Re: [SI-LIST] : Earth Ground

Howard Johnson (howiej@sigcon.com)
Wed, 18 Feb 1998 14:49:30 -0800

Hi Melinda,

First, thanks for using my book as the basis of your course. Few
university professors realize the importance of teaching high-speed
signal concepts to digital engineers. I'm glad you're doing it (regardless
of whose book you use).

About teaching the subject of grounding...
In my opinion the most important point to make with regard to grounding is
that the input to every digital logic gate is a DIFFERENTIAL amplifier.
That's right, a differential amplifier. This differential amplifier
compares the digital input signal to some local reference (often generated
inside the chip), and decides which is bigger (more positive).
It couldn't be any other way. It's not like we could make a chip
that had a private wire leading out of the package to the center
of the earth, to pick up some "ideal" ground voltage. No, every chip
evaluates its inputs with respect to its local reference. I don't move
onto even looking at grounding structures until I'm sure my students
get this point, and get it so well they will never forget it.

Another way to put it is this: every logic signal on a chip's datasheet
actually has TWO pins that can activate it: the input pin, and
the ground pin (or, for ECL families, the more positive power rail).