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I have GNDC (chassis GND) and GND (digital GND) on my PC motherboard design.

All connectors in my design (RJ45, USB-TypeC, USB-TypeA, HDMI, DP, etc.) are connected to the chassis through the GNDC plane through the mounting holes (see last picture below.)

During my work, I've looked at previous designs and I saw the following connection between GNDC and GND (on PCB):

enter image description here

Question 1:

Why is this circuit useful? Should I keep it in my PC motherboard design?

Here is the circuit for my DC jack power connector circuit:

enter image description here

Note: The N-MOSFET is for reverse polarity protection.

Question 2:

Is the way I connected GNDC to the drain of the N-MOSFET and GND on the source with the ferrite beads inbetween acceptable?

As far as I kniow, the the chassis shouldn't have any current pass through it. How does this GNDC connection guarantee that current goes only inside the DC jack connector and not to the outer chassis itself, which might cause safety hazards to the consumer?

These are the mounting holes that connect to chassis and the GNDC plane:

enter image description here

Question 3:

I understood that GNDC should be the cleanest ground reference on the board, and digital GND is the one with all the "garbage/noise". First of all, is this statement true? If the answer is "yes," to what this clean GNDC is a good reference for?

I also got advice from an experienced co-worker that GNDC plane should not be close to any other plane, especially GND so that it will not absorb all the noise coming form near planes, like GND and power. The preferred way is to make GNDC shared between all layers, to prevent overlap.

My layer stackup is eight layers. I have layer 2: GND, layer3: signals, layer 4: 3.3V and I have GNDC connection shared between all layers, except a plane on layer 3 (see orange rectangle,) which is just on layer 3.

This means that this plane selected in orange can be affected by noise from layer 2 and 4. What harm can this do, and how critical is it?

I can remove this connection plane and just leave the connection to the lower mounting hole.

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  • \$\begingroup\$ "why is this circuit useful? should I keep it in my PC motherboard design?" I would imagine because you don't want noise from chassis into the signal ground plane. But it will be far more important where you connect those grounds together physically. Standard practice is to do so near where the supply ground enters the PCB but obviously the specific mechanics for the certain project has to be taken in account. \$\endgroup\$
    – Lundin
    Commented Mar 23, 2023 at 11:13
  • \$\begingroup\$ "I understood that GNDC should be the cleanest ground reference on the board, and digital GND is the one with all the "garbage/noise". First of all, is this statement true?" Yes and no. In case of radiated susceptibility, the noise goes to the chassis ground and should be kept away from signal ground. In case of radiated emissions, the noise comes from your signal ground. All EMC concerns tends to go in both directions. \$\endgroup\$
    – Lundin
    Commented Mar 23, 2023 at 11:18
  • \$\begingroup\$ @Lundin I am a little confused about the ferrite inductors/beads between the two grounds, and then a capacitor between them as well... I'm guessing the capacitor connects their common point, while the inductors prevent HF noise elsewhere along the line. \$\endgroup\$
    – Edin Fifić
    Commented Mar 23, 2023 at 11:23
  • \$\begingroup\$ On those previous designs, have you checked the bills of material to see what actual parts were fitted Firas? \$\endgroup\$
    – Andy aka
    Commented Mar 23, 2023 at 12:06
  • \$\begingroup\$ In those designs, the only way not assemble parts is to place DNP (Do not populate) in the part properties. and in these designs the DNP wasn't entered, which means they were assembled for sure (talking about ferrite in parallel with the 1000 pf) \$\endgroup\$
    – Firas Abd El Gani
    Commented Mar 23, 2023 at 12:42

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