Bob Smith chassis connection for plastic enclosure?

[philby]

I am designing an ethernet product that will be fitted into a plastic housing.

Until now, I’ve always had a metal enclosure to treat as chassis ground and connect my bob smith termination to through a 1nF 2kV capacitor.
 
I’m having trouble finding any application notes on what to do with this termination for plastic enclosures.

I’ve found these two application notes that suggest the high voltage capacitor “helps in filtering the noise residue managing to leak out through the transformer” and “provides a discharge path on the unused pairs”.

https://www.ic72.com/pdf_file/l/151743.pdf
https://ww1.microchip.com/downloads/en/Appnotes/VPPD-01740.pdf

Connecting the 1nF 2kV capacitor to my 0V signal plane seems the wrong thing to do, but the first application note above suggests “A chassis ground plane can be added to the layer stack. Place this plane directly next to a ground plane to create a very tight capacitive coupling between the two planes”.

Any thoughts or advice would be much appreciated.
Kind regards
Phil

[Siwastaja]

You can connect the "chassis GND" to your ground plane but then you lose the galvanic isolation which is one of the big advantages of Ethernet, IMHO. Raspberry Pi does this.

I had similar situation (100M ethernet, plastic case) and just left the chassis ground as its own small region under the connector, and quite obviously, with just plastic enclosure, it does not connect anywhere further. Was first-time pass in emissions and immunity, radiated and conducted and ESD. With 2.2ohm series resistors for edge rate control and 100M Ethernet compatible ESD TVS array. Did not test without these components.

I connected CHASSIS_GND to GND using two of the 2kV MLCCs just to reduce impedance.

Note the possibility of confusion: the bob smith termination network in itself contains a 2kV capacitor to CHASSIS_GND. This does not mean it has to be the only 2kV MLCC in the system. You can further connect CHASSIS_GND to GND with another 2kV MLCC, maintaining functional insulation (DC) between the grounds.

[philby]

You can connect the "chassis GND" to your ground plane but then you lose the galvanic isolation which is one of the big advantages of Ethernet, IMHO. Raspberry Pi does this.

I'm sorry but I don't follow. If I connect the 2kV cap to my 0V plane rather than to what would normally be my chassis ground, how is the galvanic isolation lost? Are you assuming/suggesting that the RJ45 shield would also be connected to 0V plane? I was just going to leave the connector shield unconnected.

I had similar situation (100M ethernet, plastic case) and just left the chassis ground as its own small region under the connector, and quite obviously, with just plastic enclosure, it does not connect anywhere further. Was first-time pass in emissions and immunity, radiated and conducted and ESD. With 2.2ohm series resistors for edge rate control and 100M Ethernet compatible ESD TVS array. Did not test without these components.

Thanks for the insight. My feeling has been that it is not really necessary that that kind of confirms it.

I connected CHASSIS_GND to GND using two of the 2kV MLCCs just to reduce impedance.

Note the possibility of confusion: the bob smith termination network in itself contains a 2kV capacitor to CHASSIS_GND. This does not mean it has to be the only 2kV MLCC in the system. You can further connect CHASSIS_GND to GND with another 2kV MLCC, maintaining functional insulation (DC) between the grounds.

I have been considering that too.