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4 Measurement of Biosignals and Analog Signal Processing

magnitude / dB

magnitude

phase

phase / °

Fig. 4.30: Magnitude and phase frequency response of the notch filter according to the circuit from

Figure 4.29: The notch frequency is exactly at 50 Hz and has an attenuation of 45 dB there.

time t / s

ECG input signal

ECG output signal

Fig. 4.31: ECG before (blue) and after (red) application of the notch filter from Figure 4.29: The weak

50 Hz mains hum, which is still present in the original signal, was completely eliminated by the

notch filter. However, the curve of the filtered signal in the area of the R-prong is changed a little

compared to the original curve.

ing transmission through the filter than signal components up to 100 Hz. A steep edge

in the signal curve is therefore transmitted differently than signal components with a

low gradient. Consequently, the signal is distorted there. In general, the group delay

changes with frequency more the higher the filter order and the greater the edge steep-

ness is. In medical measurement technology, Bessel filters are preferably used, be-

cause their group delay times vary least with frequency compared to other filter types

such as Butterworth, Chebyshev or Cauer filters.