[sdiy] Synth applications of CMOS inverter as SPDT switch?

Olivier Gillet ol.gillet at gmail.com
Wed Oct 31 01:38:29 CET 2012

On a related note, a way of building a cheap computer-controlled
attenuator for unipolar signals is to feed the signal to the Vref pin
of a DAC, and change the DAC value to control the attenuation factor.
For a polarized signal and if you don't want control feedthrough it's
a bit more complicated - you need to use a digital pot (most of them
having poor specs compared to DAC), bias your signal at Vcc/2, and tie
the other end of the pot to Vcc/2. I've seen this used in the RockIt
synth kit. http://hackmeopen.com/Downloads/Rockit.pdf. I am not sure
how they solved the dynamic range problem though - probably through
these resistors connecting the wiper to one end of the pot to skew the
response. This same project uses the "shutdown" pin of some op-amps to
turn them into switches. This is a very convoluted filter -
ironically, the same thing using the more traditional "DAC to generate
CVs -> classic voltage-controlled SVF" has a lower part count!

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