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I am connecting the chassis ground of the RJ-45 connector to the board's digital ground.

In that case, I saw an Evaluation board design that uses a 1000pF 2kV cap to connect both the grounds.

My question is instead of that capacitor, can I use a ferrite bead? If so, can someone tell me a safe range of values?

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    \$\begingroup\$ This depends on what is the purpose you want to have. Do you want the connector chassis to be AC coupled or DC coupled? It also depends on if you have a metal chassis which is anyway connected to both connector and signal ground. And it also depends on what the connector is for. RJ-45 is not really a connector, and actually RJ-45 as a wiring standard does not even exist. If you mean e.g. Gigabit Ethernet interface with 8P8C connector, then edit that and other information in the question. Sometimes the cap is just useless if the connector metal shell already is connected to device metal chassis \$\endgroup\$
    – Justme
    Jan 5, 2023 at 18:16

3 Answers 3

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Typically, the capacitor is used to ensure that testing carried out on a design to EN 61000-4-5 (indirect lightning surges) will be passed. During testing a surge will be applied to the other end of RJ45 cable (via its shield) and, the kV surge (generated by specialized test equipment) will be significantly attenuated by the capacitor.

Actually the way the surge is applied is a little more complicated than what I said but, in essence, my previous description is good enough for this question.

This then limits voltages on the inner conductors AND makes the devices at both ends less prone to latch-up or over-voltage failure. The capacitor is also useful for ESD in some applications although, it's main use is against indirect lightning surges.

My question is instead of that capacitor, can I use a ferrite bead?

No, most unlikely to be of any use. Does a ferrite bead have a 2 kV rating (no it doesn't). It needs to be a capacitor. An FB or inductor doesn't even come close to satisfying the needs.

Additionally, the a shield connected via a ferrite bead would also provide an earth path/loop from one piece of equipment to another rendering the ethernet comms (if that's what the RJ45 cable is used for) to interference from potentially tens of amps of current flowing instead of through the normal earth protection wires for equipment.

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    \$\begingroup\$ Thank you for your answer! So, ferrite bead is not a replacement for a capacitor for this application. \$\endgroup\$
    – user220456
    Jan 5, 2023 at 18:04
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    \$\begingroup\$ It's not even close to doing what the capacitor does @Newbie \$\endgroup\$
    – Andy aka
    Jan 5, 2023 at 18:04
  • \$\begingroup\$ But one thing I noticed is that, let me show you. ww1.microchip.com/downloads/en/DeviceDoc/…. On Figure 4-1 of this document, a ferrite bead is used right? I also see a capacitor. But why ferrite bead? \$\endgroup\$
    – user220456
    Jan 5, 2023 at 18:06
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    \$\begingroup\$ There is an FB but that is between earth chassis and ground and not from the RJ45 shield connections. The capacitor shown is from the shield (or unused) connections. \$\endgroup\$
    – Andy aka
    Jan 5, 2023 at 18:09
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    \$\begingroup\$ The ferrite is on the ground side of the capacitor and ties the chassis and system grounds together while preventing noise from coupling between them. \$\endgroup\$
    – vir
    Jan 5, 2023 at 18:10
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I think you have confusion over what's being grounded, or why.

10/100 Mb/s Twisted Pair Interface

(Image source: Texas Instruments - AN-1469 PHYTER Design & Layout Guide SNLA079D)

There are three commons (not necessarily "ground") in a typical Ethernet connection: PHY / digital / main circuit ground (show above as the three-line "ground" symbol); chassis or shield ground (shown as the rake-like "earth" symbol); and the Bob Smith termination "ground" (common) formed by the resistors.

The capacitor in question is only used to connect the BS common to an appropriate sink for ESD current. For equipment in a metal chassis, this will most likely be the chassis itself. For plastic enclosures, this will probably be the PCB ground plane.

Note the above shows 0-ohm jumpers, which can be removed if circuit-chassis isolation is required. In that case, most likely the 1500pF capacitor should be returned to chassis, not circuit GND. (With the jumpers in, obviously it doesn't much matter.)

The purpose is to close the loop as short as is possible. ESD comes in from outside, not necessarily on wires at all, but also as displacement current to nearby metal (chassis, ground plane, cabling), or waves entirely. The shortest loop you can close on that, is back to the same things: chassis, ground plane, cabling. If circuit ground is inside a chassis, returning the capacitor to that would needlessly invite ESD inside the enclosure, partially defeating the purpose of the enclosure(!).

Another point against returning the cap to DGND: since you mention "digital ground" -- I assume making the distinction to general circuit, signal or power ground -- that implies DGND may be noisy (due to digital signals running across/into it). While there is a separate conversation to be had about the merits of such a segmented-ground design, the relevant aspect here would concern noise on the ground with respect to chassis or cabling. Point in case, putting a capacitor here would literally couple noise from that plane directly into external connections, making a big emissions hazard.

Ferrite Bead

As for ferrite beads -- you might use one when you need DC grounding between circuit and chassis grounds, but need AC isolation, or some impedance between them anyway (as opposed to a very low impedance from jumpers, or direct bonding). This might be relevant when you have other connections, daughterboards say, in a larger system, and there's some amount of common mode noise sloshing around between them, and no single board really has a good "ground" basis with respect to the enclosure around it. But this probably isn't a very common situation, because all those cards will be dropping DC across the same connections, so the common mode isn't limited to AC alone. And a few ~mV of DC will be enough to saturate a ferrite bead (again giving a low impedance); that is, if the FB itself doesn't melt from excess current! Generally, multiple current return paths is not a good plan, especially in systems like that. Keep loops small, both at AC and DC. In that case, probably an R+C network is all the bypassing you'd want to use, between circuit and chassis grounds, when you need to make a connection between them at all.

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Obviously a ferrite bead is never a useful replacement for a capacitor..One could argue that they indeed fulfil opposite roles in some regards.

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  • \$\begingroup\$ I guess it depends on situation. For example an Ethernet board may have multiple ways of grounding different parts of the system, such as metal chassis, connector shell, and digital signal ground. Sometimes you may want DC connections between them, sometimes you want AC connections, or a mixture, and be able to change the connections depending on if the board will be used with a metal case or plastic. But the question is a bit unclear and thankfully some manufacturers have appnotes how to select a method. \$\endgroup\$
    – Justme
    Jan 6, 2023 at 11:07

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