S/5 EEG Module, M-EEG and S/5 Headbox, N-EEG
Document No. 800 1011-1
5
of the brain. To overcome this the stimulus is repeated several times (100...2000), and average of
all responses is calculated. Responses containing big artefacts are removed from the average to
improve the signal to noise ratio.
2.1.4
Impedance measurement
The impedance measurement is done from one channel at a time and the EEG or EP measurement
is stopped during the impedance measurement.
Differences in electrode impedance of the electrodes causes common mode noise coupling to
measured signal. To minimise this the electrode impedance is measured and a warning of
unsatisfactory impedance level is generated when necessary. The impedance of an electrode is
measured by applying a known current through the electrode and measuring the voltage drop over
the electrode. This way the impedance of a single electrode can be resolved instead of a sum
impedance of an electrode pair.
2.2
Main components
2.2.1
Neuro board
The Neuro board consists of the following functional sections:
•
audio stimulator
•
microprocessor for stimulation and measurement control, and for counting the
measurement results
•
two serial communication drivers
The serial bus speed to monitor is 500 kbps and the bus itself is half duplex, i.e. data can be
transferred to both directions but only one way at a time.
The serial bus speed to the headbox is 500 kbps and the bus is full duplex i.e. data can be
transferred to both directions at the same time.
Module
Keys
RS485
Driver
Reset
Data
Power
+15 V
±5 V
±12V
CPU
3048
Mo
du
le
B
us
Pa
tien
t Is
ola
tio
n 5 kV
Power
supply
Opto
Isolation
EEPROM
AEP
Stimulator
RS-422
driver
+15 V
Data
AEP stimulus
To
To
EEG
EEG
Head-
Head-
box
box
RAM
Figure 2
Neuro board block diagram
