How to Route Differential Pairs

Many guidelines suggest routing the two traces of a differential pair tightly together, implying they act as a single transmission line that references itself and does not need a continuous ground plane.

 

On a circuit board, this is not the case. Differential pairs don’t behave like twisted-pair cables. Here is where differential pairs are often misunderstood:

 

• Field Coupling: It is often assumed the electromagnetic fields of the two traces primarily interact with each other. In reality, most of the field coupling from each trace is directed to the reference plane directly below it.

 

• Return Paths: If the pair is properly balanced, no return current for the positive trace flows in the negative trace. A differential pair is simply two single-ended transmission lines carrying equal amplitude and opposite polarity. The energy for each signal is in the dielectric between the trace and the reference plane.

 

• Crosstalk Rejection: A frequent justification for tight coupling is that it rejects crosstalk. However, if an aggressor trace routes near a tightly coupled pair, it will couple more with the closer trace, and less with the further trace. This difference means the crosstalk is not fully rejected by the receiver. It leads to common-mode current and radiated emissions.

 

• Skin Effect: To maintain a target differential impedance while bringing the traces close together, the trace width must be reduced. Narrower traces increase skin effect losses at high frequencies, which can degrade edge rates and close the eye diagram.

 

Differential pairs do not need to be tightly coupled to function properly. The primary reason to route them side-by-side is to keep them in a similar environment and at a similar length.

 

Instead of focusing solely on the gap between the traces, ensure the reference plane below them is continuous. If a differential pair crosses a gap in the ground plane or changes layers without adjacent return vias, the fields will spread and radiate, just like any other single-ended signal.