The input stage for measuring bipolar electrophysiological signals is configured as an instrumentation amplifier: the difference between a ‘plus’ and ‘minus’ signal is amplified.
The advantage of a bipolar channel is that two electrodes can be used in one input channel, whereas two input channels are needed when using unipolar channels. The downside of using a bipolar channel is that one cannot perform (re-)montages afterwards, as is possible with unipolar channels. The patient ground electrode is required to keep patient potential and the amplifier at the same level.
The input impedance of the active channels is very high. The influence of electrode impedance is therefore very small. It should not be necessary to perform electrode impedance measurements. Inputs that are not connected to one or both of the electrode cables automatically switch off (i.e. will show a zero/flatline signal).
All electrode cables are shielded with the average of the ‘plus’ and ‘minus’ electrode signal (active shielding). This ensures a disturbance free measurement. Mains interference and cable movement artifacts are reduced to a minimum.
After the first amplifier stage (gain=20 or lower) the signals go directly to the ADC. No high pass or low pass filters that can cause signal phase shifts or filter overflows are present.