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4 Measurement of Biosignals and Analog Signal Processing

Amplifier

L1

A

B

Fig. 4.16: Schematic representation of inductive mains

coupling: Starting from the current in the mains cable

L1, a magnetic field is generated, which penetrates

the conductor loop consisting of the two measuring

cables, the body and the measuring amplifier and

induces an AC voltage at the input of the measuring

amplifier.

Fig. 4.17: LTspice simulation of the galvanic network disturbance with network coupling via the insu-

lation resistance Ris = 10 GΩ; the resistor Rs is needed for simulation purposes to give a reference

potential to the source UECG.

Galvanic Coupling

The galvanic coupling of mains disturbance requires a limited resistance between the

measurement electronics and the mains. At first glance, this does not seem to be a real-

istic situation, since both –also for reasons of electrical safety –must be sufficiently

isolated from each other. However, even at large insulation resistance, a significant

network disturbance can occur, as shown by the simulation result in Figure 4.18. The

insulation resistance is sometimes lowered by creepage paths. For simulation pur-

pose, the insulation resistance Ris and a line voltage were inserted in the electron-

ics circuit (Figure 4.17). An effective countermeasure is to encapsulate the electronic

components that carry the mains current (e.g. transformer).