Analog and digital grounds

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engr_david_ee

Joined Mar 10, 2023
123
Hi, I have heard about two types of ground planes in PCB stackups. One is analog ground plane and the other is digital ground plane. I have seen some layouts in which at some pint in the PCB these both ground planes are connected through a via close to the power supply connector. I do not have the layout now with me but I am wondering how it is beneficial to use capacitors between analog ground plane and digital ground plane ? The reason these two ground are separate that switching in the digital ground reference remains isolated then the analog ground reference. There can be slightly different reference level. We normally use de-coupling capacitors between power and ground planes. How about using capacitors between analog ground and digital ground ?
 

dl324

Joined Mar 30, 2015
16,688
How about using capacitors between analog ground and digital ground ?
That would defeat the purpose of having analog and digital grounds isolated as much as possible.

It depends on the circuitry, but I'd be inclined to connect the two grounds at one point near the power supply unless I had reasons to do otherwise.
 

MrChips

Joined Oct 2, 2009
30,488
No. One does not use capacitors between ground planes.
All ground connections are meant to be at equi-potential, i.e. the same 0V reference or COMMON voltage.

In general, one needs to use a "star" connection to avoid ground loops. All ground connections terminate at a common node, usually close to the power supply COMMON.

The purpose of separate analog and digital ground planes is to prevent digital switching noise in the supply chain from reaching sensitive analog circuitry.
 

WBahn

Joined Mar 31, 2012
29,865
Hi, I have heard about two types of ground planes in PCB stackups. One is analog ground plane and the other is digital ground plane. I have seen some layouts in which at some pint in the PCB these both ground planes are connected through a via close to the power supply connector. I do not have the layout now with me but I am wondering how it is beneficial to use capacitors between analog ground plane and digital ground plane ? The reason these two ground are separate that switching in the digital ground reference remains isolated then the analog ground reference. There can be slightly different reference level. We normally use de-coupling capacitors between power and ground planes. How about using capacitors between analog ground and digital ground ?
The reason that you might use a capacitor between the analog and digital ground planes is to control the loop size of image currents for signals crossing a split between the planes. But doing so places at risk the isolation you have tried to achieve by splitting the planes in the first place. It would be much better to route your signals so that they don't cross the gaps. If that is unavoidable, then stitching capacitors between the planes are an option and if you have taken care to provide good local bypassing on both the analog and the digital planes, then the crosstalk will be minimal.
 

crutschow

Joined Mar 14, 2008
34,047
If there is a mixed-signal device on the PCB, such as A/D or D/A converters, than it may be preferable to connect the two grounds together near those devices.
Sometimes a split analog/digital ground is used with the split going underneath the mixed-signal devices.
 
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