Electronic – Proper power design for a USB device

groundloopsisolationpowerusb

I'm designing a USB device that I want to be powered eintirely from a wall wart and have a couple questions concerning the power connections and ground loops.

1) If I connect the USB GND pin to the ground of the wall wart I should be safe from ground loops, right? My understanding so far is that the wart's DC output should be floating with respect to earth ground so I can safely connect it to the USB ground, since the PC's supply is earth grounded from mains.

2) If this is the case, can I just leave the USB's 5V pin unconnected?

3) Also, is the USB GND necessary? Would it be acceptable to only connect D- & D+, or is the USB GND needed for these signals?

4) In what cases would optical isolation be necessary? I see a lot of designs using isolation, is this necessary for all self-powered USB devices?

I've looked through other answers and other USB designs, but I'm not entirely confident yet and so some help would be greatly appreciated.

Best Answer

  1. You are correct about the floating nature of the wall wart output.
  2. There's no harm in leaving USB +5V unconnected. On the other hand, you might have to connect it to your controller, because that's part of its scheme for detecting the USB connection.
  3. Connecting USB GND is necessary. D- & D+ carry a differential signal, however the common mode needs to be within the operational range.
  4. Self-powered USB by itself doesn't required galvanic isolation. Whether or not your instrument requires galvanic isolation (and where to have it) depends on the application (?) of your instrument.

A sketch of a self-powered USB device can be found here see 6.2 on p.24.

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