Power line interference in ambulatory biopotential measurements

Stray capacitances between body and (grounded) power lines (C_p and C_b) influence the interference in biopotential measurements. Those capacitances can be estimated by a simple measurement method, and some values have been reported for grounded equipment. These values, however, are not necessarily the same under ambulatory conditions, thus resulting in different interfering voltages. The aim of this work is to evaluate C_p and C_h, and, using an interference model, assess the interfering voltages likely to appear on measurements obtained from battery-supplied ambulatory biopotential amplifiers. Results show that C_p and C_b greatly vary depending on the situation, as opposed to the more constant values for fixed equipment. Furthermore, the interference model shows that differential-mode interference dominates over common-mode interference, even when the CMRR is relatively small. This explains the presence of power-line interference even in battery-supplied equipment.