I agree that grounding is a complex topic which can't be fully covered within
this forum. However, I must make a general comment with regard to single point
ground versus multi-point ground for a digital board. There is in fact no such
thing as a single point ground for a digital board because of distributed
capacitive coupling between the digital board and the system reference plane.
If a single-point ground is attempted, you end up instead with large RF current
return loop areas which become antennas for radiation and susceptibility. To
minimize the loops, you must "short out" the capacitive coupling by connecting
the digital circuit ground plane in the board directly to the reference plane of
your system at multi-points. Each connection creates its own little "loop"
between the circuit ground plane and reference plane, but these loops are
smaller and more controlled than the loops that are formed with the distributed
capacitive coupling.
Methods of multi-point grounding and other details of circuit board design can
be found in a number of publications. I recommend "Printed Circuit Board Design
Techniques for EMC Complance" by Mark Montrose or "Noise Reduction Techniques in
Electronic Systems" by Henry Ott.
Jim Hulbert
Senior Engineer - EMC
Pitney Bowes
brian_kunde <[email protected]> on 12/06/2000 11:36:04 AM
Please respond to brian_kunde <[email protected]>
To: PRao <[email protected]>, emc-pstc <[email protected]>
cc: (bcc: Jim Hulbert/MSD/US/PBI)
Subject: Re:Grounding
Hello Praveen,
The topic in which your question addresses is "Grounding". Grounding is one of
the most complex topics EMC engineers have to deal with especially in a design
that has both Analog and Digital circuitry. Most every case is different and
there are thousands of variables to consider. I will address the most basic
points and hope they will help you out.
The concept of Ground Loops or Earth Loops is more of an analog issue than
digital. Lower frequency and usually low voltage analog signals can be very
sensitive to Ground Loops.
On the other hand, digital circuitry is less effected by ground loops and in
most cases it is desired to keep the loop areas as short as possible. This is
usually accomplished by grounding digital circuitry as often as possible.
Bypass caps and other de-coupling components are also reducing loop area.
It is still a debatable topic on whether Digital PC cards should have a single
point ground or multipoint. I think in most cases it just depends on the
application. But, I think most everyone will agree that getting a good ground
connection on both ends of a digital I/O cable is a must. You also need to have
a good PE ground connection between equipment and its peripherals or it will
have trouble passing the immunity tests you listed below (not to mention
possible radiated emissions problems).
My advice it this.... internal to your design you should do a good job of
isolating Digital and Analog circuitry to keep digital noise out of your analog.
Digital I/O should be grounded well at BOTH ends of the cable providing a good
return path for signals and providing a common Earth Reference to between the
main unit and the peripherals.
Like I said, these are only the basic concepts that apply most of the time.
Good luck,
Brian
____________________Reply Separator____________________
Subject: Grounding
Author: Praveen Rao <[email protected]>
List-Post: [email protected]
Date: 12/6/00 6:52 PM
Hello Group,
We currently have a system which has modules in a box(metal) with floating
digital and analog grounds.It is a Class 1 (safety) equipment.
We intend to connect the 0V of each board to protective earth (chassis) at a
common single point at the power supply.
By doing this, the conducted emissions, ESD, Surges, etc show better
results.
We have a serial port (DB9)on the device which is only used for configuring
Our concern is when a PC is connected to this port, cause in PCs the signal
ground (pin 5) is normally connected to PE.
So, when a PC is connected to the serial port of our system we may end up
with signal grounds on each end of this serial cable
connected to it's own device PE. This may cause earth loops. Since this is
only a config port, approvals may not be an issue
but functionally in the field this may create problems.
Is it common to connect digital grounds to a single point chassi ground
directly at the power supply ?
Should there be any concern from functionality point of view ? especially
when a PC is connected to our serial port.
Your comments will be greatly appreciated.
Praveen Rao
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