How should I route USB Connector shield on PCB? Should it be connected to GND plane right where USB is placed, or should the shield be isolated from GND, or should it be connected to ground through ESD protection chip, high resistance resistor or fuse?
PS. Should I put the shield connections on schematic, or just route it on PCB?
Answer
For the shield to be effective, it requires as low impedance connection as possible to your shield ground. I think those recommending resistors, or not connecting it to ground at all, or strictly talking about your digital logic ground, and assuming you have a separate shield ground. If you have a metal enclosure, this will be your shield ground. At some point, your digital ground must connect to your shield ground. For EMI reasons, this single point should be close to your I/O area. This means it's best to place your USB connector with any other I/O connectors around one section of the board and locate your shield to logic ground point at that location. There are some exceptions to the single point, rule, if you have a solid metal enclosure without any apertures, for example, multiple connection points can be helpful. In any case, at shield to circuit ground connection, some may recommend using a resistor or capacitor (or both) but rarely is there a reasonable reason to do this. You want a low inductance connection between the two to provide a path for common mode noise. Why divert noise though parasitic capacitance (e.g. radiate it out into the environment)? The only reason usually given for such tactics is to prevent ground loops, but you're talking about USB, ground loops most likely won't be an issue for most USB applications. Granted, such tactics will prevent ground loops, but they will also rend your shielding all but ineffective.
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