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In our design we've used a parallel RC circuit for chassis ground and digital ground isolation

The resistor value is 100 Ω. The capacitor is 1000 pF 4 kV.

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I was told that it's basically for "EMI, EMC and ESD protection".

Why exactly is it used? How does the the parallel circuit precisely work and help in preventing ESD and EMI/EMC noise?

winny
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Hari
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  • It might depend what your product is, what are the vulnerabilities of your circuit and, what the EMI threats are. – Andy aka Mar 04 '23 at 17:03
  • The exact and detailed reason would be known by the designers you asked. Without knowing anything about how the circuit looks like, what other connections it has and where, and it having a metal or plastic case, or grounded or ungrounded power supply etc, we can only say that according to your designers, that's the best way to connect things, or at least, good enough to pass official EMI/EMC tests, so it might just be the least worst way to do it. – Justme Mar 04 '23 at 17:03
  • What is 100E? Is it a typo? – RussellH Mar 04 '23 at 17:44
  • 100 ohm,it's a typo – Hari Mar 04 '23 at 18:14
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    @RussellH - Hi, Re: "*What is 100E? Is it a typo?*" There is some info in [this Q&A](https://electronics.stackexchange.com/questions/251598/what-does-it-mean-that-a-resistor-has-value-of-56e). – SamGibson Mar 04 '23 at 22:57
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    @SamGibson: Thanks. 50 years in the business and I never came across it. – RussellH Mar 05 '23 at 02:23

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The circuit is a standard solution to improve ESD discharge robustness of your board/device. A metal connector housing is the most likely area for an ESD discharge either directly by touching or indirectly via the connected shielded cable. By coupling the connector housing via an parallel RC circuit and not directly to PCB ground you limit the peek of ESD energy injected into your PCB during an ESD event so improving your PCB ESD robustness. And because of the capacitor you still have good hi frequency coupling of your connector housing and connected shielded cable to ground so your shielding still performs its function. Its similar in the usage in PCB ground connection to device chassis ground, to on one side provide coupling for EMC shielding and on the other hand to reduce ESD discharge energy to your PCB in an ESD event comparing to direct connection. Sometimes its also only used as capacitor only especially when you have an AC connected device where you don't want no DC coupling at all...

So in short for a simple explanation, but as always you can make more science out of it, end there can be some interesting EMC related usage as well :)

Dexter
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