Are these ground planes handle right ?

[ltourist]

Hi,

I'm currently working on a prototype that will basically route audio through 8 successive loops. I'm using latching relays to route the audio, as my initial prototype shows me that is does not produce any noise when switching.

Anyway, today topic is about ground planes. I look through the internet and the conclusion seems to be that if you can, you should use ground planes. Now, as I'm using relays, digital and analog signals are close and I'm looking for a way to layout DGND and AGND right.

Is the attached picture a good layout idea, especially when it comes to mix two different ground planes in such an unusual shape ? Should I rather leave a "blank" zone under the whole relay zone ? I'm not realy found of the small regions that layout produce.

Thanks

[T3sl4co1l]

What do you need DGND bathing your analog signals for?

A better question is even: why do you need DGND at all?  Or even: why do you *think* you need it at all?

Nice thing about relays is, they have contacts on one side, coil on the other, so you get proximity in your favor to begin with.  And latching coils in particular, you don't apply any power whatsoever when they're inactive, so you don't even have analog or digital or thermal noise in the area!  Of course, you'd want the coil to be powered from analog connections, so what little digital noise remains is isolated by the coil driver.

I don't see any digital signals in the area, just coil drive, which can be logic level but filtered to clean it up if really necessary; or buffered to analog level to get similar isolation action.  So, I see no reason to have DGND over there.

There's rarely any reason to split grounds, and very good reasons not to.  Reading articles that say "isolate analog and digital!!!" without going into the reasons is, at best, reckless.  Understanding the reason is key to implementing a good design, whether split or not.  It's usually easy to avoid crossing digital currents over analog signal paths

Also, don't forget to stitch the ground pours.  All those traces cutting up the polygon make it a far worse ground than even a properly executed analog/digital split (and by proper, I mean if necessary at all).

Tim

[ltourist]

First, thanks for answering me. As you can tell, I'm in a learning phase here and want to understand as much as I can what I'm doing (that's the whole point !). If I wasn't, I would have already completed the PCB a month ago. I hope my response isn't too long for you to take the time to help me again.

I probably did not entirely understand your points as my technical vocabulary on that matter is not really up to speed.

What do you need DGND bathing your analog signals for?

As far as I can see, my DGND is not bathing my analog signals. Analog signals only pass through the middle part of each relay (where DGND is not). But I could have misunderstood your question. :/

A better question is even: why do you need DGND at all?  Or even: why do you *think* you need it at all?

I originally thought that I needed copper fills to reduce PCB production costs. The final form factor of this project will be a 19" 1U rack. As I want to protect it from interferences which could come from other racks (is that even a risk ?) and for the analog part to avoid those copper fills to become antennas and induce hum, I went for the ground planes.

And latching coils in particular, you don't apply any power whatsoever when they're inactive

That's precisely why I choosed to use latching relays 

Of course, you'd want the coil to be powered from analog connections, so what little digital noise remains is isolated by the coil driver.

All those analog connections are entirely passive. They're just audio jacks and relays is only used to route the signal to a loop or to bypass it. That being said, I do not see how I can power the coil from analog connections.
Perhaps I did something wrong though : I only have one VCC for the entire board. But I do not use that VCC for any of the analog part except for the relays which I thought would be fine if they were powered from that line as coil are physically separated from the analog signal.

I don't see any digital signals in the area, just coil drive, which can be logic level but filtered to clean it up if really necessary; or buffered to analog level to get similar isolation action.  So, I see no reason to have DGND over there.

There's rarely any reason to split grounds, and very good reasons not to.  Reading articles that say "isolate analog and digital!!!" without going into the reasons is, at best, reckless.  Understanding the reason is key to implementing a good design, whether split or not.  It's usually easy to avoid crossing digital currents over analog signal paths

That sound dummy, but you make me realize that I indeed don't need DGND in that area. Relays are controlled from an I2c chip far away, and if I start to grasp the whole thing correctly, that's where I would eventually need a DGND. The whole idea is to avoid hum in the audio signal and to protect both analog and digital traces from other equipements interferences. Isn't that good reasons to use ground planes ? (again, beginner here)

Also, don't forget to stitch the ground pours.  All those traces cutting up the polygon make it a far worse ground than even a properly executed analog/digital split (and by proper, I mean if necessary at all).

That screenshot is an early version of the PCB I'm working on and learning how to it right at the same time. Kicad need some fake trace to be able to place via from bottom to top layer and I obviously make those traces in a wrong way... if you're talking about the the zone I attached to this post. If not, I did not understand your point 

Congratulations if you read this far !

[ltourist]

Well, for those looking for a detailled answer : http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

The conclusion seems to be that in most cases, splitting ground case is not necessary nor or good idea.
What could and should be done instead seems to split analog and digital VCC.

Should I spot a second voltage regulator and dedicated its use to powering relay coils ? If not, is the fact that my two zones cannot be clearly delimited (relay sides vs middle, I therefore don't have a "straight" line) an issue ?

Or Am I simply over complicating things ? 

EDIT : that answer also worth reading