common-mode current it can induce can be significant sources of signal integrity or EMC failures.

Trace pairs routed on a densely populated board often must turn corners and change layers making the control of their lengths more challenging. Designers often specify the maximum allowable difference in the lengths of each trace. However, often these specifications are based on what is possible rather than what is necessary in a given application.

Common-mode voltages and currents are the result of "imbalance" in a differential circuit. Unequal trace length is only one of many possible sources of imbalance. Driver skew, asymmetries in the geometry or position of each trace, and termination imbalance are other sources. The overall balance in the system will never be perfect and will generally degrade as frequency increases. Therefore, the advantage of a differential transmission scheme versus a single-ended transmission scheme will disappear at higher frequencies. It is only necessary to maintain balance in the lengths of the differential trace pairs to the degree necessary to maintain sufficient balance at the maximum frequency where the balance wasn't degraded by other sources.

This topic is further explored and quantified in the publication referenced below.