REFA FAMILY USER MANUAL
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19
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5
OPERATIONAL PRINCIPLES
Unipolar input channels
5.1
The input stage for measuring unipolar electrophysiological signals is configured as
a so called average reference amplifier. All signals are amplified against the
average of all connected unipolar inputs. Inputs that are not connected to an
electrode cable are not used in the average reference and automatically switch off.
These channels will show a flat line signal on screen. The input impedance of the
active channels is very high (100 M
). The influence of electrode impedance is
therefore very small and no electrode impedance measurement is required.
The Patient Ground electrode is required, but it is not an active input. It is meant as
a way to keep subjectt potential and amplifier potential at about the same level.
All electrode cables are shielded with the electrode signal itself (active shielding).
The active shielding ensures that disturbances such as cable movement artefacts
and mains interference (50/60 Hz) are reduced to a minimum.
No high pass or low pass filters that can cause signal phase shifts or filter
overflows are present in the device.
Bipolar input channels
5.2
The input stage for measuring bipolar electrophysiological signals is configured as
an instrumentatio
n amplifier. The difference between a ‘plus’ and ‘minus’ signal is
amplified. The Patient Ground electrode is required to keep subjectt 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 artefacts 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.
Auxiliary input channels
5.3
Each 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.
Filtering
5.4
A 1
st
order low pass filter is placed before the analog-to-digital converter (ADC)
with a -3 dB point at 6.8 kHz. Next to that, the ADC of the device has a digital sinc5
filter with a cutoff frequency of 0.2 * sample frequency. The maximum sampling
frequency depends on the Refa configuration. Refer to the technical specifications
of your device which can be downloaded from
