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In my system, I have 3 Grounds:

DGND, AGND and Shielding GND,

What steps should I take to isolate shielding ground from the other grounds that exist? Should I connect them with ferrites or use some of the solutions listed in this question about connecting DGND and AGND?

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  • \$\begingroup\$ You want to connect grounds with capacitors???? \$\endgroup\$ – stevenvh Aug 11 '12 at 14:21
  • \$\begingroup\$ @stevenvh I read a lot about splitting and not splitting grounds ,, it's a debate that will never end. But my question was specifically about isolating Shielding GND. \$\endgroup\$ – Abdella Aug 11 '12 at 14:24
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    \$\begingroup\$ @stevenvh you can decouple different ground planes with capacitors to allow higher frequency noise a more direct return path while still keeping them isolated. This can make it easier to couple noise from one section to another but it can also make noise that does couple less catastrophic. \$\endgroup\$ – Kortuk Aug 11 '12 at 14:29
  • \$\begingroup\$ @Kortuk - But a capacitor can't give you a lower impedance than connecting them directly, which is the best option. \$\endgroup\$ – stevenvh Aug 11 '12 at 14:31
  • \$\begingroup\$ @stevenvh As such, you strong support not isolating different ground planes? If I connect it everywhere it will take the absolute easiest path for the energy, which can cause noise in other areas. If I dont connect them anywhere near eachother then I might have noise that wants to couple, but cant, and radiates. If I put a capacitor which is not a dead short, but acts somewhat close to it for certain frequencies it gives me almost a termination between them and they are able to couple if the energy really has no path but otherwise should favor other paths. \$\endgroup\$ – Kortuk Aug 11 '12 at 14:41
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The shielding ground, by which i presume you mean the Earth and chassis as so forth, doesn't necessarily require a low impedance path to the circuit. You could short them together, but if you do make sure its done at a single point, possibly close to the power supply.

You should, however ensure that it's at a fixed voltage with respect to your system ground. A resistor to the system ground shod do the trick. It would hold the chassis at close to the system ground and would also provide a more direct path to earth from any noise or discharge the chassis may experience from the environment, than going through potentially sensitive circuitry.

I am assuming that your system ground is otherwise isolated from the Earth through a transformer.

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