In a bipolar measurement, the potential difference between a pair of electrodes is amplified by one amplifier channel. In a unipolar measurement the output signals are formed by several input electrodes that are all amplified against one so called reference. This reference can be an electrode (the common reference electrode), or a calculated internal reference potential consisting of two or more electrode signals. This is never done by one amplifier channel, but always in a multichannel set-up (with a minimum of two channels). 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.
As a result of the above, two types of unipolar amplifier principles can be distinguished:
The common reference amplifier amplifies the signal of each unipolar electrode against the signal from one common electrode, which is thus present in each of the outputs. In EEG, a variant of this is sometimes used by amplifying each unipolar electrode against the mean of the two ear-electrodes, the so called ‘linked-ears’.
In the average reference amplifier there is no electrode that acts as the reference for the measurement system. Instead, each of the unipolar electrodes is amplified against the average of all the connected unipolar electrodes.
The average reference principle has several advantages over the common reference principle. The first one is very obvious: if the reference electrode is bad (or worse yet, falls off entirely), it does not invalidate the entire recording. Also, from an amplifier-technical point of view, it is beneficial not to have one electrode ‘distributed’ over all input channels. The average-reference principle, as employed by TMSi, can be seen as the multi-channel counterpart of the well know instrumentation amplifier that has been used in electrophysiology for decades.
It should be stressed, that the way in which signals are recorded and the way they are analyzed are two different things: as long as electrodes are included in the measurement set-up, they can always be ‘re-montaged’ in software. Thus, common mode montages can be derived from average reference recordings and vice versa. Also, special montages like the so-called Wilson Central Terminal in ECG can easily be derived.
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