What are the advantages of Active electrodes with respect to shielding ?

Please note that the principles of shielded electrode cables were already fully analyzed in a scientific paper by the BioSemi group more than 13 years ago. It is remarkable to see that the technique is regularly presented as something "new" by competitors. Development at BioSemi on the other hand has not stood still during the last decade, resulting in the very successful Active Electrode. The Active Electrode is a superior solution for the problem of interference pickup of the cables, providing the best possible suppression of interference by impedance transformation directly on the electrode. The Active Electrode has an output impedance of less than 1 Ohm, ensuring that the signal in the cable is fully insensitive to interference (compare this 1 Ohm with the tens or even hundreds of KOhms of a passive electrode and you get an idea of the effectiveness of the principle.) In addition, the Active Electrode has crucial advantages with respect to shielding and guarding (see note) techniques:

	Highest possible input impedance because of minimal signal path length between electrode and first amplifier stage.
	Minimal stray-capacitances of the input.
	Low impedance output of the electrode allows the use of reliable, cost-effective light-weight cables (no need for expensive low-noise heavy coax cables).
	Low impedance output of the electrode combined with matched low input impedance of the AD-box eliminates contact problems in the electrode connectors (no "cracking" when you move the electrode cables or connectors as seen with all passive electrode systems).
	No need for delicate expensive coaxial plugs. Active electrode systems can use convenient multi-pole plugs. As used in the BioSemi setup with 32 electrodes per connector. (Imagine having a spaghetti of 128 electrode cables going to your headbox)
	Low impedance output eliminates artifacts by cable movements. (all systems with shielded coax cable are particularly sensitive to this problem because of micro phony and piezo electric effects generated by the isolation layer between the inner core and the shield).
	No danger for amplifier instability because of the absence of complicated feedback loops with unknown parameters (i.e. electrode impedances and subject stray-capacitances). A cable shield driven by a guarding circuit (see note) forms a loop with positive feedback which may become unstable with particular combinations of electrode impedances and subject stray-capacitances (see this paper for more information).

 

Note:

Active shielding is nothing else than the 50 year old principle of guarding as described in every textbook on electronic engineering and application notes from several semiconductor companies (Analog devices, Linear Technology). The use of the word 'active' is misleading in this context because the electrode remains fully passive. No electronics are integrated in the electrode. There is no impedance transformation on the electrode.