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How do you protect a differential signal line (like Ethernet) without a common ground from high voltage or ESD? Do you use TVS-diodes from Tx+ to Tx-?

I thought of using TVS-diode arrays due to their low capacitance, but they all require a ground connection.

Any suggestions?

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2 Answers 2

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If you must protect the Ethernet cable side of the transformer, you simply might put a TVS between signals of the pair. That will protect the transformer from differential voltage within a pair. For PoE purposes, you might need ESD protection between the center taps on the cable side too, which means protection between pairs.

Typically you would protect the PHY chip side of the transformer which is referenced to local ground, and for that purpose a standard ESD protection with connections from all data pins to ground is fine.

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How do you protect a differential signal line (like Ethernet) without a common ground from high voltage or esd?

This is pretty much achieved by the magnetic interfaces at both ends of the cable. Some of the magnetic interfaces are rated up to 6 kV isolation. Of course there may be a little common-mode-to-differential surge transference but, this is largely minimized by keeping an impedance balance on the cable and, of course, the magnetic interfaces at each end are pretty-well balanced.

ESD and indirect lightning surges produce a common-mode voltage (for your information). The magnetic interface will transfer these surges through to the electronic components via parasitic capacitance (that isn't going to be more than a few pico-farads) and, it will do so fairly evenly on both connections. The energy level will also be quite low because of the small amount of parasitic capacitance.

But, should the need arise for some protection on the circuit-component side of the magnetic interface, then a small TVS on both lines could be used.

I cannot envisage any need for common-mode protection circuits due to the magnetic interfaces.

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  • \$\begingroup\$ Okay thanks, so that means if I don't have any magnetics a TVS diode between the data lines should do the trick \$\endgroup\$
    – VicTic
    Feb 4, 2022 at 13:20
  • \$\begingroup\$ If you don't have magnetics then you can't say they are ethernet lines and therefore you cannot assume they don't use a common ground. Different beast entirely @VicTic - Ethernet requires magnetics for precisely the reasons in my answer (and more). \$\endgroup\$
    – Andy aka
    Feb 4, 2022 at 13:22
  • \$\begingroup\$ Well I'm working on a device that I want to put between an Ethernet cable with the wires directly on the device without magnetics. That's why I was asking how to protect it from esd since there is no ground wire, just the 2 data lines \$\endgroup\$
    – VicTic
    Feb 4, 2022 at 13:26
  • \$\begingroup\$ It isn't ethernet then so, if you want to protect it, do exactly what ethernet does and.... use magnetics. Not using magnetics is not sensible if you want an ethernet-like system. \$\endgroup\$
    – Andy aka
    Feb 4, 2022 at 13:27
  • \$\begingroup\$ Yeah I can't put 2 more magnetics inside, that would damp the signal and it would not be compliant anymore. In this case I had to use some kind of amplifier \$\endgroup\$
    – VicTic
    Feb 4, 2022 at 13:32

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