So we all know certain things:
- DGND (and subsequently AGND) needs to be coupled to Chassis ground, or Earth at one position, at least, in a device.
- The Earth is otherwise considered as a hazardous source of stray current and noise - and needs isolation from the electronics.
- The System ground and earth are usually capacitively coupled.
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?
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?
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.)
Are there any other recommended EARTH to DGND connection schemes recommended other than capacitive for special needs/applications?