How to Read ADC value without Common GND ? (Possible or not !) I want to Read external signal with the microcontroller. 

But the problem is that ADC does not work because the signal GND and the microcontroller are not Common,

My question is, have you encountered such a problem? Is there a way to do it?
The solution is to always use at least 2 electrodes. You need 1 electrode, that you connect to ground. You can put this electrode to somewhere neutral (this instructable suggest connecting it to your leg, but I don't know, if that position is really good).

An ADC measures voltage and voltage is the difference in potential between 2 points, so to measure voltage you by definition need 2 points/connections.



Note: The voltages from the EEG electrodes are normally very low, so that they completely vanish in the noise. To really get something useful, you need to amplify the signal. I think there are many tutorials out there, that explain how to do this.