If you need a safety earth connection, you need to ensure that your AC/DC
converter provides such an Earth connection.
Your exposed metal should be connected to this Earth.
You can then connect this to the 0V return of the AC/DC converter, but only
in one place with a solid, low inductance connection.
Connecting it in multiple places only gives rise to multiple return path
options - bad idea!
The idea of using multiple connections with either caps or 0R resistors has
been passed down through the ages...
Each engineer was told a different theory by their mentor/master/guru who
insisted that their method was the key to eliminate all the EMI woes...
The use of a land pattern would allow the engineer to experiment with
different values and parts in order to find the "silver bullet" for that
particular design...
So, each time a new design would be started, the next engineer would take
inspiration from the previous design, and copy the same mounting hole
connection strategy, foolishly thinking it would work for a completely
different application...
Regards,
Dr. Tim
On Thu, May 10, 2018 at 7:10 AM, Lyndell Asbenson <
Lyndell.Asbenson@xxxxxxxxxxx> wrote:
This is , and has been debated tons of times in the arena of high end
semiconductor test equipment. High end meaning > $3M/unit. I have seen tons
of book and tons of theory but, like most things in engineering it depends
on what your goals are?
Intel at one time had a big problem with its memory chip 1103 testing
showed
it was good but when engineers attempted to use it, it was unreliable.
David
House came to Intel from a systems background he solved the problem through
1000s of experiments the solutions opened ended up requiring going from a 2
layer PCB to 4 layer PCB. Having power and ground planes on the inner
layers, also requiring caps .o1 .001 uf for noise decoupling etc.
-Regards,
Lyndell Lee Asbenson
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of leeritchey@xxxxxxxxxxxxx
Sent: Wednesday, May 9, 2018 10:12 AM
To: joel@xxxxxxxxxx; gurushankara@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Logic ground and chassis/safety ground
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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