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Electronics

Basics of Measuring Bio-Electricity

Bioelectric signals are provoked by electrically active tissue like the heart, the muscles or the brain. This active tissue can cause some concentration differences in the extra-cellular fluid that includes ions like Na+, K+, and Cl-. That is the reason that one can measure signals like ECG, EEG or EMG from outside the body on the surface of the skin, by means of electrodes.

The electrode builds an interface between the extra-cellular fluid and the metal of the wire. The electrode is a sensor consisting of a metal and often a salt-bridge, which converts the local differences of the concentration of charged ions into an electrical signal. The bioelectric signal measured from the surface of the skin is mostly in the range of 0-2000 ┬ÁV (2 mV).

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Measurement Principles

In the field of electrophysiological measurements, we distinguish two types of measurement principles: the bipolar and the unipolar measurement principle. In a bipolar measurement, the potential difference between a pair of electrodes is amplified.

In a unipolar measurement the output signals are formed by several input electrodes that are amplified against one so called reference. This can be an electrode, or a calculated internal reference potential. This is not a two channel recording, but a multichannel measurement. This type of recording is often used when measuring EEG or multichannel ECG. A new field of unipolar measurements is the high density surface EMG, where for instance 128 channels are measured using so called grid electrodes.

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DC offset

The electrical signal of an electrode is not only caused by the concentration differences of ions, resulting from physiological processes. When two or more metals are immersed in an electrolyte, this will always result in a DC potential. The extra-cellular fluid is more or less equivalent to seawater, so can act as an electrolyte. The DC offset may also vary over time because the double layer of the electrode-skin interface can change a little, for example by sweat or movement of the electrode.

The DC potential is often not stable, but can vary within a large range.  Actually, one can regard the offset potential as a battery. When Ag/AgCl electrodes are used, the variation of the offset will be limited (<100 mV). But when using for instance a stainless steel electrode in combination with a gold electrode, the offset can go up to several hundreds of milivolts (>500 mV). For this reason, one should always use just one type of electrode during a measurement and even then, the DC gain should be limited.

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Mains interference

The second signal that is found on the electrode is coming from the mains, 220 V / 50 Hz in most of the European countries, 110 V / 60 Hz in many others.

Suppose the body is completely floating and the capacitive coupling to the mains and to the earth ground is equal. Then the potential of the body really is 110 V with reference to the earth ground. This is the reason that TMSi amplifiers always use a floating measurement system, which means that the zero of the power supply of the measurement system is floating on the same 110 V. If there is a moderate difference between the zero of the power supply and the potential of the human body, the amplifier cannot amplify the signal anymore, the signal will be out of range. For this reason, the zero of the power supply is connected to the human body so that under most circumstances the signals will be in range.

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Movement Artifacts

Very often a signal is disturbed by artifacts and most of these are so called movement artifacts. Movement artifacts originate from two different phenomena:

  • Movement of the cables
  • Movement of the electrode

The movement artifacts from the cables can be solved in a very simple way; shielding the cables with the electrode signal itself.

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