simple sample/hold circuit?

[Ian Fritz]

Hi --

My computer-controlled analog synth uses an interface similar to Rene's
(actually, largely taken from Hal Chamberlin's book). It is run by a Z80
micro/DAC0800 and uses 8 4051's to scan 64 analog CV's and 5 9334
latches to set the patch matrix. I used 20 nF caps and TL074 buffers.
The system refreshes at about 100 Hz, using a continuous loop. It polls
once per loop for a request from a hardware programming unit and can
also be interupted by the system-monitor's interrupt.

I've never been very happy with the 4051's.  They often have too much
cross talk and/or digital feedthrough.  I had to deselect about half of
them.  If I were doing it again, I'd look for something better. Does
anyone know of a pin-for-pin replacement with better performance? SSM
used to sell a drop-in replacement MUX with the storage cap and buffer
built in.

  Ian

Rene Schmitz wrote:
> 
> Hi Andy!
> 
> At 11:58 02.10.99 -0500, you wrote:
> >My question is, what's a good way to
> >do sample and hold on the DAC output so that I can scan through all the
> >outputs using a single DAC?  I guess it doesn't need to hold for very long,
> >but I haven't written the PIC code yet so I don't know how long it needs to
> >hold for.
> 
> In my multi output (up to 80!) printerport CV convertor I've used 4051s
> with 22n MKT (=mylar) caps and TL084 as buffers. Works pretty good, the
> hardware side is done, but my software is not yet ready to publish it.  The
> caps hold a few seconds without serious droop. I think an update frequency
> of 10-100Hz will be more than sufficient.
> 
> With the PIC I would not use the timer, but just do a refresh loop forever,
> and use an interrupt to jump into the routine which receives and processes
> the data. Otherwise one would run into problems doing a software uart.
> Constancy of the update frequency is not required.
> 
> Bye
>  René
> 
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