I'm working on a sensitive analog measurement board. The design currently has split ground planes for analog and digital which are tied under the ADCs, as described in Analog Devices note MT-031. There are several ADCs. While this seems like a good way to handle ground, I'm not sure how to distribute power to the analog sections.

This is for a relatively low-frequency (< 1kHz) but very high precision (ENOB > 18) analog application.

Here are sketches of three possible layouts. Black is ground ties (connecting the split planes), red is power distribution traces. You can assume each domain is appropriately decoupled, nothing crosses the gaps in ground planes, there is a power plane in each domain, and the digital outputs from the analog section go directly over the ground tie (and are as short as possible). Analog and digital power are produced by separate LDO regulators.

Layout 1: Power to the analog domain goes through the digital domain and enters next to (or over) the ground tie. While power is kept as far away from digital signal traces (and on the other side of a ground plane), there may still be some noise pickup.

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Layout 2: Power goes from the power domain to each analog domain directly. This necessarily crosses some of the splits in ground planes.

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Layout 3: Power goes from the power domain to each analog domain directly, and is paired with ground connections for the return current. This necessarily creates some loops in the ground.

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Layout 4: Each analog domain has its own LDO regulators for power, located within it. These are fed from the unregulated power (battery) in the same way as in 1. Obviously less preferred since that increases the number of power components by a lot.

Which of these is likely to produce the least noise, and why?

  • \$\begingroup\$ In your third picture, why are you including the two horizontal ground connections? They are not needed. \$\endgroup\$ – Vladimir Cravero Oct 7 '15 at 20:50
  • \$\begingroup\$ @VladimirCravero: They are there so that the digital signals that cross over at that point can be routed over the ground tie. It is true, they may not be needed; but if they aren't there the return current would have to take the long way around. Could you please explain more what you have in mind? \$\endgroup\$ – Alex I Oct 7 '15 at 20:59
  • \$\begingroup\$ these return paths should be connected to the digital ground of your analog IC and not to the analog ground plane. any 18 ENOB or whatever analog IC should have a separate ground for the digital circuitry. \$\endgroup\$ – Vladimir Cravero Oct 7 '15 at 21:02
  • \$\begingroup\$ @VladimirCravero: From Analog Devices MT-031: "the AGND and DGND pins should be joined together externally to the analog ground plane". This is certainly a confusing subject :) \$\endgroup\$ – Alex I Oct 7 '15 at 21:32

I'd go for number 1 because there is no chance that spikes caused by digital chips on the "red" rail can enter the analoge part of the red. BUT BUT BUT...

You need to explain what external analogue inputs there are and how they retain isolation from ANY ground local or otherwise. This could be a killer without decent differential inputs or some other form of isolation.

You also need to explain what may or may not be grounded on the circuit board and the capacitance across the transformer that creates the basic DC supply. If battery powered then no probs.

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  • \$\begingroup\$ The whole thing is floating / battery powered; the analog inputs are also from a floating source, and are differential, if that helps. \$\endgroup\$ – Alex I Oct 7 '15 at 20:53
  • \$\begingroup\$ Sounds cool to me dude. \$\endgroup\$ – Andy aka Oct 7 '15 at 22:28

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