When I was looking over the schematics of the Leonardo I noticed the USB GND line is labeled UGND and does not directly connect to the board's GND plane. However, the USB shield is directly connected to the boards GND plane. The UGND line connects to the processor's UGND pin (5) and also to a capacitor which connects to the processor's UCAP pin (6). I was wondering if there is a specific reason the UGND line isn't connected directly to the board's GND plane, and what the UCAP is for? Looking at some of the other boards' schematics, the Nano for example, the USB GND line connects directly to the board's GND plane, along with the USB shield. I've looked at the datasheet for the Atmega32u4 and didn't find anything useful in explaining the reason they're separate. I'm still learning about electrical design so I probably missed something on the datasheet. Any help explaining these differences or pointing me in the right direction would be appreciated.
I'm pretty sure it's because that separate ground runs through a ferrite core to eliminate radio signal amplification.
Thanks for the reply @er_name_not_found. I think I kind of understand why the ferrite core is there, for eliminating high frequency noise? However, I still don't understand the difference between the Leonardo's USB grounding method and the USB grounding method used on the Nano. Couldn't the Leonardo UGND connect to the board's GND plane through the ferrite core, the UGND pin on the process just be a GND pin, and the UGND side of the capacitor just be GND? This way all return paths went through the ferrite core back to the USB line. Instead the USB GND connects to the the ferrite core, the capacitor, and the UGND pin on the processor. I'm just wondering if there is any benefit in this type of grounding method (not directly linking the USB GND line to the boards GND plane) as opposed to the method used on the Nano( the USB GND is connected directly to the GND plane on the board)? Is it important for one of the return paths from the Atmega32u4 to not pass through the ferrite core? Thanks for the help!
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