EMC Question of the Week: January 1, 2024
When a circuit board with a ground plane is mounted to a metal supporting structure, the ground plane and the supporting structure should almost always be connected
- directly
- through capacitors
- with a large resistor
- at a single point
Answer
The best answer is “a.” It is generally important to have a good connection between a circuit board's ground plane and any metal supporting structure. Unacceptable conducted and radiated emissions may occur if the structure is at a different high-frequency potential than the circuit board's ground plane (and everything referenced to it). Also, in order to comply with many immunity requirements, noise currents coming onto the board on an attached cable need to be routed directly to the supporting structure through a low impedance. Making a good connection between a circuit board's ground plane and the supporting structure near every I/O connection is a key part of ensuring EMC compliance.
Of course, there are some situations where a direct connection to the supporting structure is not allowed. For example, some (but not all) medical and aerospace products will not permit any connections that would allow a low-frequency current to flow on a chassis. These products will often employ a capacitive connection between circuit board ground and chassis ground, but this can be troublesome and should only be done when absolutely necessary. Capacitive connections typically have higher impedance, can introduce resonances, and are less reliable than direct connections to the ground structure. They also add cost and reduce the available space on a board.
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