u "micro" data acquisition unit

[mrjoda]

Hello eevblog members,

i would like to introduce you one of my new hobby project. Work name is simply uDAC. I had some LTC circuits in my drawer so... i want to test it. It is first version and i want to continue with this project and made some "adult" DAQ. NI DAQ are nice, but they are quite expensive and i need to educate myself

This project will be open source, if it will works

Basic idea : take ADC, DAC, reference a put it in one board, isolated power supply and SPI interface.

ADC is older, but still OK 24bit LTC2410
DAC - LTC1650
Main reference - LTC 6655 2V5
SPI isolator - SI8662AB-B-IS1
power supply DC/DC AIMTEC 5V to 2x7,2V
positive supply rail LDO - ultra low noise high psrr LP38798SD
negaitve supply rail LDO -  ADP7182A 5V fixed (very hard to find )

Steps :
1) Find,collect and solder all parts- DONE
2) Do some long time stability measurements on supply rail and reference. Check electromagnetic emissions and noise spectrum on supply rail, tune supply filter.
3) Write some test code to STM32
4) Do some stability measurements on DAC and ADC. Evaluate ENOB, errors, nonlinearity atc.
5) Make some changes for better results.


I will put results here. It take some time because i dont have GPIB controller and cable to my HP34401 so i must take it to my university lab.


After all steps i post here altium design files, C test code and all results.

At the end, excuse my english level i am not an native.

[BFX]

Nice
One PCB is waiting also on my desk
I'm looking forward to see first results   
 

[mrjoda]

so... after few first baby steps...

i found small error in design. Signal marked as CS/LD at the isolated side (chip select) for DAC is connected to INPUT of isolator. I accidently swapped MISO and CS/LD and I didn't notice this mistake .

I tried to easiest part - DAC.

there are some results. I am going to do better measurements such as noise, long time stability, DNL, INL, ENOB, offset, transfer characteristic, maybe more. From results i want to modify an algorithm and try to set a correct requested value of voltage. Dinamic signal will be next level. This DAC is quite slow so generating signal is not primary purpose.