Discrete R2R DAC (part 1)

Discrete R2R DAC (part 1)

I wanted to design a R2R DAC long time ago. I’m aiming for simple discrete design with 74 series logic ICs. After a long planning, I finally built 2 versions for testing and I’m amazed with its performance.

Kicad R2R DAC layout, Kicad, PCB, DAC, R2R, DIY, 24bit
Kicad R2R PCB layout

I read a lot of technical article about I2S signal , and the easiest way for me to process digital signal in my R2R design is right justified format. I choose 74 series logics and the first stage is 74HC164, a serial to parallel shift register. The shift register driving octal D type flip flop such as 74HC374 ( I used 74AC574 in my final design ) will directly connected to R2R network and directly connected to simple RC filter and then to output socket, without any op-amp or line stage.

 Kicad, PCB, DAC, R2R, DIY, 24bit
Kicad R2R DAC layout, Kicad, PCB, DAC, R2R, DIY, 24bit

The received right justified signal are actually 2’s complement binary, if I can find a Spdif to right justified output IC with COB ( Complementary Offset Binary ) format, that will be much easier to design because COB can be directly output to R2R network from D type flip-flip. Since I can’t find Spdif to right justified + COB output ICs, I choose classic Cirrus Logic CS8416 instead, with 2’s complement output. I will write another article regarding my DIY Cirrus Logic CS8416 board after this.

Kicad R2R DAC layout, Kicad, PCB, DAC, R2R, DIY, 24bit

I need to convert 2’s complement to COB , by either reverse all bits accept MSB bit, or just reverse the MSB bit. I used 7414 inverter here.

The most difficult parts are matching resistors, need to matched down to 0.001% or better to ensure linearity of the DAC. I bought 2 spools of 5000 pcs SMD 0.1% resistor to match down to accuracy I wanted, and it take me quite a long while to complete it.

The whole projects actually facing quite a lot of problem which I need to solve them one by one, switching noises are enemy of DAC, and PCB layout and signal grounding are very critical. Power supply need to be clean and my DAC design are very sensitive to power supply design. NiMh Battery power supply had been tried with good result.

I will split into several parts for this DAC article to cover every design and building detail. The finished DAC already in use for several months without any problem, I need some time to complete this DAC article to share with you all. Please stay tuned and subscribe to my youtube channel for all my projects.

Here’s my test video , but I need to re- record a proper one using better microphone though :

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Part 2 :

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