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I am currently designing a wiring loom which includes USB 2.0 cable within it. It is critical that the thickness of this leg is as thin as possible due to the constraints of my client. Unfortunately, the USB 2.0 cable currently specified (PIC USB2624) is much thicker that I would ideally like for it to be, having an outside diameter of 4mm.

This has lead me to the idea of splitting the D+/D- wires from the GND/Vcc wires in order to reduce the profile of this USB 2.0 cable. The plan would be to use a 90Ω Twinax cable (Molex 1000680085) for the D+/D- signal pair and then to use 2 primary wires for the GND and Vcc connections. All of the wires would be starting in the same connector and terminating into the same connector, its just that they wouldn't be within the same cable. Would this be possible? And would I need to do anything to keep the USB 2.0 functional?

Images of this can be seen below...

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Any advice is appreciated

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  • \$\begingroup\$ IMO, this is completely fine. Keep the two extra wires both close to each other, and close to the data pair. (For USB 2) \$\endgroup\$
    – tobalt
    Commented Jul 18, 2023 at 18:08
  • \$\begingroup\$ What kind of connectors are you considering? \$\endgroup\$
    – Tim Williams
    Commented Jul 18, 2023 at 22:09
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    \$\begingroup\$ Hi Tim, this loom is for a motorsport client so will be using Deutsch Autosport connectors, much different from a standard USB connector. I can easily connector the shield to the housings of these connectors. \$\endgroup\$
    – 5ealy
    Commented Jul 19, 2023 at 10:43

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Indeed, USB cable is normally two-pair, with at least one screen on the data pair. Usually the whole cable is also covered by braid.

The characteristic impedance of the power lines is largely irrelevant of course (it still matters, to a much looser extent, due to the low-frequency equivalent parameters i.e. inductance per length; but it's difficult to even construct a cable with a grossly different value), so the power lines can be placed pretty much how ever you like, in or outside the shield. Within is preferred, as noise on the power line can be an emissions concern. If outside, they should be bundled closely with the data pair to reduce the loop area between them.

Note that the data pair must still be shielded, the shield conductor must cover the majority of the route, and the shield must be (AC or EMI) grounded at both ends.

The shield can be used as the ground return (without an independent GND wire) if it is low resistance and/or the return currents are relatively small. (Mind, such a configuration would not be compatible with a USB-compatible connector; this would be suitable for custom/internal use, where GND and shield would be tied at the connector anyway.)

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  • \$\begingroup\$ Standard USB cable shield anyway connects to the connector shells only. So it would be up to the devices how they use the shell, not up to the cable. Devices may choose to hardwire shell to ground (like on any normal PC), leave it floating, or anything between them. \$\endgroup\$
    – Justme
    Commented Jul 18, 2023 at 20:16
  • \$\begingroup\$ @Justme Exactly. Of course, the ones that leave it floating are making their own problems, but that's another matter. \$\endgroup\$
    – Tim Williams
    Commented Jul 18, 2023 at 22:09

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