6 Feb
2012
6 Feb
'12
4:29 a.m.
On Feb 5, 2012, at 3:23 PM, Mouse wrote:
And, in the same sense, earthed ground is very important for lots of things. There are
two good reasons ECL runs at negative voltages:
- The output drivers are emitter-followers, so shorting a positive voltage to ground (0v)
by accident can cause big problems (I've popped the tops off PECL chips that way).
With an emitter follower, shorting something driving high (in ECL, let's call it
-0.7v) to something driving even higher (0v) does nothing. Given the general saturation
of most cabinet environments with grounded metal, this just makes a lot of common sense.
- The system "ground" is typically more stable, and this provides rather
important noise immunity (or at least improvements) for ECL circuits. This has a lot more
to do with the way we treat what we want to call "ground" than any reason
relating to its absolute potential, of course.
- Dave
I beelive one
of Vonada's laws is 'There is no such thing as
ground'.
Well...in a sense. It's a reasonable approximation when dealnig with
low voltages, such as generally occur in computers.
But there _is_ a measurable-in-isolation ground potential in the sense
of "not charged"; if you're working with electrostatics, which usually
means (from computer points of view) very high voltages and very low
currents, this can matter in a practical sense.