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4 Measurement of Biosignals and Analog Signal Processing

common-mode signal (50 Hz mains hum). For an EEG measurement a larger gain is

necessary, which can be achieved e.g. by increasing R9. It should be noted that the

cutoff frequency of the second low-pass filter is then reduced accordingly.

4.2 Signal Interference

Biosignals are generally superimposed by interfering signals. In the worst case, a dia-

gnostic evaluation of the biosignal is impossible. In addition, the interfering signal

often lies partly or completely within the spectral range of the biosignal and there-

fore cannot be easily removed with filters without changing the biosignal at the same

time. For this reason, precise knowledge of the causes of interfering signals is indis-

pensable, in order to be able to take countermeasures from the outset.

4.2.1 Network Disturbances

In the normal measurement situation, the electronics as well as the test person itself

are surrounded by electrical power lines. Even if these are not in direct contact with

each other, an interference signal can couple into the signal from the power lines. This

is also called mains hum. The mains hum is like the causal power line a harmonic

alternating voltage with the same frequency, i.e. 50 Hz in Europe resp. 60 Hz in North

America. It can be very effectively eleminated by a narrowband digital bandstop filter

(notch filter). However, since the mains hum lies within the bandwidth of biosignals,

this is accompanied by signal distortion.

Capacitive Coupling

From an electrical point of view, the human body can be regarded as an inhomogen-

eous conductive electrolyte container. Between the conductor of the electric network

(L1) and the body, a stray capacitance is formed, as well as between the body and the

neutral conductor N. Figure 4.13 represents the situation schematically.

The capacitance value of the stray capacitances depends strongly on the distance

between body and L1 or N. Following the plate capacitor formula, the capacitance

increases with decreasing distance. For further consideration, values between 1 pF

and 100 pF can be assumed. Furthermore, stray capacitances also occur between the

measuring cables and the power lines. As long as the mains interference couples into

the electronics via the two electrodes with the same amplitude and phase, i.e. as a

common-mode signal, and the two lead lines up to the subtractor of the amplifier

are completely symmetrical to each other, this situation does not pose a problem. As

already explained in the previous section, only the differential signal between its two

inputs is amplified by the instrumentation amplifier. The common mode signal is sup-