DAC to Line Out

Thread Starter

upand_at_them

Joined May 15, 2010
940
I want to experiment with microcontroller DDS. I have a couple Microchip MCP4822 12-bit DAC's, which I'll run at either 3.3V or 5V so I assume the output will max out close to that operating voltage. I found a commercial product that uses a DAC to Line Out to see how I might design my circuit, but I'm confused by the schematic. That DAC runs at 3.3V. The datasheet for the IC lists the typical output as 2.1V and the DAC board is not attenuating it. There's no resistive voltage divider or op-amp divider, just a simple RC low-pass filter (470R and 2.2nF). Isn't Line Out supposed to be 1V max? Should I buffer and reduce my own DAC output to no more than 1V? I'm probably going to send my DAC output to my Bose SoundLink Color, since it has an Aux input, is portable, and has pretty good sound.

Here's the commercial product (although it uses a 32-bit DAC):
https://shop.pimoroni.com/products/pico-audio-pack?variant=32369490853971

The schematic:
https://cdn.shopify.com/s/files/1/0174/1800/files/pico_audio_pack_schematic.pdf

And that DAC's datasheet:
https://www.ti.com/lit/gpn/pcm5100a
 

Ian0

Joined Aug 7, 2020
6,660
A 3.3V DAC will give 1.167V rms. Line out is 0.775V rms, into a 600Ω load, so it won't quite make "line level" with a 470Ω series resistor, but nobody uses 600Ω loads any more. It's probably close enough!
 

Thread Starter

upand_at_them

Joined May 15, 2010
940
Oh, so is this simply a voltage divider calculation?...Upper resistor is 470, lower is 600, Vin is 1.167V, so Vout is 0.654V?
 

Ian0

Joined Aug 7, 2020
6,660
Do I need to shift my 0-3.3V DAC output to zero-offset AC via a series cap?
Yes.
I'd buffer it with a unity-gain op-amp as well, you don't really want a 470Ω output impedance, nor do you want the capacitance of the cable changing the cut-off frequency of the filter.
And if you're adding an op-amp, make it a Sallen & Key 2nd order filter. Most audio DACs will recommend a 2nd order filter in their datasheets.
 
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