[sdiy] using a dac as a digitally controlled amplifier

[Ingo Debus]

> Am 04.08.2024 um 19:13 schrieb Benjamin Tremblay via Synth-diy <synth-diy at synth-diy.org>:
> 
> I can imagine making a state variable filter using a dual DAC as the ‘gain' stages.  

Looong time ago (past millennium) I built a state variable filter like this, using the DAC8408. The circuit is in the data sheet, see page 13:
https://www.analog.com/media/en/technical-documentation/data-sheets/DAC8408.pdf <https://www.analog.com/media/en/technical-documentation/data-sheets/DAC8408.pdf>

One caveat, if you build the integrators as shown in the circuit (R from DAC8408, discrete C): The absolute value of R is quite inaccurate, see „Input resistance" on page 2: 6 to 14 kOhms! It was necessary to build a rig to measure the resistance of each DAC chip and calculate the proper C values.
If you use the internal „feedback resistor“, the problem goes away, since the feedback resistor is well matched to the other resistors that form the DAC. But for a programmable integrator you need an additional Opamp then.

@Chris: that circuit works well with bipolar signals.

Ingo
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