Tristate Buffer Output if Input is High-Z? - test-drb@ccmp.vtda.org

29 Sep 2005

...
 From: "Paul Koning" <pkoning at
equallogic.com>

 >>>> "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 
Hi
 The problem is as Tony mentioned, you get no buffering.
This means you can't expect to put the mux too far from
the inputs on either side. You are really better off
using something like '245's.
Dwight