11
\$\begingroup\$

I have read through many recommendations articles, and many StackExchange questions, but haven't come up with any clear understanding as there are many conflicting information out there. The simple question is, should a USB shield be connected to GND? If yes, should it be connected directly, through a resistor, or through a capacitor?

My case: Computer peripheral with a metal enclosure and USB-C connector. The thing however is, I cannot guarantee that everyone will use shielded USB cables to connect the device. Of course, I can make shielded cable a requirement in the manual, but I'd rather be predicting.

What I've read and tried to take into consideration:

  • Using metal enclosure as EMI shield (both radiated and received)
  • Using metal enclosure as ESD shield (safe ESD path away from the electronics)
  • Apparently, some motherboards do have tied GND to Shield on the host end, and some don't
\$\endgroup\$
8
  • 1
    \$\begingroup\$ @tobalt USB cables are certainly not always shielded. About a third of all cables I tested were not shielded. Users must use a USB cable, but I can't guarantee they will use shielded ones. And even if all USB cables were shielded, I still don't know if the user's motherboard has tied shield to GND on the host end. \$\endgroup\$
    – Marek
    Commented Aug 10, 2023 at 6:38
  • 1
    \$\begingroup\$ Did you look up the shielding requirements in the USB standard? What does it say? \$\endgroup\$ Commented Aug 10, 2023 at 6:42
  • 6
    \$\begingroup\$ @TimWilliams To be fair, there is such thing as unshielded USB cable. They just are used in low-speed devices and are non-detachable. So any detachable USB cable assembly with plugs on both ends should, by specification, be shielded. Of course, the non-shielded ones, if they exist, cannot be officially called USB cables. \$\endgroup\$
    – Justme
    Commented Aug 10, 2023 at 8:29
  • 2
    \$\begingroup\$ @Justme Precisely! \$\endgroup\$ Commented Aug 10, 2023 at 8:30
  • 2
    \$\begingroup\$ Could you please check this electronics.stackexchange.com/questions/389972/… \$\endgroup\$
    – Confused
    Commented Aug 10, 2023 at 9:00

4 Answers 4

15
\$\begingroup\$

I don't think there's a lot of conflicting information, but rather a lot of information that depends on context or EMC notions.

To understand, it's important not to confuse these symbols. Zero volt (equipotentiality), ground chassis (EMC) and earth (safety).

enter image description here

The general vision is as follows :

In theory, shielding can be floating. If it is hermetically sealed, the Faraday cage effect is obtained and the common-mode currents are drained off by parasitic capacitances.

enter image description here

Sometimes for safety reasons (class I devices requirements, reducing the accumulation of static electricity, ...) the enclosure must be earthed, and the parasitic capacitance (enclosure/earth) disappears, improving the efficiency of common-mode current evacuation.

enter image description here

In your electronic design, you always have to deal with the problem of internal EMC, and the parasitic capacitance between the printed circuit board and the metal enclosure is not ideal. A solution consist to connect directly the zero volt to the ground enclosure.

enter image description here

That's a general view of the problem ! There's a lot to be said for compromises in context. For example, if the two devices are in different buildings and with different earths, this configuration poses a problem because the 0V would be different. There's also a lot to say about one-point grounding or multipoint grounding inside the enclosure versus distributed stray capacitances in HF. The ground loop influence, etc...

\$\endgroup\$
12
  • 1
    \$\begingroup\$ …and this awesome book by Henry Ott covers your "etc...": Electromagnetic Compatibility Engineering \$\endgroup\$
    – pipe
    Commented Aug 10, 2023 at 11:42
  • \$\begingroup\$ "For safety reasons, the enclosure must be earthed" says who? Or are you saying to design a device that will work in a circuit having earthed enclosures, even if not all such enclosures will be grounded to earth? \$\endgroup\$
    – Ben Voigt
    Commented Aug 10, 2023 at 14:44
  • \$\begingroup\$ @BenVoigt I think that sentence works better with a "sometimes" at the beginning :) \$\endgroup\$
    – hobbs
    Commented Aug 10, 2023 at 15:43
  • \$\begingroup\$ @hobbs, you're right. I will edit my message and add "sometimes (class I devices requirements or reducing the accumulation of static electricity, ...)" \$\endgroup\$
    – Vincent
    Commented Aug 10, 2023 at 18:33
  • 2
    \$\begingroup\$ Also, the English language has a clear distinction between "security" (protection from bad people) and "safety" (protection from accidents); earth ground is obviously for safety, not security \$\endgroup\$ Commented Aug 11, 2023 at 1:43
8
\$\begingroup\$

There isn't a simple answer in generic case so that is why you read conflicting information because the correct answer how to connect it depends on many things.

However, since your device has Type-C receptacle, the Type-C standard mandates that receptacle shell shall be connected to the PCB ground plane.

\$\endgroup\$
2
  • \$\begingroup\$ Can you link to where in the specification this is mentioned? \$\endgroup\$
    – Tinsa
    Commented Feb 7 at 10:17
  • \$\begingroup\$ @Tinsa Please read whatever is the current version of the applicable standard from USB website. Type-C cable and connector specification. \$\endgroup\$
    – Justme
    Commented Feb 7 at 10:22
5
\$\begingroup\$

You'll hear plenty of people arguing about this online, but only in outdated discussions. "USB Type-C Cable and Connector Specification" Release 2.0 says in the notes to Table 3-11:

Shield and GND grounds shall be connected within the USB Type-C plug on both ends of the cable assembly.

I've tested the continuity on a few different real USB-C cables so I've also observed this first-hand. Even if you don't connect your shield and ground on your device's connector, the USB-C cable will do it for you.

\$\endgroup\$
4
  • 2
    \$\begingroup\$ The standard also says that receptacle shell must be connected to ground. \$\endgroup\$
    – Justme
    Commented Aug 11, 2023 at 11:04
  • 1
    \$\begingroup\$ @mjt I don't think it's outdated, because technically that spec only applies to USB-C, whereas a great many people (especially on this forum) still use other USB connectors, because they don't need USB C features. \$\endgroup\$
    – BeB00
    Commented Aug 12, 2023 at 0:40
  • \$\begingroup\$ That table (Table 3-11 USB 2.0 Type-C Standard Cable Assembly Wiring) is only for USB2.0 cables using Type-C connectors though. I have not yet found any reference in that document to USB3.X regarding GND and shield. \$\endgroup\$
    – Tinsa
    Commented Feb 7 at 10:15
  • 1
    \$\begingroup\$ @Tinsa OK, but have you tried to search? That would be specified in Table 3-10 note 6 right above it on the previous PDF page. Use the search functionality to find text from PDF files, maybe CTRL-F depending on which PDF reader you are using and on which platform. \$\endgroup\$
    – Justme
    Commented Feb 7 at 12:36
3
\$\begingroup\$

I always connect USB port shields to GND. Even when designing PCBs, you can see on their footprints, that shield pins are connected to the ground plane. This is common practise for handling USB port shields, I even use USB ports as a ground source when testing something on microcontrollers like ESP32 or Arduino. It is much easier to place the GND probe on the USB port rather than on some pin, which you might accidentally short with +5V. Connect shielding directly to GND no need for resistor or capacitor.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.