124

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4 Measurement of Biosignals and Analog Signal Processing

ideal low pass

ideal high pass

ideal band pass

ideal band stop

Fig. 4.33: Ideal-selective-standard-filters: |A| denotes the magnitude of the transfer function, fD the

pass frequency of a low or high pass, fD1, fD2 the left and right pass frequencies of a bandpass or

bandstop.

Fig. 4.34: Tolerance-ranges-of-a-low-pass filter, with A0, AD as the upper and lower values of the

gain in the passband and fD, fS as the passband and stopband frequencies.

filter), or certain spectral lines such as the 50 Hz mains hum can be suppressed. De-

pending on the selectivity requirement, standard filters such as lowpass, highpass,

bandpass and bandstop are used (cf. Figure 4.33).

Of course, the properties of ideal filters cannot be realised with real filters, but

the tolerance range in which the magnitude frequency response must lie can be spe-

cified. This is for example shown in Figure 4.34 for a low-pass filter. The magnitude

of the transfer function |A| of the designed filter must lie within the non-shaded area.

Furthermore, with the standard filters it can be specified whether the course of the