After turning on power on VIN and GND I measured an stabilized 5V output, and it decreases over approx 5-10 minutes and finaly stabilized on some point a little different for different units i tested at lease 5 units all have the same behavior, voltage drops from 40mV to 10mV it depends from specific unit. I test all units with no other pins connected.
I need very stable and accurate reference voltage just from beginning when device is powered on, ho i can achieve that ?
So why not use the VREF source ?
Or an external source, such as
If you have >=4.75V, can get a very stable 4.5V out. Use that for Vref.
So if i connect a external voltage stabilization to VREF i will have a stable "5V" on 5V output ?
No, the 5V on the power header comes from the 5V regulator. If that is varying, you are stuck, the only recourse being to change the 5V regulator to something different.
I believe this is the regulator used
You can see it has a range from 4.95V to 5.05V, so 5V +/-50mV (1%). The 10 to 40mV you see is in that range.
If you are powering from the computer USB port, I believe that can have an even wider range +/- 0.25V, so 4.75V to 5.25V (or 5%).
Putting a stable voltage on Aref gives you a stable voltage for performing analogRead's, nothing more.