A new technique for line interference monitoring and reduction in biopotential amplifiers

It has been reported that power line frequency must be accurately known if line interference is to be accurately subtracted from the output of a bipotential amplifier. In this paper, hardware developed to record the common mode line frequency signal on the body simultaneously with the ECG lead signals of a 15-channel computerized cardiograph is described. This interference reference signal and its quadrature, obtained by software, are linearly combined to be subtracted from any one of the channels to reduce line interference to below the quantization level of the 12 b A/D converter. Coefficients of the linear combination are estimated using linear regression which is applied to the relatively isoelectric regions of the data, excluding the QRS complexes. Since the interference reference signal is available in real time, simultaneously with the ECG signals, another software approach is also adopted in which an adaptive interference reduction algorithm is used to cope with varying interference. A recursive least squares algorithm with forgetting factor is used to update the coefficients. This updating mechanism is gated by the output of a software QRS detector. Results regarding the performance of both the off-line and the adaptive algorithms are given, and the effects of nonisoelectric portions of the ECG lead signals on the estimation of the coefficients are quantified.