Second and third order analogue high-pass filters for diagnostic quality ECG

An analogue high-pass single-pole filter with a 0.05 Hz cut-off frequency has been used for decades as the comparison milestone for the low frequency performance of electrocardiographs. The aim of this work is to prove that it is possible to design other analogue high-pass filters that allow undistorted recordings of the ECG signal and yet have better rejection characteristics than this reference first order filter. A constrained numerical optimisation procedure is implemented to derive the parameters of second and third order filters with a 3 dB point at 0.05 Hz that satisfy international standards for the measurement of diagnostic quality electrocardiograms. The magnitude response of the optimised filters shows indeed a narrower transition region than the 0.05 Hz single-pole system for different levels of stop-band attenuation, whereas the phase characteristic of all the filters analysed is practically the same over the ECG frequency spectrum.