Ecg filter and slew rate limiter for filtering an ecg signal

Abstract

A linear phase high pass filter is disclosed using a slew rate limiter and a linear phase low pass filter in parallel with a delay for removing baseline wander from an ECG signal. The linear phase low pass filter is preferably a digital Infinite Impulse Response (IIR) filter. The slew rate limiter preferably includes a clipper and a unit delay. An input ECG signal is simultaneously presented to a slew rate limiter and IIR low pass filter combination and an electronic delay. Thereafter, the outputs from the slew rate limiter and IIR low pass filter combination and the delay are passed to a summer where the output from the slew rate limiter and the low pass filter is subtracted from the output of the delay to produce the output ECG signal representation. The combination of the output from the slew rate limiter and the low pass filter being subtracted from a delayed input signal produces a high pass filter. The resulting high pass filter has a relatively high -3 dB cutoff frequency of about 0.5 Hertz while providing a virtually constant group delay for the ECG signal passed through the filter. In this way, distortion of the input ECG signal by the filter is minimized so that the baseline wander portion of the input ECG signals is removed while the part of the input ECG signal representative of the heart function is preserved. The high pass filter is preferably implemented on a microprocessor which may be part of an ECG processing system. The ECG system may also perform such functions as filtering muscle artifact signals or supplying sample points to a video display system, or both.