EMI/EMC Academy ~35 min Interactive

Fixing Susceptibility at the Source

Stop susceptibility at the victim: filter analog inputs, shorten high-impedance nodes, fix references and layout, and shield where needed.

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The highest-value fix is usually a small low-pass filter at the susceptible input: a series resistor/ferrite plus a capacitor to a clean reference rolls the RF off before it reaches the nonlinear junction that would demodulate it. On op-amp and ADC inputs, a modest RC (or ferrite+cap) often eliminates the failure outright.

Place the filter cap's ground return right at the device reference with minimal loop — a filter referenced to a noisy ground just re-injects the disturbance.

Shorten & guard. Keep high-impedance nodes short, guard them with ground, and avoid running them near connectors/cables where RF enters.

Clean references. Give sensitive analog stages a solid, quiet reference; RF on the reference is indistinguishable from RF on the signal.

Where filtering isn't enough, add local shielding over the sensitive stage (a can bonded to ground) — the immunity counterpart of the shielding techniques in the Pre-Compliance course. Bond the shield properly, or it can make things worse.

Common mistakes: adding a filter cap but returning it to a noisy ground; shielding a stage but leaving its cable unfiltered (the RF just rides in on the wire); or increasing gain to ‘overcome noise’, which also amplifies the demodulated disturbance.

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