4051 mux problem?

[Grant Richter]

That would be the current out of the op-amp input.
Remember, even if they are FET inputs, they still require
some finite amount of current flow to stay active.

In this case they source current, so the voltage drifts up.
If they sank current the voltage would drift down to -V.

Typically in this arrangement you would write an interrupt service routine
that scans a certain number of memory locations into the
sample and holds. That way, the main code routine only has to write
values to the RAM buffers and the interrupt routine takes
care for scanning them out. For MIDI, you would like the service routine
to refresh all channels within one millisecond.

One basic assumption is that time differences less than a millisecond
are not perceptible. Possibly wrong, but that is the reason MIDI has
such a low baud rate. If your triggers are within a millisecond of
each other, it should have minimum "flam" effect.

----------
> From: Stewart Pye <stew at uq.net.au>
> To: synth-diy at mailhost.bpa.nl
> Subject: 4051 mux problem?
> Date: Thursday, July 22, 1999 6:58 PM
> 
> Hi...
> 
> I've built a midi to drum trigger unit. The output of the micro goes to a
> DAC and then to a 4051 mux. The mux then goes to an opamp buffer with
> parrallel cap.(S&H similar to paia midiCV) The problem is:
> 
> If I send 00h to the dac then switch through each channel of the DAC to
> clear the triggers so no outputs trigger, they stay low for a while and
> then start rising which causes oscillation of the drum oscillators. I
would
> have thaught that they would stay low. 
> 
> The 4051 supply is +12V and gnd. Could this be the problem? Maybe I
should
> use -12V instead of ground. It's not really a problem because I just keep
> refreshing the mux. I'd really like to know why this is happening though.
> Any ideas?? 
> 
> Regards
> Stewart.