How to Handle Reference Plane Transitions in high-speed differential pairs: Keeping Return Currents Happy

High-speed differential pairs, such as those found on HDMI lanes are more than just tightly coupled traces. At multi-gigabit speeds, the return path under those traces is just as important as the signals themselves. Ignore it, and you’ll end up with eye diagram collapse, EMI headaches, and compliance failures.

So, what Makes a Strong Reference Plane?

A strong plane provides a low-impedance path for return currents right under the signal. Think of it as a mirror of the trace, keeping electromagnetic fields contained.

• Continuous copper: A solid ground plane without gaps or splits.

• Directly bonded: Ground planes tied together with stitching vias keep them at the same potential.

• Close coupling: The plane should sit directly below the HDMI pair in the stackup to minimize loop area.

  
  

With a strong plane, return currents flow naturally, hugging the path under the signal.

What Makes a Weak Reference Plane?

A weak plane forces the return current to detour, which breaks the “tight coupling” between signal and return. That creates radiation and impedance bumps.

• Plane splits: If your HDMI pair crosses a gap between planes, the return current has to find another way back — often far away.

• Switching planes without stitching: Dropping a signal to a new layer without nearby ground vias strands the return current.

• Referencing power instead of ground: At high speed, power planes aren’t quiet references; they carry noise from regulators and load transients.

  
  

Weak planes don’t just hurt signal integrity — they turn differential energy into common-mode EMI.

Sometimes you can’t avoid swapping layers. When you do, always remember to:

• Add stitching vias: Place multiple ground vias tight to the signal vias (within ~1 mm) so the return current can jump planes directly.

• Keep symmetry: Both lines in the pair must use identical via structures and reference conditions.

• Back-drill vias: Remove unused via stubs to avoid little “antennas” hanging off the line.

  
  

So remember; An HDMI differential pair is never just two traces — it’s traces plus the return path in the plane below. Strong planes are solid, continuous, and grounded. Weak planes are split, noisy, or poorly stitched. Design as if the return current is another trace you have to route. If you keep it happy, your HDMI link will pass compliance with wide-open eyes.

References:

Texas Instruments - HDMI Design Guide. https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-138-01-00-00-10-65-80/Texas-Instruments-HDMI-Design-Guide.pdf