Typically, the grounds eventually do need to all come together at one point.

edit:

Here's a good overview: Analog.com.

Analog, digital, and power grounds will be isolated from each other as needed to minimize noise and ground loops.

Note that I didn't use 'shall'. It sounds like you don't really have a performance specification driving this, at least not by the details provided so far. There's no point in putting a value on the isolation unless it is driven by a requirement.

Z

Thank you VE1BLL

Appreciate your response.

Single point ground is indeed the mantra, but there are times when such edicts need to be violated, particularly when subcircuits are actually isolated from system grounds. Transient common mode voltages can be a bitch.

TTFN (ta ta for now)
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Since the OP is writing requirements, perhaps such requirements shouldn't be such low level and so prescriptive. On the other hand, perhaps he's part of the design team for the box, and is tasked with writing such low level requirements.

Unless the requirements writer is participating in the system design, then usually it would be up to the system or circuit designer to design such details.

For aerospace applications, perhaps one could look up the applicable design standard and just reference that.

Also worth noting that 'testable' is only one of four methods of providing proof of compliance. The traditional list is: Inspection, Demonstration, Test, and Analysis. A good overview here.

As others have reminded us, some circuits will require isolation. They'll often be associated with signals being input via transformers or optocouplers.

Aircraft wiring typically has explicit signal returns, explicit power returns, shielding and safety grounds. Aircraft are also a good example when dealing with people confused by the "necessity" for grounding to Earth (the Earth being tens of thousands of feet below).

That's a good point. By his description it doesn't sound like he is looking for isolation but instead for a star-ground; I should have said "separate but connected". I'm in the middle of a Power over Ethernet design requiring 1500V isolation (from a supply isolated from earth ground) so that's where my mind was.

Z

Isolated power can generate tons of common mode, enough to fry differential inputs.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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