4.1 Measurement of Electrical Biosignals

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same structure and should therefore have the same electrode potentials, which then

compensate each other in the measuring situation. In practice, however, a differential

voltage of a few mV often remains, which overlays the measurement signal as an offset.

This can be caused by a different ion concentration at the phase boundary of the two

electrodes. As will be shown, this offset can be eliminated by a high-pass filter in the

measurement electronics.

RE2

CE

RE1

Fig. 4.3: Disposable adhesive electrode and suction electrode (left). Electrical equivalent circuit of

the electrode (right): RE1 represents the lead resistance in the contact wire and electrolyte. RE2 and

CE describe the electrical behavior of the phase boundary between metal and electrolyte (Helmholtz

layer).

Electrically, the electrode can be represented by a series connection (cf. Figure 4.3,

right), where RE1 represents the lead resistance of the contact wire and the elec-

trode gel. The parallel circuit of RE2 and CE describes the phase boundary between

electrolyte and metal. The capacitive behavior is explained by the layer sequence

metal, Helmholtz layer, electrolyte, which corresponds to a capacitor arrangement

(conductor-insulator-conductor). The resistance RE2 represents the adsorption and

desorption processes that take place between Ag and AgCl. As in the electrical de-

scription of the skin, the magnitudes of the individual equivalent circuit components

depend on various factors. For simulation purposes, RE1 = 30 Ω, RE1 = 100 Ω, and

CE = 30 μF can be assumed. Overall, the equivalent circuit for skin and electrode is

given in Figure 4.4.

At low frequencies below 100 Hz, the impedance of the capacitors CH and CE is

much larger than the parallel resistors RH and RE2. In that case, the two capacitors can

be neglected in the equivalent circuit. What remains are the three resistors, which are

now in series and can be combined into one resistor. Since RE1 and RE2 are smaller

than RH by more than an order of magnitude, the total impedance of the skin and

electrode in this approximation is resistive and determined by the skin resistance RH.

This resistance will be called transition resistance in the following.

In electromyography (EMG), needle electrode are used as an alternative to flat

conduction electrodes. These enable the contacting of individual motor units of the