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.

  • \$\begingroup\$ Is the USB device only for powering, or will you need it to transfer any data? \$\endgroup\$ – Adel Bibi Jan 11 '14 at 23:25
  • \$\begingroup\$ No, I need to send data as well, but the current draw would be out of spec for USB so I'm thinking a wall wart is the way to go. \$\endgroup\$ – user2312466 Jan 11 '14 at 23:38
  • \$\begingroup\$ I can't post this as an answer, because I'm unable to answer all your questions. As for the ground, both the red line and ground have to be connected to your power supply. This will never work if you didn't connect either of the two lines. If you are only going for powering, this will be enough for you. In the case you want to send data, you will have to connect the two other pins (D+,D-) to the corresponding on the other side. \$\endgroup\$ – Adel Bibi Jan 11 '14 at 23:51
  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.

  • \$\begingroup\$ For 4), The application is a controller board for an Ipad Retina Display to be used as a DisplayPort monitor, so really the USB data is just for sending a few commands to an MCU via an FT232RL. \$\endgroup\$ – user2312466 Jan 12 '14 at 1:34

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