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So we all know certain things:

  1. DGND (and subsequently AGND) needs to be coupled to Chassis ground, or Earth at one position, at least, in a device.
  2. The Earth is otherwise considered as a hazardous source of stray current and noise - and needs isolation from the electronics.
  3. The System ground and earth are usually capacitively coupled.

Questions:

  1. I have seen AGND and DGND coupled with ferrite beads - primarily to reduce HF coupling yet keeping the DC bias the same. So capacitively coupling the DGND to Earth is to make sure DC power does not flow into the system but the HF components flow out of the system?

  2. Usually its a 100nF or 10nF cap that i have seen be used for this purpose - is that the general rule of thumb, or does the value of the cap change based on the application/design of the system?

  3. What about HF transients on the Earth coming into the DGND through the cap - is protection from such events implemented in hazardous environments? (Read electricity substations, lightening towers, etc.)

  4. Are there any other recommended EARTH to DGND connection schemes recommended other than capacitive for special needs/applications?

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    \$\begingroup\$ Hey, thank you for your comment. I understand that you do not agree with the points made, but i do not think you got to the questions explicitly listed below... In the questions, you would notice that Question 4 is inquiring about the validity of point 3, and the other questions are independent. Nonetheless, thank you for taking the time to respond... \$\endgroup\$
    – Raman
    Jul 30, 2020 at 11:56
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    \$\begingroup\$ The question is still fundamentally flawed in that the assertions you make (we all know certain things) are untrue. It's the wrong way to begin a question and you won't get answers this way very effectively. I never reached the questions because I was too dogged down by the errors to proceed. Many folk will be the same. \$\endgroup\$
    – Andy aka
    Jul 30, 2020 at 12:00

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Typically mains ground is your best bet, assuming that the connection to ground is good (in the building) and the wiring in the building is good, it's as good as you can get. A ferrite on ground will actually give you more problems, because any impedance that restricts current on the mains ground will create a voltage on the ground plane.

Make sure the connection between ground and the PCB ground is low impedance (that means low resistance) to prevent common mode voltages.

The System ground and earth are usually capacitively coupled.

No, in all commercial products with no isolation, the ground is ground and should be for safety reasons.

I have seen AGND and DGND coupled with ferrite beads - primarily to reduce HF coupling yet keeping the DC bias the same. So capacitively coupling the DGND to Earth is to make sure DC power does not flow into the system but the HF components flow out of the system?

A properly designed board does not need separation between AGND and DGND. It's about managing currents on the PCB. Currents create voltages through the ground plane. Currents usually arise from bad placement of connectors that are carrying currents or shield currents (shield currents should more often then not directed to chassis ground).

A properly designed board does not have switching return currents running through the analog section (the currents from the GND pin of an IC will return to ground, usually through the shortest distance, and will create voltages in conjunction with the resistance of the ground plane. So place components with large or switching return currents away from sensitive analog components.

Another source of currents is EMI capacitivly coupling with the PCB, and returning to ground in which proper shielding would take care of that problem.

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