4 Measurement of Biosignals and Analog Signal

Processing

After the electrophysiological processes in cells as the origin of biosignals were dis-

cussed in chapter 3 this chapter deals with the measurement of biosignals and their

processing using analog techniques. Signal processing or signal conditioning is an es-

sential step before digital signal processing (chapter 5), because biosignals, captured

at the body surface are, on the one hand, too weak for direct analog-to-digital con-

version and, on the other hand, usually overlaid by disturbances such as the 50 Hz

mains hum. First, the measurement of electrical biosignals (e.g. electrocardiogram,

-encephalogram, -myogram) will be discussed in detail, before the measurement of

non-electrical biosignals is discussed in the back part of this chapter.

4.1 Measurement of Electrical Biosignals

Electrical biosignals occur as a potential difference (electrical voltage) between two

points on the body surface and can be derived there with electrodes. However, signal

capture within the body is also possible. For example, in the electrocorticogram elec-

trodes are placed directly on the brain tissue at the open skull, in order to be able to

investigate the dynamics of bioelectrical processes in certain diseases such as epilepsy

with a comparatively large signal-to-noise ratio and high spatial resolution. Electrodes

made of platinum-iridium in a matrix arrangement on silicone or latex are used for this

purpose. However, this type of invasive measurement is associated with a high level

of stress for the patient and an enormous medical and metrological effort, which is

why it is only used in exceptional cases. For a more detailed discussion of this topic,

please refer to the relevant literature [27]. In the vast majority of cases, biosignals are

derived as potential difference at the body surface via skin electrodes. The starting

point for the potential difference is an electrically excited cell area within the body.

As explained in chapter 3, in the excited cell area, the ion concentration in the cyto-

plasm (intracellular space) and in the surrounding interstitium (extracellular space)

is different from that in cell areas with resting potential. As a result, a potential differ-

ence is formed within the body, which can be regarded as an internal electrical voltage

source. Since this voltage source is surrounded by more or less conductive tissue (cf.

Table 4.1), ionic compensating currents occur within the body. The current paths penet-

rate large areas of the body and reach the surface of the skin. An electric potential can

be attributed to each point on the current paths. The path of the equipotential surfaces

is complex because of the different conductivity of the various tissue types within the

body. Figure 4.1 shows a simulation result for the potential distribution at the body sur-

face starting from the cardiac electrical activity. Thus, two randomly selected points

usually have different potential. The potential difference of these two points is related

https://doi.org/10.1515/9783110736298-004