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EMI / EMC

EMI = electromagnetic interference (the noise a board emits or picks up). EMC = electromagnetic compatibility (the board both behaves and tolerates its environment). Products must pass EMC testing to be sold β€” and even prototypes that fail EMC often self-interfere.

The model: source β†’ coupling β†’ victim​

Fix EMI by breaking any link in this chain:

  • Source β€” switching regulators, fast clocks/edges, current loops.
  • Coupling β€” radiated (antennas: loops & stubs) or conducted (through cables/power).
  • Victim β€” sensitive analog, the same board's own clock, or another device.

Reduce emissions at the source​

  • Shrink current loops β€” small loop area radiates far less (this is mostly a return-path job).
  • Slow the edges you don't need fast (gate/series resistors) β€” faster edges = more high- frequency energy.
  • Keep switching regulators' hot loops tiny and local.

Stop coupling​

  • Solid ground planes and continuous return paths (no plane splits under fast nets).
  • Filter conducted noise on cables/power: ferrites, common-mode chokes, Ο€-filters.
  • Shield where needed (cans over RF, shielded cables); ground the shield correctly.
  • Guard I/O and connectors with TVS/ESD parts and ground stitching.

Protect the victim (immunity)​

  • ESD protection on every externally exposed pin.
  • Filtering and good grounding on sensitive analog inputs.

Design-for-EMC checklist​

  • Small loop areas; solid return planes
  • No fast trace crosses a plane split
  • Switching regulator loops tight and shielded by ground
  • Edge rates only as fast as needed
  • Filtering + ESD on every cable/connector
  • Clocks routed short, away from board edges and I/O
EMC is cheap early, expensive late

Fixing EMI after a failed compliance test usually means a re-spin. Bake in loop control, grounding and filtering from the first layout.

See also​