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4 Measurement of Biosignals and Analog Signal Processing

aR3

R3

LT1179

C1

R2

C2

Ue1

R1

5V

-5V

Ua

5V

-5V

Fig. 4.27: 2nd order low pass filter; the filter type is set via the a value (cf. Table 4.4).

Tab. 4.4: Assignment of the value a (cf. Figure 4.27) to filter types.

Filter type

Bessel

Butterworth

Chebyshev

a

0,268

0,586

1,234

previous considerations referred to low-pass filters. High-pass filters can be created

simply by exchanging R1 with C1 in the circuit according to Figure 4.25, or analogously

R1,2 with C1,2 in the circuit according to Figure 4.27.

At the beginning of this section it has already been discussed that parts of the biosig-

nal can also be suppressed by filters. In the unfavorable case, the resulting change

in the signal curve can lead to misinterpretations. In some application areas, how-

ever, signal distortion is not problematic. For example, in fitness equipment for pulse

determination by means of ECG, it is sufficient if the time interval between successive

R prongs can be reliably determined. can be reliably determined. In such applications,

it is quite reasonable to suppress the 50 or 60 Hz mains hum with very narrowband

bandstop filters, so-called notch filters (notch filter). A bandstop can be realized from

the parallel connection of low-pass and high-pass filters.

Figure 4.29 shows a possible circuit design, Figure 4.30 the corresponding mag-

nitude and phase frequency response. The notch frequency is attenuated here by

45 dB. In Figure 4.31 the effect on an ECG weakly overlaid with 50 Hz mains hum

can be seen. The 50 Hz mains hum has completely disappeared due to the filtering.

However, the ECG waveform has also changed somewhat: In particular, the R-prong

is slightly shifted.

The previous considerations focused on the frequency-selective behavior of ana-

log filters, which is described by the magnitude of the transfer function. However, the

filter phase response is also important. Each filter causes a phase shift between input

and output signal. This would be unproblematic, if all frequency components of the