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How to create power planes for this high speed mixed signal board and connect them together in Eagle PCB?

Former Member

I have read that for a board with analogue and digital grounds, they must be kept separate and joined at one point only. I cannot for the life of my find out how to this in Eagle PCB design which is now at version 7.

 

I have a board with data converters which means that I shall have 3.3V digital and 5V analogue supply and also a Vref. This will result in 2 ground planes. They will be need to be connected at 1 point on the PCB. What is the orthodox way to ->connect<- these two planes within the Eagle PCB software? I mean it is easy to have 2 separate nets with names like DGND and AGND but how to connect them in the board layout editor? Also, will the DRC raise error that the net on the component should be GND but is DGND or AGND?

 

Finally, besides this question on how to connect the two ground planes, could you tell me if the top planes would also be ground or they need to be 5V and 3.3V planes which will directly connect to the data converters supply pins rather than me have floating top planes and only use tracks to the connect the 3.3V and 5V pins on the data converters?

 

Please help... I can't find this anywhere.

  • 0

    Hi,

     

    On a plane layer, use two 'polygons', isolated from one another. Associate

    one with Analog GND, and the other with Digital GND. Place a resistor

    between AGND and DGND on your schematic, and populate it to connect the two

    grounds on your board. There are probably other ways to do this, but this

    is certainly one way!

     

    John

    --

    Web access to CadSoft support forums at www.eaglecentral.ca.  Where the CadSoft EAGLE community meets.

     

  • 0

    Hassan Iqbal wrote on Tue, 30 June 2015 18:37

    I have read that for a board with analogue and digital grounds, they

    must be kept separate and joined at one point only.

     

    Yes, that is usually a good idea.  However, it's important that you

    understand why this is good instead of just doing something you read

    somewhere.

     

    Quote:

    I cannot for the life of my find out how to this in Eagle PCB design

    which is now at version 7.

     

    I have a board with data converters which means that I shall have 3.3V

    digital and 5V analogue supply and also a Vref. This will result in 2

    ground planes. They will be need to be connected at 1 point on the PCB.

    What is the orthodox way to ->connect<- these two planes within the

    Eagle PCB software?

     

     

    Use one of my "short" parts.  That's a part with two pads that simply

    connects them.  The PCB will just have copper where you expect, and on the

    schematic the short is shown as a slightly thicker line.  To Eagle, the two

    copper areas on either side of the short are different nets, so it won't

    automatically flow copper between them.  You make each net a separate

    polygon to act as a ground plane for that section.

     

    One issue with this method is that you will get a lot of DRC errors about

    the two pads of the short being shorted.  You will have to approve these

    errors.  If you move the short on the board, you will have to approve the

    errors all over again.

     

    The shorts (and a bunch of other Eagle parts and utilities) is available in

    my Eagle Tools release at http://www.embedinc.com/pic/dload.htm.  Others

    have copied this and you can find various versions of my shorts library out

    there.  That might be easier if you just want the library.

     

    --

    Web access to CadSoft support forums at www.eaglecentral.ca.  Where the CadSoft EAGLE community meets.