A/d conversion is the least of the issues with mobile eeg monitoring. The signals are on the order of 10-300 micro volts so they need lots of amplification. If the amp is at the other end of an electrode motion of the wire through the earth's magnetic field generates signals of a comparable size. So ideally you would need to place the amp on or in the electrode, which is a bit more of a challenge. You also might have to worry about muscle signals like eye movement artifact