RE: [SI-LIST] : Dealing with Chassis and Digital Ground

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From: Ron Mancuso (ron.mancuso@corrent.com)
Date: Mon Jan 22 2001 - 14:56:19 PST


Before you design it, verify 1) the power adaptor is a brute force
transformer with a full-wave rectifier, 2) the core of the transformer
is tied to earth ground which in turn is tied to DC ground. If the
power adaptor is a switcher, you have to know the electrical
characteristics which will be beyond the scope of this discussion.
 
With those points verified, some things to remember. Earth ground is
generally noisy. The goal is to keep a 'dirty' area to dump noise
transients to better protect the elements within your circuit that are
sensitive to threat. And the fact the DC barrel connectors are
relatively high impedance paths to ground doesn't help much. Now as
ground enters the card from the DC barrel connector, isolate a chassis
ground area of the pwb that will run the perimeter of the PWB (for the
external I/O connectors). Sink any chassis ground connections to this
ground. You hadn't mentioned ESD or fast transient protection, but this
should be included over chassis ground. Avoid capacitive coupling
system ground to chassis ground.
 
To construct the system ground, the DC power and ground is run through a
common-mode choke. The inductance doesn't matter (as much) as the
permeability of the core. It should be highly permeable. Once past the
choke, system ground will occupy the whole interior of the PWB.
Assuming you're using an on-board switcher to construct your digital
voltage(s); a double-pi filter before the switcher has worked well for
me in the past. System bypass should be done PAST the switcher stage.
 
In this scheme, conducting the heat to the chassis becomes more
difficult. I never recommend using stand-offs connected to system
ground. Over time, the contacts electrical characteristics change as
they're confronted with various environmental threats (humidity, thermal
expansion/contraction, etc.). Ignoring that, tying system ground to
chassis ground allows for system ground transients to radiate through
chassis ground (via stand-off) or system ground to accept radiation.
For pure thermal conduction, define a circular keep-out area that is
isolated electrically from the rest of the board (leave room for the
screw). Each layer will have a full layer of copper that will 'touch'
the stand-off. Liberally run via's between layers. Use thermal paste,
if necessary. These steps have worked well in past designs.
 

Thank you,
Ron Mancuso

Corrent Corporation
phone: (602) 748-1401 x255
fax: (602) 748-1402

-----Original Message-----
From: Chris Bobek [mailto:cbobek@tality.com]
Sent: Monday, January 22, 2001 2:04 PM
To: Si-list
Subject: [SI-LIST] : Dealing with Chassis and Digital Ground

I am designing a board with metal enclosure that has several I/O
connectors on it (coax). The power input to the enclosure is a standard
DC barrel plug (GND and 7VDC). The coax inputs are connected to
transformers on the board. Each input is supposed to have the ability
to connect its shield to earth ground via a high voltage cap (selectable
via a stuffing option).

My problem is, I only have one ground, as mentioned earlier. I believe
this ground is tied to earth through the external AC/DC power adapter.
So, what is a good practice for creating the chassis ground?

1) Do I simply make a point connection where power enters the board
between ground and the chassis? If so, do I use a cap or resistor to
make the connection, or do I just wire them together?

2) Assuming I make the ground connection to the enclosure, I have
several standoffs on my board that I want to use to take the heat away
from the planes and conduct it to the enclosure (the enclosure is
sealed). I was just going to use metal standoffs that connect
electrically to the ground planes, then fasten it to the enclosure.
However, if I do this, I will have ground loops! What is good practice
for tackling this problem?

3) My idea for solving the problem is:

a) Connect the DC Power Jack's ground to the board at a single point.
b) Add 4 metal standoffs, one in each corner, electrically tied to
board ground and the chassis.
c) Create a moat or ground split around 3 sides of the circuitry, with
the drawbridge facing the power jack.
d) No signals cross the moat, except the transformer coupled signals.

I think this will work because any noise picked up from the cable and
chassis will be conducted through the standoffs, around the circuitry
(because of the moat) and back to the power jack. What do people think
of this from an SI and EMI point of view?
  

Thanks,

Chris

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