EMC Question of the Week: April 7, 2025
A circuit board mounted on a metal chassis communicates with another circuit board mounted on a different metal chassis several meters away. The 100-Mbps differential communication signal is conveyed over a shielded twisted wire pair. If the metal chassis of the second board is not connected to system ground, the cable shield should be connected to the chassis of
- the first board only
- the second board only
- both boards
- neither board
Answer
The best answer is “c.” If both boards are mounted on a metal chassis, the cable shield should always be connected to the chassis at both ends. This would be the case regardless of whether one, both, or neither chassis was connected to system ground. The only exception to this rule would be if making a DC connection to the chassis at both ends presented a safety issue. And even in these situations, a high-frequency connection at both ends would be required.
Of course, if the shield carried intentional or significant unintentional signal currents, it would also need to be connected to the board EMC ground at both ends. And, if either board had no chassis, the shield would simply connect to the printed circuit board EMC ground at that end.
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