Gregory's Serverincidentally this approach does another good thing -- putting all the connectors on one side of the board. we're trying to build a circuit that plugs into a USB jack, not a dipole antenna!
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 naturally the topic gets even more complex when you add in ESD protection. some folks mentioned adding a "bleeder" resistor in parallel with the coupling capacitor. i'm leery of adding series impedance to limit the current, typically you want that ESD out of there without giving it opportunities to current share with sensitive signal returns. also it turns out that many resistors can get destroyed by an ESD pulse, so there's another good reason to avoid this approach. you might be OK if you add a shield ground ring around the board, near any gaps in your enclosure, so that ESD strikes will hit that rather than your main board ground. you should also protect any buttons or switches. for example, some tact switches come with a shield ground ring that goes to a 5th pin, which should be tied to your shield ground. earlier i said that the tl;dr for hobbyists is to just tie the shield to your board ground. for professionals who aren't experts in EMI but work for big companies who have EMI folks on staff, you might just want to add a generic "series component" between the two grounds and populate it with a 0 ohm jumper. the EMI people (during precompliance testing) may need to play around with that connection, and this makes it easy. @tubetime  @tubetime Ooooh, that's what these lines on PCBs are for. Makes sense. |
here's a fantastic real-world example of this design technique. here's a Macintosh 512K motherboard. with a bright light behind it, you can see the divide between the "clean" IO ground and the "dirty" logic ground.
(they did break the rule slightly with the keyboard connector on the front, but they've also extended the cut in the ground plane along the right edge of the board.)