Correct way to terminate connector shielding

[marko.severin]

Hi guys,
I'm getting a bit confused on how to correctly terminate connector shielding (like USB, D-SUB, or similar). For instance, most of the board I've seen uses the RC variant, like this one:


However, Texas Instruments uses a ferrite bead instead in one of their designs (Sitara Starterkit):


These are two completley different approaches. One shorts high frequencies to GND, the other blocks them. Which one is correct, or better said, when do I use one and when the other?

Thanks and best regards,
Marko

 

[crutschow]

The first one uses capacitors for coupling to a single ground.

The second is using a ferrite bead to isolate the high-frequency noise on the digital ground from getting to the chassis/USB ground while still connecting the two grounds together for DC. That's a common technique to help isolate analog and digital grounds while still allowing them to be tied together at one place for DC or lower frequency AC continuity between grounds.