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.
Two types of unipolar measurement principles are known:
- Common reference
- Average reference
The common reference amplifier just has one electrode, which is used as one part of every bipolar input and several other electrodes that form the other parts of the bipolar inputs.
In the average reference amplifier there is no electrode that acts as the reference for the measurement system. Instead, the multichannel bipolar derivatives are made by using one electrode as one part of the input of each bipolar amplifier, and the mean of all the connected electrodes as the other input of each bipolar amplifier.
The average reference principle has several advantages over the common reference principle. It should be stressed, however, that the way in which the signals are recorded and the way they are analyzed are two different things. For some purposes, it may be better to re-montage the data in software.