MUX

[patchell]

Mikko Helin wrote:

> On Fri, 3 Dec 1999, Guido Goebertus wrote:
>
> >
> > Can I use standard CMOS 4051/4067 for the MUX? Or is it better to use
> > HCT series? What would be a good choice for the hold caps (type,value)?
>
> Use the CMOS muxes as you can use them with >5 V voltages, and
> the max. CV out is about 10 V - Vref for 127 MIDI notes in 1/2 steps
> is 127/12 = 10.583333... V, so 12V is a good Vcc for the mux.
>
> Use 0.1 uF caps with TL082 or TL084 op amps. I think polyester
> caps are OK in S/H's, though there might be better ones, but
> ceramic caps are not recommended (though I don't know why).
>

    There are several good reasons not to use ceramic capacitors for
sample/hold's or anywhere else in a signal change.  Number one in my book is
they have a memory.  Several years ago I was debugging a data aquisition
system that was designed by somebody else.  It had a 16 bit ADC and I was
only getting 8 bit accuracy.  I went down the signal chain looking for
potential problems.  The guy had used a ceramic capacitor as a low pass
filter, he figured it did not matter.  But the dang things remember the
voltage they were at and try to go back to that value.  I replaced it with a
polystyrene, and it fixed they thing right up.

>
> I think at PAiA site (www.paia.com) has schematics of the
> MIDI2VC8 converter, see how the S/H's are done there.
>
> -Mikko
>
> >
> > Guido
> > --
> >
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> >        (o o)
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> >
> > ------Guido Goebertus THD Studio Amsterdam------
> > ---------->Total Harmonic Distortion<-----------
> >
> >

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