106

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4 Measurement of Biosignals and Analog Signal Processing

time t / s

ECG with superimposed system incident

Fig. 4.14: ECG with superimposed mains disturbance: simulation of the output signal of the circuit

according to Figure 4.11 with Ru1 = 4 kΩand Ru2 = 2 kΩ.

One way to reduce the network disturbance is the principle of reference potential con-

trol, also called driven right leg. Here, the common mode signal is tapped behind the

first stage of the instrumentation amplifier, inverted (phase shift by 180°), and re-

turned to the body via a third electrode. By superimposing the common-mode signal

with the opposite-phase common-mode signal from the third electrode, the two ideally

cancel each other out. In practice, however, the two amplitudes of common mode sig-

nal and the opposite-phase common-mode signal are different, which means that the

common-mode signal is not completely eliminated. Nevertheless, the disturbance can

be significantly reduced in this way. Figure 4.15 shows the extension of the circuit ac-

cording to Figure 4.10 by the reference potential control.

The common mode signal is tapped between the two resistors R11,12 and fed to the

impedance converter OPV5. Behind it an inverting amplifier follows, formed by OPV6

and R13,14. There the common mode signal is phase shifted by 180° and amplified by

a factor of 22. The amplification is intended to compensate for the voltage drop from

the injection point to the electrodes.

Inductive coupling

According to the Biot-Savart law, the current through a power line generates a mag-

netic field that surrounds the conductor in a circle. For a straight conductor surroun-

ded by air, the magnetic flux density B is calculated to be

B(t) = ^μ0

2πr ^I(t)eφ

(4.7)

with the vacuum permeability μ0, the perpendicular distance to the conductor r and

the current I(t). Since I is an alternating current, B must also be an alternating quant-

ity. According to the law of induction, an alternating magnetic field induces a voltage