COMPARATIVE-ANALYSIS OF POWER-LINE INTERFERENCE BETWEEN 2-ELECTRODE OR 3-ELECTRODE BIOPOTENTIAL AMPLIFIERS

Differential input amplifiers are commonly used in the measurement of biopotential signals. This is because of their ability to reject commo n-mode signals, for example power-line interference. Rejection is furt her enhanced by referencing the subject to the instrumentation system. One such referencing technique uses a third subject electrode connect ed either to circuit common or to ground. However, these techniques ar e rarely explained or quantified in the literature. Four recording con figurations are examined, isolated and non-isolated amplifiers with tw o and with three electrodes, and existing models are extended. From a brief assessment of power-line interference sources, it is concluded t hat only displacement currents induced into the leads and the body nee d be considered. The effect of these on the four configurations is ana lysed using SPICE. Results show that interference from displacement cu rrents induced into the leads and into the body is dependent upon the recording configuration used and is significantly lower in three-elect rode than in two-electrode amplifiers. Isolation also reduces levels o f interference. It is concluded that the choice of referencing may aff ect interference levels on the biopotential signal significantly. Expe rimental and further analytical work is planned to confirm this.