14
5
Operational principles
5.1
Bipolar input channels
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
patient ground electrode is required to keep patient potential and amplifier potential at about the
same level.
All electrode cables are shielded with the average of the ‘plus’ and ‘minus’ electrode signal (active
shielding). The active shielding ensures that disturbances such as cable movement artifacts and
mains interference (50/60 Hz) are reduced to a minimum.
After the first amplifier stage (gain = 20) 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.
The NeXus-4 has one dual bipolar input. This connector leads to two bipolar input stages for in total
two differential channels.
5.2
Auxiliary input channel
The Auxiliary input has a 5-pin connector. Signals on this connector are +5V output, -5V output,
GND, +signal input and -signal input. The +5V/-5V/GND pins can be used to power an active sensor.
The + and - inputs are connected to an instrumentation amplifier with a gain of 1. The output of the
amplifier goes to the ADCs without any filtering.
5.3
Filtering
The ADC of the device has a digital sinc filter with a cutoff frequency of 0.2 * sample frequency.
Supported sampling rates are defined in
chapter 8
. There is a 1
st
order low pass filter before the ADC
with a -3db point at 408Hz.
Besides above no other filtering is applied.
