Electromagnetic interference (EMI) refers to noise induced by any electric or magnetic source in PCB design. It’s deeply connected with problems in routing and grounding.
Caring about EMI-induced noise may be required by law, especially on how much radiation a device can emit/conduct/receive. We can solve interference problems with:
- Following key guidelines for routing and grounding in the board layout.
- i.e., don’t route over a split ground plane, except in rare instances.
- Enforce impedance rules, allow for proper spacing between traces.
- Redesigning the stack-up or applying proper grounding in the design. Sometimes bridging connections to the same ground is necessary.
- Filter circuits that are strategically placed (often at the power input or at the output of a supply/regulator circuit) can help reduce conducted EMI.
- Ferrite choke: placed around the outside of a power cord can filter out common- and differential-mode noise. Don’t use this on the output of a DC power regulator.
- Common-mode choke coil: can reduce one type of noise depending on what they’re designed for.
- LC filters: which allow noise to pass to ground instead of to other circuit components.
- Smart enclosure design that grounds the enclosure/board/exposed conductor.
- If the enclosure has metal, ground it to the same input power ground.
- Electrostatic discharge protection circuits.
Radiated EMI comes from time-varying electromagnetic fields. One device will emit some radio frequency energy, which is received by another device as noise. In design, we need to reduce the radiated EMI.
Conducted EMI can affect internal components. A key problem is electrostatic discharge, which essentially has a large current pulse on exposed conductors. So the idea is that these exposed conductors should be grounded.