Bio-electricity, or 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. It is therefore good to understand where these potentials originate from and what the challenges are that one will face when measuring them.
The bioelectric signals that we measure are provoked by electrically active tissue like the heart, the muscles or the brain. This active tissue causes concentration differences of ions such as Na+, K+, and Cl- in the extra-cellular fluid. These concentration differences make that we can measure signals like ECG, EEG or EMG from outside the body on the surface of the skin, by means of electrodes.
The electrodes form the interface in between the extra-cellular fluid and the metal of the wire. An electrode is a sensor consisting of a metal and often a salt-bridge, which converts the local differences in 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).
There are some phenomena that make measuring this potential a challenge, especially if the signal is small. Two important ones are the DC offset which is due to the electrochemical potential of the electrodes and the 50 or 60 Hz mains interference due to a different capacitive coupling of the patient and the amplifier to the mains.
In addition, the electrodes will add noise to a signal and can even potentially deform the shape of the signal. The cables between the electrode and the measurement system will introduce a lot of movement artifacts and noise and the processes inside the body other than the processes that we want to measure will also add a large amount of undesired artifacts to the measurement.
The measurement configuration has to deal with all these signals and noise sources 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 any 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.