Optimised design of the front-end analogue high-pass filter for a diagnostic quality ECG monitoring system

Even though an analogue high-pass single pole filter with 0.05Hz cut-off has been used for decades to define the low frequency response of electrocardiographs, in recent years the requirements for diagnostic quality ECG recordings have been expressed in terms of the system magnitude characteristic and its response to a rectangular pulse. The objective of this work is to design an analogue high-pass filter for the front-end of an ECG monitoring system directly from these new specifications. A constrained numerical optimisation procedure is implemented to determine the parameters of second and third order filters having the best rejection properties allowed by the requirements for distortion-free ECG recording. The outcome of the process, aimed at maximising the filter 3dB point, is a system having better attenuation characteristics than the reference 0.05Hz first order filter. The cut-off frequency of the optimised second order filter is indeed equal to 0.068Hz, whereas for the third order system the 3dB point can be as high as 0.123Hz.