De. Wood et al., COMPARATIVE-ANALYSIS OF POWER-LINE INTERFERENCE BETWEEN 2-ELECTRODE OR 3-ELECTRODE BIOPOTENTIAL AMPLIFIERS, Medical & biological engineering & computing, 33(1), 1995, pp. 63-68
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.