In theory, and at first blush, ground is ground is ground. After all, they
all are at zero volt potential (usually).
But then, even on a single ground plane there can be 250 mv difference from
side to side because of IR drop caused by current flows across the plane.
All circuits on a card may each require +5 volts. But some may be digital,
some may be analog, some may be very high gain, some may be very sensitive
differential switching circuits, some may be high power electromagnetic
switching circuits. Therefore, it may be desirable for EACH ONE to have it's
own, separately regulated + 5 volt power supply. It is not hard to
intuitively understand that this would lead to separate, dedicated power
traces, even power planes, for each type of circuit. It is not as
intuitively clear that each one SHOULD ALSO have its own ground return
(plane). Even though the grounds are at the same potential (zero volts),
they are still kept separate. The reason is to prevent the noise on the
ground system from one type of circuit from coupling into another circuit.
It is common to have analog and digital circuits on the same board --- hence
analog and digital grounds. But don't let the terms "analog" and "digital"
confuse you. You could (and we do) just as easily have power 1 and ground 1,
power 2 and ground 2, power 3 and ground 3, etc. And all these grounds
should be kept separate, just as the power leads are kept separate. Again,
the reason is to prevent coupling of noise from one part of the circuit to
Since all grounds are (usually) at the same nominal potential (zero volts)
they are usually tied together. But if tied at more than one point, there is
a possibility for a (current)(ground)(noise) LOOP to exist, causing (you
guessed it) noise. So they are tied together at ONE point --- to prevent loops.
Chassis ground is the (often)(single) point where the grounds meet the
outside world (if they do). Usually, chassis ground is the same as "earth"
ground, which is the same as "signal" ground for a radiated (radio) wave.
Thus, one place to tie analog and chassis grounds together is at the antenna
The reason for separate grounds, again, is to prevent coupling between
sensitive circuits. That is why you don't ever allow:
1. overlapping grounds
2. overlapping power planes
3. one power plane to overlap another's ground
4. signals to flow across an unrelated power or ground plane.
Hope that helps
UltraCAD Design, Inc
At 06:03 PM 2/16/98 -0700, you wrote:
>I am teaching a class out of "High Speed Digital Design" to first year
>grad students. We recently discussed grounding issues and there was a
>lot of confusion about "digital" ground vs "analog" ground vs chassis
>/ earth ground. Grounding in a cellular system .. etc..
>If anyone has clever ways to explain these concepts intuitively I (and
>my students) would appreciate the help. How do you view these issues
>from a practical everyday point of view?
>University of Colorado at Boulder
>ECE Department , Campus Box 425
>Boulder, CO 80309-0425
>(303) 492-7448 Fax: (303) 492-2758
>Office: Engineering Center OT4-14