Design considerations for eurorack DAC - Page 1
Forum section: electronics / projects

author: MoffleMusic
author_rank: Newbie
timestamp: November 11, 2023, 04:02:30 am
content: Greetings,I have been reading for a while but this is my first post on here so appologies in advance for any mistake on my part.I am currently developping a Eurorack module, but i am somewhat new at working with larger DACs, my idea calls for 12 output channels (6CV + 6 Audio), controlled by a microcontroler (STM32H7 for now but would be downsized in the future).I have noticed two chips that could be interesting for my intended use :ADAU1962 and PCM1690 (i know it only has 8 channels), however while reading both of their datasheets some questions came up that i wasn't able to answer even after further searches.1-ADAU1962-The datasheet states that if supplied with 5V (AVDD) the nominal output voltage will be 2.25V and swing +-2.12VWhat i gather from this is that if the digital value is 0 (24 bits) the output voltage of my pin will be 2.25-2.12 = 0.13V (roughly)and if my value is 24 bits all 1s i would get 2.25+2.12 = 4.37V (roughly)The differential pin for that channel would get the opposite value, so 0.13v for 4.37v on the positive side or 4.37v for 0.13v on the positive side.That feels right but i am unsure with how the theory of operation page talks about the Vrms values for differential mode.-I am also unsure about the clocking needs of the chip, i am intending to be able to get a 48kHz sample rate on all 12 of my channels, so my bit clock should be 48k*2*24 = 2.304 MHz,This would be while pushing the proper bits on all 6 of the data channels, but the minimum clock for the chip for a 48kHz samplerate is given as 12.288mhz per table 12.My question is: how synchronized do those different elements have to be (mcu & dac) do i need to use a buffer for the mcu data channels for synchronization purposes?As for the PCM1690, the datasheet seems even stricter about all the clocking... So if anybody has some useful ressources i could read about that, they would be much appreciatedAlso if my idea seems terrible, any bit of feedback or criticism is more than welcome.Thank you very much for reading

author: dobsonr741
author_rank: Frequent Contributor
timestamp: November 11, 2023, 04:23:14 am
content: I would suggest looking at the Daisy family. Audio and CV usually not done on the same type of DAC. I found there is good community and software support for these:https://www.electro-smith.com/daisy/patchinit

author: Victorman222
author_rank: Regular Contributor
timestamp: November 12, 2023, 01:43:38 am
content: As for your first question about how the Vrms values are written about in the datasheet. Since i don't know what you know and don't know lets back up a bit.What is RMS voltage? Also called the effective value of AC, it is defined as being the value of direct current (DC) which in the time equal to one period of our alternating current will perform the same work (i.e. heating a resistor) as the AC. Basically 3Vrms heats resistor same as 3V DC. For a sine wave the rms value is the peak value divided by the square root of 2 (1.414). For example a 5V peak AC voltage (10V peak to peak) will be 5/1.414 = 3.54 Vrms.Coming back to the question the datasheet says that the rms differential voltage is 3Vrms with 0dBFS input (0dBFS input means maximum input). The difference of the maximum and minimum output voltage 4.37V-0.13V = 4.24 will be the peak differential AC voltage. Now we take that and divide by the square root of 2 and get 4.24/1.414=2.99=3.

author: cncjerry
author_rank: Unknown
timestamp: November 12, 2023, 04:47:26 am
content: Take a look at the sure electronics boards. they use ADAU1701 and a AD14xx series DSPs and they are supported by SigmaStudio, a visual programming environment for those two and other chips. Yes, more than your requirements, but if you look at the AD14xx series board you can see how the DSP uses I2C to connect to outboard DACs. Then look at the audio specific DACs like the crystal semiconductor series because all those are much more suited to music reproduction than standard DACs, which of course can also be used. But if you get a crystal semi outboard DAC board, all the clocking, interface, etc is taken care of for you. the other advantage of using those chips and boards is they are easily interfaced with AES or SPDIF for digital connections. You don't have to program all the framing, etc.I like the Sure products. they are dirt cheap, have various programming interface boards with controls for bluetooth and can be programmed easily using SigmaStudio. I was playing around with building Eurorack modules based on these boards, bought a set of Electronotes from Bernie Hutchins, and was really into designing all my own stuff. Bought a few synths and I set the projects aside for retirement. By the way, the sure boards have auxADC inputs so you can bring in CV and decode it for keyboards, or better yet, bring midi directly in though I never tried that with SigmaStudio but it should work.