4.4 Interference Suppression and Analog Filtering

|

115

R1

270k

C1

22n

U1

Photodiode

Fig. 4.22: Principle of SpO2 measurement in transmission (left), exemplary equivalent circuit of a

transimpedance amplifier with low-pass filter (center) and pulse curve measurement with a reflec-

tion sensor (right): Control and readout are performed via a PC measuring card.

time t /s

voltage U /mV

Fig. 4.23: Photoplethysmogram (PPG) from the measurement with the reflectance sensor of Fig-

ure 4.22.

temple become accessible to a SpO2 measurement, which are excluded for a transmis-

sion measurement. With regard to mobile applications in the form of wearable func-

tional textiles, this aspect is becoming increasingly important. Figure 4.23 shows a

pulse curve recording with a reflectance sensor.

4.4 Interference Suppression and Analog Filtering

The measurement of biosignals is usually overlaid by disturbances. In section 4.2 dif-

ferent causes were presented. In this section, possibilities of interference suppression

by means of analog filters are discussed. The essential function of filters is the suppres-

sion of defined frequency components. When using filters, it should be noted that the

biosignal is distorted if the stopband of the filter overlaps with the biosignal spectrum,

and thus the corresponding frequency components of the biosignal are suppressed in

addition to the interference signal. In this case it has to be checked whether the res-

ulting signal distortion is tolerable with regard to the medical-diagnostic statement,