Absolutely! There have been several designs that worked (and I mean
functionally!) when in a metal chassis that did not when the chassis
was redesigned to plastic.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Istvan Novak
Sent: Friday, May 11, 2018 7:24 AM
To: doug@xxxxxxxxxx; leeritchey@xxxxxxxxxxxxx
Cc: joel@xxxxxxxxxx; gurushankara@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Logic ground and chassis/safety ground
This message originated outside of DISH and was sent by:
si-list-bounce@xxxxxxxxxxxxx
All this having said, we should also remember that multiple connections to a
chassis ground is no substitute for good design practices within each board
and/or module (like switching power supplies) that gets connected to the
chassis. If we allow significant 'functional' current flowing through the
chassis instead of dedicated return paths in the boards/subsystems, it will
likely create signal and power integrity issues.
Regards,
Istvan Novak
On 5/11/2018 2:29 AM, Douglas Smith wrote:
But Lee I am talking about solving operational and ESD problems for over 40
years now, having been in engineering for iced 55 years.
Solutions are not unique but carry different costs. Multiple grounds result
in the least cost solutions most of the time and the highest PCB to chassis
resonant frequencies (hopefully beyond what would be a problem. All the
boards and chassis members are connected whether you want or not by
impedances on thd order of 30 Ohms at high frequencies. Single point ground
guarantees strong and low frequency resonance. Only with multiple grounds do
you have the option of killing the Q of the PCB/chassis resonance by adding
loss. I demonstrate this effect in all of my classes. It is easy to measure
and prove. I demonstrated the advantages of multiple grounds at AT&T Bell
Labs about 40 years ago.
You may have designs that work but I can show they cost more than they should
to achieve good ESD performance. This is easy to show which I do every six
weeks at my courses. A single pF of capacitance will couple Amps of ESD
current at very low Voltages.
There are about a half dozen articles on my website ( http://emcesd.com ;)
that prove my point with measurements and analysis.
I am not saying your designs don’t work, only that single point ground is
not achievable above tens on MHz and attempting single point ground raises
system cost to make it work.
Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office:
702-570-6108 Email: doug@xxxxxxxxxx Website: http://dsmith.org On Thu, May ;
10, 2018 at 8:50, leeritchey@xxxxxxxxxxxxx <leeritchey@xxxxxxxxxxxxx> wrote:
Sorry Doug. Been doing this a long time and have had very good results. Have
fixed many EMI problems by removing all but one connection between logic
ground and the case or Faraday cage- which many seem to want to call chassis
ground.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf Of Douglas Smith
Sent: Wednesday, May 9, 2018 9:33 PM
To: leeritchey@xxxxxxxxxxxxx
Cc: joel@xxxxxxxxxx; gurushankara@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Logic ground and chassis/safety ground
Hi All,
I respectfully disagree with Lee’s comments. It almost guarantees an
ESD problem. I have fixed hundreds of ESD problems by connecting the two
grounds in as many places as possible.
Also doing so significantly reduces the cost of switching supplies operating
from the mains, lowering induced noise by 30 dB on average for a given
supply. I have lost count on this kind of fix.
One connection also guarantees the lowest resonant frequency of the
chassis-PCB resonant circuit (which WILL exist) and prevents the simple
solution to kill the Q of that resonance. Multiple connections raise this
frequency but the best is multiple connections and then to de-Q the resonance
you will always have between the PCB and the chassis.
After 40 years of fixing chassis-PCB interactions I have developed quite a
base of solutions.
Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108
Email: doug@xxxxxxxxxx Website: http://dsmith.org On Wed, May 9, 2018 at ;
10:12, leeritchey@xxxxxxxxxxxxx <leeritchey@xxxxxxxxxxxxx> wrote:
There is no electrical reason why you need to connect these two. If you do
connect them, do it in only one place and do it with a DC connection. I make
that connection on the side of the PCB where the unshielded wires exit as a
way to minimize potential EMI escaping on those unshielded wires.
Don't know where all of the rules of thumb come from about capacitors and
multiple connections.
Lee RItchey
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf Of Joel Brown
Sent: Wednesday, May 9, 2018 9:47 AM
To: gurushankara@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Logic ground and chassis/safety ground
Unless you have a requirement or other reason to keep them isolated It would
be best to connect them together.
If you decide to keep them them isolated then add multiple capacitors between
signal and chassis ground at each chassis connection point that can be
populated with different value capacitors or zero ohm resistors. Another
option is to have a chassis ground plane layer in your PCB that is connected
to signal ground at multiple points with capacitors. If you don't have a
solid RF connection between signal and chassis ground then there can be an RF
potential between the two grounds that can radiate if you have antenna
structures such as slots in your enclosure or external cables.
Joel
On Wed, May 9, 2018 at 9:16 AM, Guru Shankara
<gurushankara@xxxxxxxxxxx>
wrote:
Hi Experts,chassis.
I am working on a modem design that is powered by an AC/DC supply
(isolated).
The design includes a base board that hosts a mezzanine/daughter card
and both the baseboard and the mezzanine cards are powered by 12V DC.
The chassis walls and misc. mechanical hardware have a conductive
coating on the inside and will be connected to the
chassis/safety/earth ground of the AC/DC power supply.
As the name implies, the chassis/safety/earth ground provides for
safety of the operator if there is a situation where the higher
voltage side line of the AC/DC supply rips off and makes contact with
the
So, I prefer NOT to connect the board logic ground to the
chassis/safety/earth ground of the supply.
Any thoughts on if I should connect them together or not?
Also, if they need to be connected, is there a recommended method?
Really appreciate your help.
Thank you,
Gurushankar
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