USB hub devices can be implemented in the following ways:
• Embedded with a host controller
• Embedded with a mix of inaccessible ports with embedded devices and user-accessible ports
• Embedded with a mix of a host, inaccessible ports with embedded devices, and user-accessible ports
• A stand-alone hub
Each of these implementation details can affect the best method to use for connecting the chassis and USB cable shields to digital ground. The best way to make these connections will depend on the design’s system details. Refer to the many reference documents and white papers on this topic available on the Internet, especially those published by members of the USB consortium and providers of USB-enabled motherboards and devices.
Microchip has observed positive EMI and ESD behavior on stand-alone designs when connecting the USB cable shield to digital ground with an RC network (330 Ω resistor and a 0.1 μF capacitor in parallel) at each USB connector.
The shield of the USB connectors (Chassis Ground) must be considered carefully. Some kind of shield-ground isolation is usually desired for EMI or ESD reasons. Embedded systems may require a direct ground connection. An RC network of 330 Ω and 0.1 μF is used on most of Microchip’s hub evaluation boards for this purpose.
Source: AN 26.2 Microchip
The circuit is a standard solution to improve ESD discharge robustness of your board/device. A metal connector housing is the most likely area for an ESD discharge either directly by touching or indirectly via the connected shielded cable. By coupling the connector housing via an parallel RC circuit and not directly to PCB ground you limit the peek of ESD energy injected into your PCB during an ESD event so improving your PCB ESD robustness. And because of the capacitor you still have good hi frequency coupling of your connector housing and connected shielded cable to ground so your shielding still performs its function. Its similar in the usage in PCB ground connection to device chassis ground, to on one side provide coupling for EMC shielding and on the other hand to reduce ESD discharge energy to your PCB in an ESD event comparing to direct connection. Sometimes its also only used as capacitor only especially when you have an AC connected device where you don’t want no DC coupling at all…
