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I am currently designing a modular device for signal acquisition and signal generation. This is not a professional/commercial device. I am only building it out of interest and to learn.

Nevertheless, the device should behave reasonably concerning EMI and I want to avoid the signal acquisition being disturbed by the mains frequency or USB. Unfortunately, I have little experience with this topic, and I hope that you can give me answers to my specific questions.

The basic structure of the device looks something like this where the questions below came to my mind:

enter image description here enter image description here

Should I connect the protective ground wire of the mains connector directly to the system ground of my backplane or only to the case shield (via a screw)? enter image description here

Should I connect the shield of the USB connectors to the system ground? (I have measured that both in my USB hub and my PC USB shield and USB GND are directly connected (0 Ohm).) enter image description here

Should the PCB to case holders (metal) next to the USB connectors directly connect the system ground to the case or should they be isolated? enter image description here

Should the ground of the single PCBs be connected to the metal front covers (case shield) or should they be isolated from the shield at that point? enter image description here

Should the ground of the backplane be connected to the case shield via screws or should they be isolated from the shield at that point?? enter image description here

Thank you in advance for your helpful comments!

Charly
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    not an answe rbut: for a hobby device, this is an impressively sturdy mechanical design. Is this some kind of standard modular chassis? I'd like to get one for my projects :) – user253751 Jul 22 '22 at 19:54
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    I think there are just too many separate questions, even if they revolve around the same subject. But we don't even know what your device does, how it does it, what is important for it, what it connects to, and with which kind of power supply. The answer could fill a book. – Justme Jul 22 '22 at 20:00
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    "The answer could fill a book." ...and probably has. If I may speak for the OP: does anyone know of one? I don't. Even though I have my biases for grounding topology (yay! puns!), I'd sure like to have a practical guide of best practical practices on my bookshelf :~) – Chris Knudsen Jul 22 '22 at 20:16
  • @user253751 This form factor is commonly used for (but not limited to) Eurocard designs. Lots of enclosure manufacturers support this type of form factor, even US-based ones! https://en.wikipedia.org/wiki/Eurocard_(printed_circuit_board) – Chris Knudsen Jul 22 '22 at 20:23
  • @ChrisKnudsen I will have to try it! – user253751 Jul 22 '22 at 20:24
  • @Justme I'm sure the answer is the same for most devices in metal chassis without special requirements. There must be a best practice. – user253751 Jul 22 '22 at 20:25
  • @user253751 That's the chassis: https://schroff.nvent.com/en-de/products/enc14575-134 – Charly Jul 22 '22 at 20:30
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    The problem with “ground” is that there is team yes and team no. First: earth is not the ideal ground en the ground of the pcb is the return plane and should be designed very well. In my opinion and lots of others, the outer shield should be connected to the chassis or floating. Why disturb your return plane (EMI etc)? And what if your device has the cable plugged in on the machine side and not on the supply side? Chassis is chassis. Ground is ground. Team no here haha. – RemyHx Jul 22 '22 at 21:23
  • @ChrisKnudsen Popular titles include Howard Johnson's Handbook(s), and Henry Ott's EMC Engineering. Lengthy and dense subjects, but highly worthy of study. – Tim Williams Jul 22 '22 at 21:37
  • EMC is a holistic study, and this question doesn't provide nearly enough information to propose a solution (and if it did, I'd want to send a bill for the time taken to analyze it anyway..!). Unfortunately, Stack's question format is a poor fit for matters as in-depth as this. – Tim Williams Jul 22 '22 at 21:38
  • @user253751 If you think there is a best practice then go ahead. But it is even unknown if the electronics should be isolated or be referenced to mains earth. It all boils down to how you want signal currents to flow, supply currents to flow, and external EMI current to flow, and how to make them flow so they don't mess up with each other. – Justme Jul 23 '22 at 11:04

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