[sdiy] using a dac as a digitally controlled amplifier

[brianw]

On Aug 5, 2024, at 8:28 AM, Ingo Debus wrote:
> Am 04.08.2024 um 19:13 schrieb Benjamin Tremblay:
>> 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

There are many ways to design a DAC, and I am particularly fond of the way the DAC8408 does it. You get balanced outputs (complementary outputs), and the conversion can be very fast. I've used similar DAC designs capable of 125 MHz sample rates (yes, 125 MHz, not only 125 kHz).

The down side is that the output is current, not voltage, but sometimes that's a feature.

Brian


> 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