How to best terminate D-SUB connector shield and best way to do board layout for it

Hi all,

We are designing a product in plastic enclosure with 24V DC supply. Supply terminals are 24V+, GND and possibly a third terminal for a "functional earth" FE. The product will have two opto-isolated CAN communication interfaces which are using the standard D-SUB 9 connector.

How should the connector shield be connected:

Possible options are:
- to a separate "frame ground" potential which is connected to the FE terminal
- to supply GND
- to isolated ground ISOGND
- one of the above but through a RC network of 1M/10nF
- or a combination of above

And what sort of plane or ground pour (if any) should be around/under the connector I/O area?

Possible options are:
- no plane and ground pour around connector I/O area
- fill and ground pour connects to ISOGND potential
- fill and ground pour connects to GND potential
- fill and ground pour connects to SHIELD potential
- fill and ground pour connects to FE potential

Always tricky!

But consider what each 'ground' is doing.

The shell of the connector is screening your circuit and the wires, so keep as much as possible any of this connection away from your electronics.
Use a short braid between the shields and possibly to any screening on your plastic case. This shield (in my view) should be connected to your FE to give a route for all picked up noise and interference to a ground.

Your circuit is now capacitively couple to the earthed shield and will pick up interference via this capacitance, so connect your 0V to this such that the electronics and shield are seeing the same signal and it is not coupled into your electronics. As there should be no dc current in this connection and RC connection as suggested should be a good one, which will prevent ground loop currents.

I'm not an expert, but have investigated this type of issue in the past and this method does 'seem' to work. It certainly helps the thought process of what is happening and what you are trying to achieve.