[sdiy] CD4051 working as Demux

[harrybissell]

NOTICE that is is no guarantee that one switch will be OPEN
before the next one closes.  There can be leakage between
adjacent switches at some times during the transition.  A really
high performance app might use the inhibit to turn off all the
switches, then change the mux address, then de-inhibit.  If you
are driving from a low impedance you should be OK...

... I did a 'trick' with the 4051 this weekend where I used the
analog and DIGITAL characteristics at the same time, ie the input
to the analog switch also selected which output it should go to.

It was a miserable few hours, tracing the oscillation that resulted...
and while it was oscillating the leakage between states caused the
input voltage to go to hell (as well :^).

Separate dropping resistors to the analog, and digital inputs cured the
problem.  This way when the mux changes state (and the switches might
overlap)... the digital inputs do not see the transient state.

So plan on getting USED to 4051 operation before you get too cozy.  Or go
for one of the Maxim or Siliconix parts instead

H^) harry

Magnus Danielson wrote:

> From: Gorka Garcia <torpedo at demadrid.com>
> Subject: [sdiy] CD4051 working as Demux
> Date: Sun, 13 Feb 2005 11:18:13 -0800 (PST)
> Message-ID: <20050213191813.30EF437CEE at sitemail.everyone.net>
>
> > Hello List!!!
>
> Hi Gorka!
>
> > How does the 4051 operate when being used as a Demux? Basically, my concerns
> > are about what does it do when more than one channel is selected and when no
> > channel is selected. Looking at the datasheet, it does not seem to explain
> > much about its functionallity as a demux.
>
> The 4051 has been successfully used as a demux of CV signals in several synths.
> Just look at the OB-8 schematics for instance, from each of the 8 taps a 22nF
> cap hangs with a TL084 op-amp as a voltage-buffer. By keeping the update rate
> sufficiently high, there is no real problem with doing this.
>
> Basically, the 8 analog gates will be high-impedance when they are not enabled
> and then only one of them can be enabled at any point in time. The inhibit
> input ensures that all 8 gates are in their off-state unless you want one of
> them enabled.
>
> Cheers,
> Magnus