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Take a six layer PCB with lots of circuitry, smaller than you'd like, parts on both sides, with very sensitive amplifiers looking for RF signals in the 100nV range, and digital circuitry.

Top layer, parts and short tracks. L2 Ground L3 long tracks L4 long tracks L5 Ground Bottom Parts and short tracks.

The digital section is running, and there are plenty of bypass caps etc, but in the end there will always be currents running around. The low noise sections are laid out very tightly, and as far away from the digital as possible. For mechanical reasons, I don't have the option of just placing circuitry where I'd like to, many part locations are dictated.

If L2 and L5 are solid ground, then current between any two points will take all possible paths, the majority will take the most direct paths and a small part of the digital noise currents will end up running through the analog sections.

If I carefully "moat" the analog sections, I think that I would get lower noise because that current would be prevented from running through my analog.

Obviously I still need to make a connection, but power in and ground can then be restricted to a single point.

Am I thinking about this correctly?
Experience in support or against? Authoritative documentation in support or against?

Thanks

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  • \$\begingroup\$ Input and output connections will dictate some things and input and output device port specifications will dictate some of these things and maybe other things too. You might need to think about individual ground islands that are star pointed then this might work better for your signals but it might prove more problematic for EMC (generation of spurious antennas). \$\endgroup\$ – Andy aka Jun 20 '17 at 18:50
  • \$\begingroup\$ Long and short tracks - where are the power tracks /planes? \$\endgroup\$ – Paul Uszak Jun 20 '17 at 21:57
  • \$\begingroup\$ Did you read this: electronics.stackexchange.com/questions/306862/… \$\endgroup\$ – analogsystemsrf Jun 21 '17 at 4:45
  • \$\begingroup\$ What are the sensitive inputs? I am trying to ascertain whether your circuit is within my domain of experience. Some people have a mistaken idea of what a "sensitive input" is. Anyway, when you force digital currents on the ground plane to go around something, you are, by definition, increasing the loop area of the digital return currents, which is counter to radiated emissions minimization. However, if you have placed your analog circuits where the digital currents are very small, as you indicate in your question, radiated emissions may not be a concern. \$\endgroup\$ – mkeith Jun 21 '17 at 5:55
  • \$\begingroup\$ Sensitive inputs are analog amplifiers looking for signals in the 100nV range. \$\endgroup\$ – user103218 Jun 21 '17 at 13:26

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