Whitepaper

Passing EMI Compliance Testing the First Time—Part 1: The Physics

Published by Analog Devices

This technical article from Analog Devices takes a physics-first approach to understanding EMI, aimed at helping engineers pass electromagnetic compatibility compliance testing on their first attempt. Rather than offering a checklist of layout rules, the article builds understanding from fundamental electromagnetic principles. It explains that EMI results only from accelerating charges, and that the magnetic field is fundamentally a relativistic manifestation of the electric field. Using Maxwell's equations and Poynting's vector, the article shows how changing electric and magnetic fields can propagate energy through free space — which is the mechanism behind radiated emissions. The core insight is that mitigating EMI is about controlling and confining electromagnetic fields so they cannot expand into free space or exert forces on nearby circuits. This confinement happens naturally when a ground plane is placed close to signal and power paths, forming a transmission line structure where Faraday's law induces cancelling currents in the ground plane, nulling external magnetic fields. The article distills this physics into three good practices: strive for zero net accelerating charge everywhere on the board, keep electric and magnetic fields confined to dedicated small spaces, and think in terms of electromagnetic fields rather than voltages and currents. It also previews common PCB layout situations where field confinement is lost, including layer transitions, parallel signal routing, field fringing, and shared ground plane crossings — topics to be covered in Parts 2 and 3 of the series.