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I have a couple of questions regarding the Basics of ESD.

Usually, when we employ ESD protection, say placing a TVS diode on the signal line, the other end of the ESD diode terminal:

a) Will be connected to the ground if the signal is a single-ended signal. b) If the signal is a differential signal, should the other end of the diode should be connected to the other signal pair of the differential signal or should the other end of the diode has to be connected to ground? What happens if connected to other signal pair or ground?

Can someone clarify this point?

Also, is there a concept of differential mode ESD protection and common mode ESD protection (similar to common mode and differential mode noise?)?

My basic understanding question is that when an ESD hits our PCB through a connector pin, should the ESD energy be dissipated through our board ground or is it better if the ESD energy is made return back to its source? Can it be returned back to its source?

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  • \$\begingroup\$ That would depend if the ESD event is between the two data pins of differential signal, or from one signal to ground. It also depends how the chip you are using handles ESD, and it depends also if you are using e.g. Ethernet which has a transformer. \$\endgroup\$
    – Justme
    Apr 17, 2023 at 10:12
  • \$\begingroup\$ @Justme, thank you for your comment. Actually I am using an Ethernet application. But I thought my question was more generic and hence didn't include the ethernet. Could you please elaborate a bit more? \$\endgroup\$
    – user220456
    Apr 17, 2023 at 10:14
  • \$\begingroup\$ That's why there can't be a generic answer because it always depends on what you are trying to protect and from what. Do you want to protect the chip, or the transformer? And there are several Ethernet ESD protection application notes from multiple vendors, have you consulted what they say? \$\endgroup\$
    – Justme
    Apr 17, 2023 at 10:16

2 Answers 2

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The ESD current usually finds the shortest path to GND. If such a path does not exist then it takes any path that is available, which could also be a destructive path for our circuitry.

The intended ESD strategy should be that the charges find the easiest way to GND such that it doesn't find a path through our internal circuitry. We also cannot always ensure a safe path to the source of the ESD pulse which keeps varying (ESD sources could be e.g. humans, metals, etc). But to the GND we can always strategize well enough such that the ESD current gets rightly taken away.

The suitable approach would be to place a separate ESD protection on each of the terminals (positive and negative) to GND.

Also in this document there are some conceptual ESD protections discussed for positive and negative pins, https://www.ti.com/lit/sszb130.

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  • \$\begingroup\$ That's wrong because in Ethernet you can't put a TVS to ground on the cable side because it is required to be isolated with a transformer. It's not ground-referenced. Except on the PHY side. \$\endgroup\$
    – Justme
    Apr 17, 2023 at 10:18
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    \$\begingroup\$ Ah okay. I do not know about this specific use case and also it was not mentioned in the question. \$\endgroup\$ Apr 17, 2023 at 10:23
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    \$\begingroup\$ Please don't delete your answer; I think it generally covers most applications as far as I can tell. The point being that ESD can be regarded as a threatening voltage source relative to earth/ground so, if you have floating circuits (as per ethernet that uses transformer isolation on signals), they tend to be less susceptible to problems. But, you can get a differential glitch coming through the transformer which may require protection on the PHY side. \$\endgroup\$
    – Andy aka
    Apr 17, 2023 at 10:30
  • \$\begingroup\$ Understand. Okay, I'm keeping it. \$\endgroup\$ Apr 17, 2023 at 11:35
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The idea of ESD is that it finds the shortest distance to discharge to the ground. So placing a TVS diode over a signal line should be in parallel to the signal line with the cathode side to the ground of the circuit.

There are two types of TVS diodes as well, unidirectional (usually for positive voltage) and bidirectional (if positive and negative voltage is used). So depending on your circuit, the TVS diode is applied accordingly.

Regardless of single or differential, each point of entry should have a TVS diode parallel to the ground of the circuit.

The ESD will not return to its source but try to discharge to the ground and dissipate through the process. So the ground plane in the circuit/PCB is important.

Hope it helps.

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