Tristate Buffer Output if Input is High-Z?
ard at p850ug1.demon.co.uk
Thu Sep 29 14:13:12 CDT 2005
> 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?
An analogue switch IC is a similar device, and those do exist. The
problem is finding one that will switch quickly enough (I would guess in
a few nanoseconds) for this application.
Also remmeber that analogue switches don't buffer the signal, you don't
get an increased drive current if you use one.
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