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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, 2016 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, 2016 at 21:01
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    \$\begingroup\$ Those aren't "back-to-back" diodes, but antiparallel diodes. \$\endgroup\$
    – tobalt
    Jun 8, 2022 at 3:41
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    \$\begingroup\$ You're better off just not splitting the ground planes and keeping the analog and digital components segregated over different parts of the plane. Since there is no bridge anywhere between ground planes connecting them, you have nowhere for signals to safely cross between analog and digital parts without causing more noise than if you just had no split plane at all....that is unless you galvanically isolate digital and analog but if you did that then you never needed to connect the planes together at all. \$\endgroup\$
    – DKNguyen
    Jun 8, 2022 at 4:07
  • \$\begingroup\$ Even if it actually works as per answers below, keeping voltage between the two grounds at safe level, I really can't see any good reason to do so. All in all you allow grounds to be millivolt apart as far as DC is concerned -this is seldom any concern, PSUs will take care instead- but the typically high capacitance of Schottky diodes will short grounds at high frequency spoiling any noise rejection and actually closing a ground loop exactly where -in frequency- we wouldn't need. \$\endgroup\$
    – carloc
    Jun 8, 2022 at 4:49

2 Answers 2

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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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The back-to-back diodes are mainly used for ESD protection but for filtering. During ESD events, the back-to-back diodes would discharge the ESD to Chip ground plane to protect Module(Analog) ground from being raised up.

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