[sdiy] DAC via Mux

[Eric Brombaugh]

On 11/19/2009 05:07 AM, Justin Owen wrote:
> Firstly - my plan is to clock the mux via pins on my micro. The sequence would be - inhibit the mux, set the DAC output value then open the correct mux output - and then just keep repeating that. I figured a clean break as the mux was inhibited is better than glitches and crap as the values were re-written. Sound right?

That should work fine. Inhibiting the mux while the DAC settles is an 
excellent idea. Also, don't forget that you'll need sample/hold caps and 
buffer amplifiers on the mux outputs - without that your output signals 
will not hold the voltage while the DAC is driving the other channels.

> Secondly - I'm wondering about the speed I should be clocking the mux at and if I should be trying to 'sync' that to the write rate of the DAC. As fast as possible? Something based on the speed of something else?

"Clocking the mux"? I assume that if you're inhibiting the mux while the 
DAC settles that you'll be naturally synchronized with the DAC. You'll 
want to make sure that the inhibit timing is long enough for the DAC to 
actually reach the desired output voltage - check the data sheet for 
that information. You'll also want to ensure that the mux is enabled for 
long enough to allow the S/H cap to settle. That depends on the DAC 
output impedance, MUX on resistance and S/H cap value. YMMV.

Check Seb Francis' Fat Controller MIDI2CV schematic for a good example 
of how this is done:

http://burnit.co.uk/sdiy/index.php?page=fatcontroller

Eric