I have a doubt regarding how to implement the star grounding config. in Eagle.
I have designed a circuit with a mixed-signal chip.
I'm planning to isolate the analog GND from the digital GND in the bottom plane, while my top plane will be the reference GND where both AGND and DGND will be joined to by vias.
In most forums I read that both of these GNDs have to be physically connected, so I downloaded a "short" pad (from here: How to you JOIN two grounds, i.e VSS
... | element14) in order to use it for joining my AGND and DGND in the schematic. This is so that I don't get one of those "same net" errors when I short them with a wire.
My doubt comes when doing the layout. I was told that I could implement a "star" config. by just drawing the polygons on both planes and then joining them with vias as I said before. If this is so, why do I need the short pad?
And also, don't I need a third GND in the schematic in order to make its ground plane in the top layer?
I'm really confused about how to implement this star config. with the "short" pad. How should I go about this?
So far I have already isolated the AGND and DGND in the bottom layer, but the "short" pad is hanging in there and I'm not sure how to use it... because if I join both grounds with the pad in the bottom layer, and then if I put a ground plane on the top layer, I would be creating a GND loop.
I have designed a circuit with a mixed-signal chip.
I'm planning to isolate the analog GND from the digital GND in the bottom plane, while my top plane will be the reference GND where both AGND and DGND will be joined to by vias.
In most forums I read that both of these GNDs have to be physically connected, so I downloaded a "short" pad (from here: How to you JOIN two grounds, i.e VSS
... | element14) in order to use it for joining my AGND and DGND in the schematic. This is so that I don't get one of those "same net" errors when I short them with a wire.
My doubt comes when doing the layout. I was told that I could implement a "star" config. by just drawing the polygons on both planes and then joining them with vias as I said before. If this is so, why do I need the short pad?
And also, don't I need a third GND in the schematic in order to make its ground plane in the top layer?
I'm really confused about how to implement this star config. with the "short" pad. How should I go about this?
So far I have already isolated the AGND and DGND in the bottom layer, but the "short" pad is hanging in there and I'm not sure how to use it... because if I join both grounds with the pad in the bottom layer, and then if I put a ground plane on the top layer, I would be creating a GND loop.
To join the grounds, overlap traces coming from each ground-plane and manually place a via in this location. This will show as an error when you run DRC so just ignore. A better solution might be to join the Ground planes with ferrite beads such as Bourns MH2029-600Y
You can do this in Eagle pretty easily by naming the grounds separately. Then, you can add supply symbols for those 'grounds' in the schematic and connect them. This way, you just route it in the board like you'd route any other components. Not sure what a 'short pad' is, and I'm certain you don't need one.
While placing vias in the board is a reasonable thing to do, it's always better to try to do things in the schematic that can then be routed on the board. It makes the schematic a lot more clear, and makes it easier to do the various design rule checks.
While placing vias in the board is a reasonable thing to do, it's always better to try to do things in the schematic that can then be routed on the board. It makes the schematic a lot more clear, and makes it easier to do the various design rule checks.
Search for mixed signal grounds, I cant find my links at the moment but look for stuff by H.Ott etc. having three grounds is silly especially if you have one full one then join the other two to it, I suspect you don't have much experience in this sort of thing. Generally if you do have to separate the GNDs (not the best solution) then they generally need connecting with a low impedance point under the active device, this means something more than a thin pad... More importantly is how the various sections of the circuit are places 9the analogue and digital sections) also how the power is derived for each section etc. having one ground is preferable and IF you need to using some slots to separate where you may think there are problems, this best done by experience. having a third ground and coupling both separated grounds to this makes it pointless having the analogue and digital grounds in the first place...
What is your layer stack, i.e. how many layers and what is the main device?
What is your layer stack, i.e. how many layers and what is the main device?
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