What is USB drain wire?
The drain wire permits a continuous, low-resistance connection to the cable’s metallic shield, resulting in very effective grounding. That places the drain wire in an ideal position to connect a shielded cable to its ground terminal.
What is the purpose of a drain wire?
Drain wires are used in cables in conjunction with a metallic shield to ensure effective grounding. The drain wire serves to complete an electrical circuit from the shield and carry unwanted electrical noise to ground away from the circuit.
How to connect USB shield to signal ground?
Here are some of the options that have been recommended. only. (1) Connect shield directly to signal ground. connector shell be tied to the ground plane. Intel. EMI Design Guidelines for USB Components. Sec 5.4 (p. 9). (2) Connect shield to signal ground through a capacitor. (Possibly with high-value parallel resistor approximately 1 Mohm.)
Are there any drawbacks to a shield to GND connection?
The only real drawback to a direct shield to GND connection is that you wouldn’t get any filtering of ground plane noise on cables, which could lead to EMI issues or potential ground loop issues, but I would think because USB cables are so short, ground loops aren’t really much of an issue.
Is the GND wire supplied by the USB host?
Obviously in USB there is the GND wire, which the USB Host supplies, so that could be the ground. What if the USB Device is self-powered and does not share ground with the USB Host?
Where is the GND trace on a USB shield?
The location of the Chassis to PCB GND tab is on a sub-PCB, with a 30thou trace to the tab. (yes, I know it sounds strange, but the space constraints were ridiculous and this was not my design!) Firstly (as a bit of a cop out) personally, in designs I always ground through a 0R resistor so that the decision can be changed.