EMI Bites: The Danger of Crossing a Split in the Reference Plane
Here’s a classic layout mistake:
A signal trace runs across the board—but the reference plane underneath is split in the middle.
What happens next?
The signal loses its return path, and the electromagnetic fields start to spread into the cavity between the two planes.
Simulations clearly show this:
Field lines burst outward, looking for a return path—and that’s when the signal starts coupling into unintended structures.
Why this causes problems:
- The split breaks the return current loop, forcing the fields to spread laterally.
- This creates a resonant cavity between the planes.
- At certain frequencies, the signal can leak out of the board, becoming a source of radiated emissions.
- Both signal integrity and EMC are compromised.
Key Insight:
Don't let a signal cross a plane split, what you save in routing, you’ll lose in compliance.
How to avoid this issue:
- Keep reference planes solid and continuous under all signal traces.
- If a split is unavoidable, use stitching capacitors or vias to maintain the return path.
- Plan your stackup and routing together to avoid split-plane crossings from the start.
- Especially with high-speed designs, validate your layout with field simulations to catch hidden cavities.
Hope this helps,
Dario
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