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I have developed a board based on STM32F207ZG MCU with Ethernet interface using:

  1. LAN8742A
  2. TVS diodes device for differential pair signaling
  3. Magnetic-transformer device
  4. Hybrid circular connector from Phoenix contact for Ethernet hybrid cable hybrid cable link

This PCB will be inside a metal case. Connector is panel mount and it is in contact with the metal case and with the PCB, through shielding pins.

As you can see, hybrid cables have Cat5 wiring standard (for 100Base-T comms.), with 4 Ethernet signal wires, 4 more wires for power and signal and metal net for shielding. Cat5 wires have a ground net connected to the ground of the entire cable and these two ground nets are connected to the circular connector shield.

Hybrid wire shielding scheme

When using an RJ45 connector it doesn't bring either drive power lines. I have in mind that common grounding way suggestions could be that one, where clearly ST is applying to its NUCLEO development boards:

RJ45 grounding typicall scheme

ST Nucleo boards is connecting the RJ connector with the magnetic transformer in a way where ground of RJ shield is not connected to any GND point present on the board by default, (but they let the possibility for connecting it to GND point).

But my circuit hasn't RJ connector. When using this kind of sollution, both cable and connector drives power lines too. PCB circuit gets the voltage from a power bank, 24V DC, arriving with Ethernet signal by this cable to the PCB. The power stages has two voltage conversions:

  1. 24V-5V
  2. 5V-3.3V

The two DC conversion steps have the same ground connection (a GND polygon I called AGND). But I separated this power GND from digital GND (a second GND polygon I called GND) using ferrite beads.

power conversion steps scheme

Now I'm wondereing how to ground the connector shield? What could be the best strategy?
Connector power ground pin is now connected to analog/power ground, and connector shield is connected to digital ground and metal case:

ethernet and circular connector scheme

I have now doubts about 3 ways of doing it. What is the difference between these 3 types of shield connection?

  1. connector shield connected to the analog GND, called AGND on my circuit,

  2. shield connected to the digital GND. The last way, case will be also connected to digital GND (GND polygon after ferrite beads),

  3. shield and case not connected to any GND point of my board. So, let the chasis (metal enclosure) connected only to the connector shield.

NOTES:

*Despite it is a prototype for comercial purposes, this circuit doesn't need to pass any EMC tests. I only want to take profit from cable ground net and to gain some noise immunity (signal integrity) connecting it correctly to the ground.

**This PCB has built having in mind two connectors for connecting some input/output signals (3 & 3) incoming by some kind of wire, from a switch button, allocated on metal case hole. MCU should be able to read low level present on this input pin. Output pins will put low level by the activation of N-MOSFETs.

***Furthermore, inside the metal case there is an enconder sending differential quadrature signals to other connector placed on PCB.

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  • \$\begingroup\$ Your question is regarding grounding, particularly connector grounding. To answer such a question, we need to know how that grounding interacts with the assembly itself (does it have a metal enclosure? what other connectors are on it? are they grounded/shielded as well?), and with the things it's connected to (same details). Finally, EMC test levels (emissions/susceptibility) would be nice to know, or at least rough product class (commercial, industrial, automotive, aerospace, etc.). \$\endgroup\$ Dec 13, 2023 at 11:20
  • \$\begingroup\$ Hi @TimWilliams, as I said, PCB is completely inside a metal case. Connector is touching the metal case and the PCB. As this connector has no other possibility for subjection (I did them connected to GND, but I can remove connection in further versions if needed). Metal case has also plated drills for PCB subjection. Ethernet cable wires goes to cable mpunt connector towards an adapter pcb with an RJ45 connector, that is not designed yet. But it will connect a switch device for being connected to a personal computer (client host). \$\endgroup\$
    – Suvi_Eu
    Dec 13, 2023 at 12:01
  • \$\begingroup\$ So there are no other connectors on the PCB or enclosure, this is the only one? \$\endgroup\$ Dec 13, 2023 at 12:09
  • \$\begingroup\$ @TimWilliams give me some time. I will try to add data on my question post regarding you are asking for. \$\endgroup\$
    – Suvi_Eu
    Dec 13, 2023 at 12:15

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I found this interesting post about 3 typical options for grounding ethernet circuitry post link. I can see there, a clear explanation about the nucleo board design strategy, a typical and common one.

Althought it's not a detailed and specific solution for my circuit, the post is answering the question I made.

I think option 2 fits with my circuit but I'm not sure if it works until pcb will be manufactured and it passes the final test.

separated grounds option with bob smith configuration

Should chassis and system ground be connected (option 1)? I don't really know. But I probably let a solder bridge for connecting 0 Ohms resistors, ferrite or any other component for joining ground planes the best way if needed. I even could connect chassis ground to system ground by connecting system ground to the chassis in a place close to the PCB power input.

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