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Q10068 Grounding and isolation on BNC connectors

Tags: Help, hardware, input, signals, connectors, BNC, enclosures, grounding

Applies to: MSXB009, MSXB010, MSXB037, industrial enclosures

BNC connectors come in two varieties, isolated and non-isolated. Which kind should I get for my signal terminations?

BNC connectors are a compromise no matter what you choose. They expose their return-side connections to the outside world, leaving you only two choices:

  1. connect the sheath to a chassis ground, using that as your signal return path, vulnerable to ground loops and bringing ground noise into proximity with your signal paths,
  2. connect the sheath to a signal ground, where interference has a potential to couple to other signal inputs through the weaker ground connection.

Despite the deficiencies, BNC connectors are very common and hard to avoid.

Manufacturers usually call the BNC connectors isolated or non-isolated, but this can be confusing. Whether the signals are really isolated or not depends on how they are connected. We use the terms insulated or non-insulated. With the non-insulated connectors you have no choice, because the sheath of the cable is routed directly to the chassis plate through the connector itself. In DAP systems, the external enclosure or industrial rack is connected through the shield of the DAP cable to the common DAP and PC host ground, so in effect, the signal ground of the BNC cable becomes one and the same with the power supply ground. There are certain applications with strong grounds and high frequencies, designed for this configuration, and then holding the sheath potential firmly at this ground is an advantage.

That situation is a rarity however. For this reason, termination boards for DAP products use insulated BNC connectors. An insulation barrier separates the connector from the enclosure panel, and separate pins route the axis and sheath conductors inside of the enclosure. To use insulated connectors, connect the axial conductor to the input termination pin, and the sheath connection to the corresponding signal ground reference pin. You give up nothing by having insulated connectors — if you have a special need, you can always strap the sheath connection to the chassis plate at the connector, giving you the same results as a non-insulated connector.