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I am just hoping to get some best practices out of this discussion:

I have two pieces of equipment that are located about 30 feet apart and bolted to a metal frame (chassis ground). They interface with each other through two cables - one carries high frequency (280Mhz) differential pair digital signals and the other carries analog voltage rails.

In designing shielding for the two cables, I had always thought that the shielding should be connected to ground on the power supply side and left floating on the other side to prevent ground loops. To which my colleague replied that I was now creating an antenna that could be used to inject noise back into my system. Thinking about this further, I realized that the shielding on USB cables is connected on both sides, so am I wrong? Or is there a trade off point between low and high frequency lines?

Thanks!

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  • \$\begingroup\$ Great article on exactly that topic: sigcon.com/Pubs/news/2_2.htm \$\endgroup\$ – Some Hardware Guy Oct 19 '12 at 18:16
  • \$\begingroup\$ USB shielding is explicitly connected straight to ground only at the host. At the device, the shield is typically AC coupled to prevent ground loops while providing an effective high-frequency shield that won't act as an antenna. electronics.stackexchange.com/questions/4515/… \$\endgroup\$ – ajs410 Oct 19 '12 at 22:05
  • \$\begingroup\$ "digital pair signals"? Do you mean "differential pair signals"? \$\endgroup\$ – Connor Wolf Dec 19 '12 at 9:04
  • \$\begingroup\$ ...and a loop is not an antenna? No kind of cable that isn't magic makes noise go away -- but maybe if you design your circuit cleverly you can make those noise currents so somewhere irrelevant. \$\endgroup\$ – Phil Frost Aug 12 '14 at 2:14
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Have a look at this tutorial made by Analog Devices. You can also go to Analog's and Kiethley's web site and search for shielding. You might find more white papers.

Your colleague was correct that the shield that's grounded at the host end can inject noise into the ground. Typically, the shield is grounded at the remote end. You could also connect the shield to the chassis ground.

Grounding scheme of the USB cable shield is not a good example, IMO. USB is designed to work with many different gadgets. Most of the USB gadgets aren't grounded by themselves. Perhaps, a trade between EMI immunity and versatility was made.

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