EMI Bites: How a Simple Voltage Drop in the "Ground" Plane Turns Into Radiated Emissions
It might not look like much, but a small impedance difference in your layout can lead to serious EMI problems.
Here’s what happens:
- At the point where impedance changes, you get a voltage drop.
- That voltage drop in the Return & Reference Plane (RRP) is now a source of noise.
- This is now connected to the whole "grounding" structure.
Why this causes problems:
The voltage drop becomes a common-mode noise source.
Once you connect a cable to the Return & Reference Plane (AKA "ground"), that cable becomes an extension of your board.
The result? An antenna—radiating the noise you just injected into the RRP.
This leads to radiated emissions that can easily push your design out of EMC compliance.
Key Insight:
Impedance mismatches + poor "grounding" = common-mode radiation.
How to avoid it:
- Keep impedance consistent at all critical interfaces—no abrupt changes.
- Ensure clean, low-impedance connections to the return reference plane.
- Treat every connected cable as a potential antenna; make sure to contain the noise before it couples.
- Review your layout for impedance discontinuities and fix them early.
To electromagnetic enlightenment,
- Dario
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