Abstract
The measurement of bioelectric signals, e.g. the electrocardiogram and electromyogram (EMG), from the body is susceptible to noise, artefacts and interference; such as power-line interference. To reduce these effects differential-input amplifiers are commonly used. Rejection is further enhanced by implementing a referencing system into the design, such as connecting a third electrode to circuit common or driving the shields. However, there has been no quantitative study into the relative merits of the different systems. For this reason, an experimental analysis and computer simulation was undertaken. In the experimental tests, a rig and instrumentation system were developed to record the surface isometric EMG from biceps brachii. Published referencing systems were used in the trials and the recorded signals were quantified using repeatable parameters. These were calculated from the frequency domain and included the median frequency and a measure of the spectral power distribution. The results showed a significant reduction in the interference when a third electrode or isolation was used. No further reduction was observed if a more complex circuit, such as a driven referencing system, was used. In the computer simulation, a SPICE computer model of the recording environment was developed. This included the preamplifier and the interference sources from displacement currents induced into the leads and the body. The different referencing systems were incorporated within this basic structure. The results confirmed those from the experimental work and, by comparison to a 1% tolerance criterion, it was concluded that the driven-right leg or the three electrode driven shield isolated systems were superior. It is concluded that referencing significantly affects interference levels on the bioelectric signal. These results may be used to facilitate a standard system to be adopted in bioelectric amplifiers. The implication for this in clinical practice is that it may improve the measurement of bioelectric signals.