Ground plane within a ground plane?

Hello all. I'm currently using Eagle 6.5.0. I want to put a ground all over my board on one of the inner layers. It will cover my entire board with the exception of one small area. In this area, I want to put another ground that is isolated from the larger ground all around it. They must be on the same layer. I tried drawing another polygon, expecting the existing all-over ground to give clearance, but that's not what happened. No clearance was given. I then drew around the area with a line and clearance was given for that. That doesn't seem like a very clean way to do this. What is the proper way to handle this?

Thanks.

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0 clem57 over 10 years ago

Why separate gnd planes? Curious George.
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0 johnbeetem over 10 years ago in reply to clem57

Clem Martins wrote:

Why separate gnd planes? Curious George.

The usual reason is to have separate analog and digital ground planes so that noisy digital signals don't couple to your analog circuits. You also sometimes do this with an oscillator for similar reasons.
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0 shabaz over 10 years ago

If the plane is a polygon, then you can create another plane as a polygon too, and give them different 'rank' values (in the properties dialog). They will have clearance as long as they are different signals.
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0 autodeskguest over 10 years ago in reply to shabaz

On 18/04/15 02:29, shabaz wrote:
If the plane is a polygon, then you can create another plane as a
polygon too, and give them different 'rank' values

The inner one being a numerically LOWER rank than the outer one.

(in the properties
dialog).

Or in the drop-down on creation, or with "CHANGE RANK"...

They will have clearance as long as they are different signals.

... which means you can't call them both "GND". You need to
differentiate them on the schematic as different nets, say DGND and
AGND. Then you need to connect them at only one point, using either a 0R
link or, better, with one of the "short" components from (Olin's?)
library for the purpose.
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0 rickford66 over 10 years ago

The reason for more than one ground plane is that one of the circuits is electrically isolated from the others. They're coupled together with opto-isolators.

I had already tried giving it a different name, as I would need to to get all the nets of that name to connect to it. Giving it a higher rank than the surrounding plane did the trick. There is now a small gap between the isolated ground and the rest of the ground. Thanks.
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0 clem57 over 10 years ago in reply to rickford66

Thanks for that. I am learning as well as helping others learn. Wink wink
Clem
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