EMC Question of the Week: March 9, 2026
A shielded cable connects to a circuit board mounted in a plastic enclosure. To minimize EM emissions and maximize EM immunity, the cable shield should generally connect to the circuit board ground planes
- directly
- through an RC filter
- through a CM choke
- through ferrites
Answer
The best answer is “a.” Cable shields serve two purposes. They block fields created by common-mode currents on the inner wires and return those currents to the board. They also block external fields which puts a common-mode current on the external surface of the shield. The shield can't serve either of these purposes effectively if it doesn't make a good high-frequency connection to the board's EMC ground.
If the common-mode currents induced on the interior of the shield are forced to flow through any impedance on their way back to their source, a common-mode voltage will be developed that drives the cable shield relative to the board. If externally induced currents are blocked from reaching the board ground, a voltage will develop between the cable shield exterior and the board. This voltage will drive the wires inside the shield and induce common-mode currents on those wires.
Of course, shielded cables work best when they connect devices that have a metal chassis or a shielded enclosure. In this situation, the cable shield should always make a good connection to the chassis or enclosure. If the shield is returning significant common-mode currents to the board, it also needs to make a good connection to the board.
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