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

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From: Chris Bobek (cbobek@tality.com)
Date: Mon Jan 22 2001 - 13:03:33 PST


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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