Isolating grounds

[mike.durian]

I have a variation on the analog ground / digital ground question. As answered in the Eagle forum, I know you can join an analog ground and a digital ground using either a 0 ohm resistor or by overlapping separate polygons and ignoring the warnings. Neither of these solutions quite address my issue.

In my case, I have a power ground area I'd like to keep separate from a regular ground fill on the top layer, only joining them by dropping vias from the power ground to a middle ground layer. Since I'm only joining grounds by vias, the 0 ohm resistor and overlapping polygon tricks won't work for me.

Since I'm using vias to connect power ground to regular ground, I need to keep both signals named GND (as opposed to GND and PGND, say). Ideally, I'd like to make my power ground polygon rank 1 and then my surrounding, top layer ground fill rank 2. If the signal names were different, my power ground would be automatically isolated from the surrounding ground. However, the names are the same and fusion will join them. Is there a way to trick Fusion in treating the two polygons as the same GND signal, but keeping them isolated from each other? Perhaps by naming them GND@1 and GND@2 or something? I suspect not.

My next thought was to draw a line in the tRestrict layer to cut out my power ground area. Basically, make a moat around the power ground area. This actually works, but I have to set isolate to zero in the large ground fill polygon. With a non-zero isolate, I get additional gap around my tRestrict lines. If isolate is zero, then the regular ground polygon bumps up against the DRC limits around other signals. I have my DRC value low to handle a couple, isolated cases, but in general want a larger isolate value for most of the ground polygon.

The last option I can think of is my least favorite. It is similar to drawing a line in the tRestrict layer, but instead, I create a moat using a number of cutout polygons. Creating the moat from a bunch of polygons is an awkward process and it appears that the cutout polygon type isn't even an option in Fusion, so I'm not sure this is even a viable option.

So, does anybody have a good solution for this scenario?

[jorge_garcia · Accepted solution]

Hi @mike.durian ,

I hope you're having a great day. As far as I understand, this is actually a scenario that would benefit from having the signals with different names.  Here's what I propose:

1. On the top layer you would have two polys a large GND poly and a small PGND poly within the the larger ground poly. Giving them different ranks will isolate them.

2. On an inner layer you will have the same two polys, the small PGND poly would ideally be directly under the PGND poly on the top layer. On the inner layer you would set the ranks to be the same.

On the inner layers the polys will short together because they have the same rank but the vias connecting the top and inner layer will work because they are both connecting PGND polys. If I've misunderstood any part of your scenario or if having differently named signals isn't an option then I don't have a better solution than what you have summarized.

Let me know if there's anything else I can do for you.

Best Regards,

[mike.durian]

Hi @jorge_garcia,

That's a great idea. The only downside is I have to remember to move the PGND in the middle layer if I move things on the top, but I think I can manage that.

With this in mind, a nice feature might be the ability to anchor one object, like say a polygon, to another object, like say a device or via, such that if one object moves, the other moves with it. Once the relationship is set up, you wouldn't have to remember to group select things before moving. It would just happen automatically.