4051 mux problem?

[Rene Schmitz]

At 09:58 23.07.99 +1000, Stewart Pye wrote:
>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. 

24V total will be too much! Maximum supply voltage is 16V. (Vdd/Vee)

>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?? 

I've been running into these problems lately while experimenting with the
such a circuit. (for my new Centronics->CV interface) 
Holding the Vee terminal a Volt below ground would perhaps help. (Didn't
try it yet myself.) The circuit drifts more arround 0V than at other
voltages, so I suspect some 
mysterious behavour of the mux, rather than the Ibias of the opamp. (I've
used the same opamps with FET switches and didn't run into that problem.)

Btw, I did use TL084s for the unity buffers and 22nF caps to GND.

Hope this helps.

 Bye 
  René



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