A Baseline Drift Canceler Using Discharging Chopper for Surface Metal Electrodes (Special Editorials : Five Selected Articles in ABE)

Many useful signals measured on the body surface contain DC and extremely low frequency components. However, surface electrodes are not ideal for DC or low frequency signal measurements because the electrochemical interaction between the body and the metal contact is unstable. We propose a substantial technique to improve DC stability for biological potential measurements on the body surface, by which unstable fluctuations are canceled by a discharging switch. The system alternates between two states. In the discharging state, two electrodes are connected to each other by an analog switch and the randomly generated charge on the electrodes is canceled. When the switch is turned off. the differential voltage is sampled and stored. The sampling frequency of the system is equal to the switching frequency because of this procedure. An experiment employing Ag/AgCl surface electrodes on human skin showed that DC shift and fluctuation were reduced to 1/4 and 1/6 or less, respectively, of the values without the canceler. Applying this technique to electrooculogram measurement, we found that absolute visual direction could be detected without major baseline drift.