I was talking to student of Bruce Wooley's yesterday, and it occurred to me that I need to nail down the requirements for a good dry eeg sensor. What's really needed?
Introduction
Three things are required to do dry electrodes. First, the input current must be small, or somewhat equivanlently, the input impedance must be large. Second, the voltage noise must be small in the frequencies of interest, which, in our case, ranges from 0.1 Hz to not more than 100 Hz. In these regions, flicker noise, or 1/f noise, usually dominates. Finally, there must be compensation for the input capacitance so that every electrode experiences exactly the same phase shift.
Input Current
Really the key, three approaches, original is a dielectric contact, stops the current from flowing, need to compensate for chargeup. This is the Richardson electrode. Second is metal contact, dielectric intermediate, then contact - this is Babak's patent. Finally, we have a capacitor made by air, with some support mechanism. This is the Sussex group. Alternative is to use something with low input current already (like the LMC662) as the basis of the sensor, then any complaints go to the chip designer, not us.
Noise
Solution is chopping. Also, we can use the same concept of chopping to take the signal further out - use two amplifiers.
Phase Shift
Need to compensate, review stuff from Bob Pease.