EMC Question of the Week: October 30, 2023

Differential trace pairs routed partially over a ground plane and partially without a ground plane

A differential signal is routed on a pair of traces 0.25 mm over a ground plane for a distance of 5 cm. The pair continues for another 5 cm beyond the edge of the ground plane. At the point where the ground plane stops, the differential signal experiences a change in   

  1. characteristic impedance
  2. electrical balance
  3. DC voltage
  4. all of the above


The best answer is “a.” The characteristic impedance of a typical differential trace pair is significantly influenced by a solid plane on an adjacent layer. If the ground plane is lost, and the geometry of the trace pair doesn't change, then the characteristic impedance of the pair will change.

On the other hand, the electrical balance of a pair of identical traces does not change when the ground plane is lost. Two identical signal traces that are the same distance above a ground plane are perfectly balanced. Two identical signal traces that are far from any other conductors are also perfectly balanced.

The presence or absence of the ground plane also has no effect on the differential DC voltage, which is mostly determined by the source and the load.

Note that most pseudo-differential sources put a significant amount of common-mode voltage on a trace pair in addition to the differential signal voltage. For the common-mode component (whose high-frequency current was returning on the ground plane), the loss of the ground plane has a profound effect on the electrical balance. In this case, the common-mode voltage effectively "drives" the trace pair relative to the ground plane like a monopole antenna at the point where the plane is lost. For this reason, it is usually very important to provide a continuous ground plane beneath trace pairs carrying high-speed signals generated by pseudo-differential sources.  

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