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.