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This is about the seemingly controversial shield to ground connection. The system is basically a body-worn intel-based computer, powered by a battery. Two USB 3.0 (or 3.1 Gen1) cables come out to a PCB that contains two USB 3.0 hub chips (TUSB8020B). Two external cameras plug in to this usb hub PCB (one camera for each hub). So the usb hub PCB has 4 usb connectors (2 upstream and 2 downstream).

The question is, what to do with each of the usb connector shields? The main directive is USB connection robustness.

I have seen many recommendations. For example:

Recommendation 1

TI's TUSB8020B hub reference design TIDA-00287 ties all shells directly to ground. enter image description here enter image description here

Intel's EMI Design Guidelines for USB components also recommends to tie to ground (although this was written for USB 2.0).

enter image description here

Recommendation 2

TI's TUSB8020B EVM (and datasheet) connect shields together, and to ground using RC filter:

enter image description here

Microchip's EVB-USB5534 also ties shields together and uses an RC filter, but a 3 orders of magnitude smaller R:

enter image description here

Recommendation 3

Cypess SuperSpeed Explorer Kit ties each shield independently to ground using either LC or L filter (chokes really):

enter image description here

The cameras themselves (off the shelf) use Cypress recommendation (LC to ground). The embedded computer seems to tie to ground from visual inspection and checking continuity to ground, but I'm not 100% sure (schematics are not available).

Now we face the shield dilemma with the hub PCB (which by the way currently does not have a metallic enclosure, it is a 3d printed plastic enclosure).

What is your say?

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It all depends on device construction, whether you have hot plug options, whether the device has exposed metal connections to electronic internals and possible ESD events, or whether your primary concern is about FCC EMI restrictions. Some details of trade-offs can be found here.

The controversial "recommendations" come from the uncertainty under which condition the particular device will be used. USB is generally a "modular" system, so when things from different manufacturers come into a system, they don't know apriori how their device will be used, and provide minimal coupling between shield and signal ground. If you are designing an integrated system, my opinion is not to connect the shield with signal ground in intermediate hubs, and only couple the shield to signal ground at the battery power joint. Typically the coupling network has unpopulated footprints, and the components are selected/adjusted based on field/certification tests results.

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