Dave, I completely agree with your arguments and removed redundunt parts from the schematic.
Kalvin, I am pretty sure this will work.
I see two reasons why things went to complicated in my case.
- it was done for a first time
- and I chased the noise source in a "wrong direction', was thinking that something is wrong with the mic-op.amp part and started to add filters (in a wrong way btw
)
The last thing that I made is I've disconnected the mic-op.amp and second power supply completly.
The only things thet are left are STM32F3-discovery power by USB from a host PC and SD card with SPI interface.
Then I made the voltage divedire from discoverys 3V and GND supply, as I did for the virtual ground for the op.amp.
And measured this with ADC. The result was next:
And this sounds exactly as the noise I was chasing.
I have plans to do more research on this and run few more tests (try external power supply instead of USB, and to to separate in time proces of reading ADC results and proces of dumping it on SD card).
All I have to add for now is the part of internal schematic of STM32F3 discovery board where Vref is shown
Have strong intention to find the reason of such behavour of my ADC and will be greatfull for any ideas about this.
Have a nice friday guys!