4.2 Signal Interference

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109

time t / s

simulated ECG output signal

Fig. 4.18: Result of the simulation of the circuit from Figure 4.17.

4.2.2 Transient Disturbances

In the previously discussed circuit according to Figure 4.10, an RC high-pass filter (C1,

R8) with a cutoff frequency of 0.16 Hz was incorporated to suppress any DC com-

ponents in the biosignal and to limit the transmission band to low frequencies. This

very low cutoff frequency is accompanied by a correspondingly large time constant

(τ = 1 s), which causes very slow balancing processes . A step-wise DC component

can occur, for example, if one of the electrodes is pressed during the measurement

to improve the contact to the body. This can change the charge of the electrode. The

resulting DC voltage overlaps the biosignal and decays exponentially with the time

constant of the RC high-pass filter. Countermeasures in the form of filters do not ex-

ist, because in this case a filter of the electronics itself is the cause of the disturbance.

As a rule, it is necessary to wait until the transient disturbance has decayed before

evaluating the biosignal.

4.2.3 High-Frequency Interference due to Electromagnetic Radiation

In many measurement situations, the electronic circuit is exposed to high-frequency

electromagnetic radiation. Sources of radiation are e.g. mobile phone networks, cord-

less phones or WLAN. The electromagnetic radiation can couple to the electronic cir-

cuit by an unshielded measuring cable which acts as an antenna. The antenna is par-

ticularly effective if the cable length is half the wavelength of the radiation or a mul-

tiple thereof. For a 1.2 m long measuring cable, this would correspond in free space

to a transmission frequency of 125 MHz. This frequency is far above the bandwidth of

biosignals.

But the high-frequency radiation can couple in by amplitude modulation. Amp-

litude modulation is a method of communications engineering, which is, however,