Hello,
first of all, thanks a lot for the input as well as for the links to the
articles.
I find it usually quite difficult to convince engineers that a single
solid "GND" plane (whatever GND is ;-) ) is the best solution for most
designs.
But there are too many application notes / design guides out that tell
different. And I think that it is not that rare to have the cases that
Todd mentioned (e. g. safety for medical applications.. ). I had quite
some interesting effects measuring designs with splitted/isolated
internal GND in the past... . And usually these designs had valid
reasons for their split/separate GND implementations.
Is it required to split the discussion into two topics?
- Separating digital and analog GND on the PCB (inside the system) e. g.
for noise sensitive circuits (guess this was the initial question)
- Separating circuit GND vs. Chassis GND at the IO shield.
Especially on Ethernet I have seen several different implementations how
to handle the second one. Especially for long cables "GND" on both ends
might be quite different.
And if you have multiple interfaces at the IO shield each one will tell
you different ways of connecting Chassis IO GND to internal GND. Some
might tell to use a high voltage capacitor, some might use some resistor
or inductors, ...
And I think the worst what one can do is to implement all solutions in
parallel ..
But is the solution really to directly connect "Chassis GND" to "digital
GND" ?
How to handle this if there is a reason to separate "Chassis GND" and
"digital GND"?
Here there are a lot different effects like EMI/EMC, ESD, Current
return, Common mode, safety, ... to be considered!
Thanks for any feedback
Hermann
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Am 15.11.2016 um 23:00 schrieb Todd Hubing:
As a few people on this list have already pointed out. Most high-speed PCBs
should have one solid ground (current return) plane. There are a few
legitimate reason for isolating grounds (e.g. safety, isolating unamplified
audio from digital, isolating amps of switching current from circuits
sensitive to millivolts of noise, etc...). All of the legitimate reasons
involve the necessity of controlling the flow of low-frequency currents. If
enclosures or cables are referenced to different isolated grounds, those
grounds will generally be connected through capacitance to control the
high-frequency voltage that can appear between them.
Isolating current return planes laterally as shown in the articles below is
almost never a good solution. Splits in a ground plane are only effective for
reducing common-resistance coupling. In most cases today, analog and digital
devices can share the same solid copper return plane without any significant
common resistance coupling. In those situations where isolation of the
current returns is necessary, the isolated returns should usually be on
separate overlapping layers; especially if signals that leave the board
reference each of the two "grounds".
Todd Hubing
LearnEMC
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Loyer, Jeff
Sent: Tuesday, November 15, 2016 2:33 PM
To: leeritchey@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
Analog Devices does a nice job of covering the topic of how to isolate your
analog and digital ground planes, as does Henry Ott.
https://urldefense.proofpoint.com/v2/url?u=http-3A__www.analog.com_en_analog-2Ddialogue_articles_staying-2Dwell-2Dgrounded.html&d=CwIDAg&c=Ngd-ta5yRYsqeUsEDgxhcqsYYY1Xs5ogLxWPA_2Wlc4&r=ftR4MROOpLbvyzclIOBnxTEyxiEvF1n3ZQy33buYK-A&m=vYIkRmJmwCyySCtlzz79C1b0oXuEqdnIliLyrTea5fY&s=iaoG-DDWbak9mANRXPh1iI-olxrvJYy2_rFNQ-TOgwA&e=
www.hottconsultants.com/techtips/split-gnd-plane.html
Jeff Loyer
Signal Integrity Lead
Amazon Web Services (AWS)
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Lee Ritchey
Sent: Tuesday, November 15, 2016 12:24 PM
To: dbrooks9@xxxxxxxxxxxxxxx; dmarc-noreply@xxxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
Sometimes I am prompted to cite Bruce Archambeault from IBM on what ground is.
Here is a quote from him.
"Ground is the place where one plants seeds in the spring time in the hope
that come summer one will get a bumper crop of tomatoes."
The reason for Agnd and Dgnd on an IC is to isolate the analog side of the IC
from the digital side for the purpose of isolating the two circuits
inside the package, not outside it. They should share the same ground
plane on the PCB. Having two different grounds on the PCB does not provide
any performance advantage to the IC.
Outside the IC, efforts should be made to insure the analog source does not
share paths with digital signals. We usually do this by using shielded
cables leading to the analog source.
Hope this clears up some of this confusion.
Lee Ritchey
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Doug Brooks
Sent: Tuesday, November 15, 2016 8:33 AM
To: dmarc-noreply@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
Occasionally some people use a capacitor between different grounds in a
misguided attempt to provide to provide a signal return path under a trace
that crosses over the plane split for signal integrity purposes.
This, of course, is usually a bad idea. The purpose for separate "grounds"
in the first place was to isolate noise. All the capacitor does is ensure the
noise has a path between the two grounds.
(Note to Jeff: Please send me a current email address off-line.)
Doug
Loyer, Jeff wrote:
I was looking at some designs and found different ground symbolsby capacitors. Can anyone explain why this might be done? Everything I've
connected
seen or heard says this is a bad thing (I would connect them directly), but I
want to be sure I'm not missing something. I think there are some A/D
devices which have specific guidelines for separating digital and analog
grounds, but I don't think they'd be connected by caps.
Thanks,
Jeff Loyer
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