Tristate Buffer Output if Input is High-Z?

[Paul Koning]

>>>>> "Tony" == Tony Duell <ard at p850ug1.demon.co.uk> writes:

 >> It sounds like the tristate buffers don't do exactly what I would
 >> like, although I think I can make it work with more logic in front
 >> of the control line for the buffer.
 >> 
 >> However, doing a bit more research, it looks like what I really
 >> need is a Transmission Gate.  The important difference is that a
 >> transmission gate will pass the input regardless of its state, so
 >> L passes L, H passes H and Z passes as Z.  The disadvantage is
 >> that if you have a noisy signal, the TG doesn't clean it up at all
 >> the way a TB will.
 >> 
 >> The catch is that I cannot find one listed anywhere as a part that
 >> one can actually buy.  Are transmission gates purchasable parts?
 >> Or are they just something they discussed in my VLSI textbook?

 Tony> An analogue switch IC is a similar device, and those do
 Tony> exist. The problem is finding one that will switch quickly
 Tony> enough (I would guess in a few nanoseconds) for this
 Tony> application.

No problem.

Quickswitches are rated to several hundred Megahertz.  If that isn't
good enough, you can use microwave switch transistors.  I don't know
the max switch rate of either, but presumably any transistor capable
of carrying microwave frequencies can switch on/off in a nanosecond or
better.

	paul