Dry Active Electrodes
To explain how dry active electrodes work, first we need to start with wet electrodes:
"Wet" electrodes setting
One of challanges of measuring EEG is that EEG signal is at level of uVolts. (To give you idea - if you will insert multimeter's leads into lemon you will get reading of around 1V - this is 1000000 times higher). Now think about electricity around you and how small EEG is in comparision.
To distinguish EEG from other surrounding signals Instrumentation Amplifiers are used. On right side of picture above you can see illustration of Instr Amp work. It takes 2 signals and rejects this part of signal which is common on both inputs. Ideally it means that it rejects only noise and leaves only EEG signal.
Now let's think why electrode/skin impedance is important in EEG measurements? One of reasons is that electrode/skin impedances are in series with input impedance of Instr Amp - the higher electrode/skin is the less signal will get to Instr Amp. So it is important to have:
1) low impedances
Other thing is to have
2) equal impedances
This is even more important becouse if you will have different impedances - signals on inputs of Instr Amp will have different amplitudes and Instr Amp will not reject all common 'noise'.
For reasons above it is recommended to abrase skin and use conductive gel when using conventional 'wet' electrodes.
How it is possible to that dry active electrodes can work without skin abrasion and conductive gel?
"Dry active" electrodes setting
Concept of active electrodes primarily involves the introduction of a unity gain operational amplifier (op-amp) physically built into the electrode. The resultant very high input impedance and the low output impedance addresses problems both (1) and (2) above.
The higher input impedance of op-amp the lesser is meaning has electrode/skin impedance. It is still important but not so much as in 'wet' electrodes.
Low and equal output impedances of op-amps addresses help in providing same signal level to instr amp. This won't help if signals will differ even before entering to op-amps but as stated above - thanks to very high input impedance - signals will not be so affected by differences in electrode/skin impedances.
This work has been supported by the Embark Initiative under Irish Council for Science and Technology.
©2008 by Radek Panek <radek.panek()gmail.com>