Another example I often relate is of a daughter card plugged in to a
large motherboard (in a 19" rack-mount configuration). When high-speed
logic on the daughter card drives current into the motherboard, the
returning signal currents, as they flow through the ground pins on the
connector, create a ground shift between the motherboard ground and
the local ground on the daughtercard. Hopefully we have bought a good enough
connector to limit the magnitude of this effect.
Now let's assume that we must place
a very sensitive analog component on the same daughtercard, and that
analog component needs to receive signals from the outside world
that are referenced to the chassis ground of the product (for
example, a video feed). I will sometimes place the sensitive analog
stuff on a ground "island" on the daughtercard and connect it to
chassis ground with ground pins on the motherboard connector that
are separate from the ground pins used to carry high-speed returning
signal currents to the main digital ground. In effect, I let the
analog circuit "reach through" the connector to touch real chassis
ground. Once this has been done, you can see that we had better not
allow the flow of any high-frequency currents between the digital
and analog sections of the daughtercard. If we do that, the return
path for those currents will circulate out the ground pins on
the analog section, and back in the ground pins of the digital
section, defeating our entire purpose. We were supposed to keep
high-frequency current OFF of the analog ground pins, not force
it ONTO those pins. The only correct way to connect the digital and
analog sections, once their grounds have been split asunder, is
to use isolating components (optical or magnetic) or extremely
well-balanced differential coupling. These methods avoid the circulation
of high-frequency signals on the analog ground pins.
That's kind of a long answer, but it's the way I like to teach the
subject in my classes.
Dr. Howard Johnson
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
Dr. Howard Johnson, Signal Consulting, Inc.
tel 425.556.0800 // fax 425.881.6149 // email firstname.lastname@example.org
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