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In a mixed-signal IC, you most likely will have two ground planes, analog ground and digital ground. This seems fair enough and easily understandable but my real question is: Why would you need to insert back to back Schottky diodes between two grounds? What purpose would they serve if the ground places are already separated? No coupling should occur if the designer properly managed to split the planes.

Here is a picture of what I am trying to get information about:

enter image description here

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    \$\begingroup\$ Basically it is a nonlinear filter that attenuates any signal smaller than 0.3V. \$\endgroup\$ – Simon Richter Dec 16 '16 at 17:17
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    \$\begingroup\$ I suspect it's to make sure that the grounds cannot float too far apart voltage wise, but still remain reasonably isolated under normal conditions, especially if the digital side is really noisy. That way, noise has a much harder time jumping domains than if both sides shared a common ground plane but either ground can't deviate too far from the other. \$\endgroup\$ – Sam Dec 16 '16 at 21:01
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What purpose would they serve if the ground places are already separated? No coupling should occur if the designer properly managed to split the planes.

And that's part of the problem. Many mixed-signal devices (ADC, DAC, etc.) will misbehave or become damaged if the two grounds are at too different potentials. The diodes prevent them from drifting apart too much.

Obviously if the grounds are supposed to be at completely different potentials (e.g. isolation) then the diodes should not be placed.

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