4.1 Measurement of Electrical Biosignals

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Electrode

Insolation

Outer electrode

Inner elctrode

Fig. 4.5: Schematic representation of

monopolar (left) and bipolar (right) elec-

trode needle (modified from [50]).

Tab. 4.2: Signal amplitude and bandwidth of selected electrical biosignals.

Biosignal

Electrode type

Signal amplitude

Bandwidth in Hz

Electrocardiogram (ECG)

Skin electrode

0,1–4 mV

0,01–250

Electroencephalogram (EEG

Skin electrode

1–100 µV

0,01–100

Electromyogram (EMG)

Needle electrode

0,1–5 µV

1–5.000

Electrooculogram (EOG)

Skin electrode

0,05–4 mV

0,01–100

Electroneurogram (ENG)

Needle electrode

0,01–3 mV

0–10.000

4.1.2 Electrical Amplifier

Depending on the biosignal and the electrode position,thedifferentialvoltagebetween

the measuring electrodes at the body surface ranges from a few microvolts for the EEG

to several millivolts for the R-wave in the ECG. The bandwidth of electrical biosignals

ranges from 0 Hz to a maximum of 10 kHz. In Table 4.2 values for signal amplitude

and bandwidth of some important biosignals are listed.

The amplifier has the task of raising the biosignal to a voltage level in the volt

range with a constant gain over the entire bandwidth. As a rule, the biosignal (useful

signal) is superimposed by interference signals such as the common-mode signal or

noise. A common-mode interference is caused, for example, by power lines located

in the vicinity of the measurement setup. The origin of such network disturbances is

treated in section 4.2. At this point it is assumed that the common-mode signal is a

harmonic alternating voltage whose amplitude may be a few volts and, in the case of

the mains disturbance, has the mains frequency (50 Hz or 60 Hz). The common-mode

signal also enters the measurement electronics. Figure 4.6 clarifies the measurement

situation. The amplitude of the common-mode signal (interference signal) often ex-

ceeds that of the biosignal by several orders of magnitude. From this, the necessity of

a very high common-mode rejection is directly derived as an important requirement

for the amplifier (Common Mode Rejection, in short: CMR). Eliminating the common-

mode signal with analog filters is not an option for most applications because the fil-

ter impedance is small and the rejection frequency band of the filter overlaps the fre-

quency band of the biosignal and would lead to distortion of the biosignal. This aspect

is discussed in detail in section 4.4.