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 µV (2 mV).
Of course there are more electrical phenomena inside the body or on the electrode. Two are very important, the DC offset of the electrode and the 50 or 60 Hz mains potential or mains interference. In addition, any measurement will show noise, produced by the body, the electrode impedance or the amplifier itself.
The potential differences that we can measure between two points on the body, can give very important information regarding the electrical activity that takes place inside the body. At the same time these measurements, especially of very small potential differences, can pose rather big challenges as well.
The electrodes can be a source of distortion. The body itself of course is a very big source of noise and finally the cables between the electrode and the measurement system can introduce a lot of artifacts and noise. The measurement configuration has to deal with all these signals and noise in such a way that the bio-electrical signals, measured on the surface of the skin, are reflected in the output signal as optimally and cleanly as possible.
At the same time all the noise signals, distortions and artifacts must be optimally suppressed and if possible, should not be measured at all. Of course some mains interference and amplifier noise will always be present in the measurement; it is hardly possible to perform a measurement without mains interference.
When an optimal amplifier system is used however, the mains interference will be common mode for all the inputs and the common mode rejection will remove all the mains interference from the measured signal.