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Should USB shield be connected to ground?
The recommendation from FTDI and the USB Spec says that, at the device end, the USB shield should not be connected to the signal ground. But they recommend connecting the USB shield to the device chassis – assuming the chassis is not connected to the signal ground.
What is Shield GND?
Dataforth Application Note. Cable shielding is used primarily to minimize or eliminate capacitively coupled interference from electric fields. When properly implemented, it can also be used to minimize inductive coupling from magnetic fields.
What is a 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.
How do I shield a USB port?
- Connect the SHIELD connection to GND through a resistor. This helps isolate it and reduces EMI and RFI emissions. Keep this resistor close to the USB connector.
- Provide a plane for the USB shield on the signal layer adjacent to the VCC plane that is no larger than the USB header.
Where do I connect the shield wire?
Signal cables The best way to wire shielded cables for screening is to ground the shield at both ends of the cable. Traditionally there existed a rule of thumb to ground only the source end of the shield to avoid ground loops. Best practice is to ground at both ends, but there is a possibility of ground loops.
What is drain wire for?
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. The drain wires are usually tinned copper conductors.
How to connect USB connector shield to host?
The location of the connector shell’s termination to the Gnd plane is critical. The connection needs to be made to the quietest area of the ground plane to prevent noise from the ground plane from coupling to the shield… The shield shouldn’t be grounded. It is grounded at the host end, of course.
How is the shield connected to the PCB?
R1 is then connecting the shield to GND, and capacitor C3 is doing the same on the other connection. The shield is connected to ground via the 100k res/100nF cap. There is a metal tab fitted to the PCB which rests on the metal chassis. According to the old ESD report, this was needed or the device failed.
What happens when USB shield is not connected to ground?
The datalogger has a metal chassis, that is bonded to the PCB board ground. When the USB shield is not directly connected to PCB board ground (for example connected by R||C or HiZ), the datalogger fails (loses memory contents).
Do you have to connect Shield at both ends?
For this reason, the lowest impedance path should be connected to shield at BOTH ends. Yes, there will be ground loops, and noise will get coupled in, but it won’t matter. In the case of high frequency, multipoint ground is prefered. Check to see if the manufacturer of your USB chip specs out what you should use.