Can voltage output DACs be used to attenuate a signal?

[Alien Brother]

I want to attenuate an input signal (audio frequencies) by an amount that I can vary; can I use a voltage output DAC for that, giving the signal at the reference input, or is there something in the design of voltage output DACs that requires the reference input to be constant?

Naively, a voltage output R-2R DAC should be like a digital potentiometer with one pin fixed to ground, right? At the same time, DACs offer more bits and specify lower glitch and digital feedthrough.

EDIT: There was a paragraph here about LTC2641, but I now realize it needs reference input above 2V, so it won't work the way I imagined. Other than that, what other issues should I know of?

[tszaboo]

It specifies the reference input range as 2V to VDD. That makes it less than ideal for this.
You normally do this with multiplying DACs. But I've seen some Microchip 8 and 10 bit DACs that potentially can do this. They have 0 to VDD reference input, reference buffer, and output buffer. Didn't try it yet though. Part numbers like MCP48FVB11

[Alien Brother]

It specifies the reference input range as 2V to VDD. That makes it less than ideal for this.

Yeah, just after posting the message I noticed this.

[Alien Brother]

You normally do this with multiplying DACs.

"Multiplying" are those specified as "current output" right? Now looking at the datasheets, I see examples of how to use them with single supply, so maybe I shouldn't be focusing on "voltage output".

But there are things that still bother me. For example, here the datasheet for AD5450 says that in single-supply configuration, input is limited to "low voltages". Without saying what is considered "low".

[TimFox]

One can terminate the current output of a traditional “multiplying DAC” with a low value resistor (which sort-of works), but usually a transimpedance amplifier (op amp with feedback resistor) is used for a voltage output.
Note that so long as the bias conditions for the multiplying DAC are met, the path from voltage input to current output is essentially a switched resistance made from L networks (so one must properly terminate the “ground” of the Ls for where the unused current goes).

[temperance]

https://www.advancedphotonix.com/media/pages/our-products/optocouplers-optoisolators/nsl-32sr2/a0b63ed33d-1718287447/ds-nsl-32sr2.pdf


Some say it sounds better than bananas while it costs peanuts.

[RoGeorge]

In theory, it might work, in practice, it won't be good for volume control.

For audio volume you need a logarithmic attenuation, not linear.  Either use an I2C (or SP) controlled dual potentiometer (digipot log, not linear)
https://eu.mouser.com/c/semiconductors/digital-potentiometer-ics/

or, if you already have a DAC on your board and want to use that, try to build a DIY volume control with diodes, in which the DAC controls the diode internal resistance, and thus the attenuation.

[Picuino]

https://www.analog.com/en/resources/design-notes/ltc1590-applications-versatility-of-dual-12bit-dac.html

[temperance]

In theory, it might work, in practice, it won't be good for volume control.

Why not? With a DAC driven by some software, you can make any kind of curvature. Or a spooky one that changes curvature unless if you did something specific with the volume control. Professional gaslighting 

[Alien Brother]

volume

Volume is a fair point, but I want amplitude.

Then, without having actual experience with this stuff, I would imagine a 10 or 12-bit DAC would be superior to a log digital pot (if we forget about zero crossing detection and what not). The highest resolution log pot I've seen on paper, was with 60-something taps spaced at 1 dB. With a 10-bit DAC, 1/1024 is about -60 dB, so more or less the same attenuation before complete mute, and I think at low dB, large steps in dB (well, 6 dB between 1/1024 and 2/1024) don't feel bad.

[Roehrenonkel]

Hi Alien Brother,

have you looked at "Logdacs"?
https://www.analog.com/en/products/ad7111.html

A bit limited with Dynamic Range: 88.5 dB
but Resolution: 0.375 dB.

Good luck

[Doctorandus_P]

PGA2311 has been the best audio attenuator I know for a long time (30 years?) It "attenuates" from +31.5dB to −95.5dB with 0.5dB Steps.

For multiplying DAC's, there is an abundant amount of easy to find info on those. Plenty of reading if you want to know more.