MYOELECTRIC ELECTRODE AMPLIFIERS: REJECTION OF UNWANTED COMMON MODE SIGNALS

It is well established that electrode amplifiers for myoelectric control require high input impedances, high common-mode rejection ratio (CMRR) and mains frequency notch filters to cut out unwanted noise. Further, they generally offer a high degree of d.c. isolation. However, it is not generally recognised that the isolation between the skin ground electrode and the common lead may reduce at higher frequencies, and this can create a path for interfering common-mode signals. Harmonics of the mains frequency may be a particular concern as they are not rejected by the notch filter. Common-mode interference can adversely affect prostheses in electrically noisy environments, as long lead runs may act as antennas. Moreover, because of the nature of common-mode, screening the leads may have little effect. Measurements were made on a commercially available electrode amplifier to quantify the effect of injected common-mode signals with and without imbalance in the electrode contact impedances. The results presented here indicate that a low impedance a.c. path between input and output can contribute to the effect of common-mode injection. Analysis of the common mode mechanism is presented and possible alternatives are discussed.