Electrode contact noise in surface biopotential measurements

Skin-Electrode interlace noise is important in high resolution surface biopotential recordings. In order to determine its contribution, we have carried out several measurements. First, we measured the noise power spectrum density of several pairs of Ag/AgCl electrodes placed face to face and then placed on the body surface (leg and arm). We simultaneously measured the impedance of these pairs of electrodes and calculated the thermal noise associated to their real part. We found that noise at low frequencies is larger than noise at high frequencies, the power spectrum density increasing as 1/f. We have also found that calculated thermal noise is lower than measured electrode noise. Next, we measured noise and impedance for different electrode areas. For electrodes placed face to face we found that noise and resistance increase when electrode area decreases. For electrodes placed on the body surface, "noise" power spectrum density (probably EMG) increases when electrode area increases. We also calculated the Probability Density Function (PDF) of electrode noise. We found that it looks gaussian for electrodes placed face to face, but not for electrodes placed on the body surface.