Hi Amaluddin Yusoff
> 1. I don't know what it means by bipolar, monopolar and multipolar
> electrodes. What is the type of electrodes implemented in OpenEEG,
> bipolar, monopolar and multipolar?
Monopolar means one electrode is used for picking up the signal from
the scalp at the location of interest, while the reference potential
is derived differently, corresponding to a potential that is connected
to the body of the person under test and considered not carrying a sig-
nificant signal, for example a connection to one or both ear lobes.
Advantage: simplicity. Disadvantages: A totally "quiet" location does
not really exist. Both the spatial separetion between the electrodes
and missing symmetry promotes interference from external sources.
Bipolar means the signal is derived from the difference between to
electrodes. There is no "quiet" potential used to act as reference.
A third electrode at a "relatively quiet" location is still required
for providing a bias potential. Advantage: No assumption of a quiet
location required for achieving the actual signal, both the symmetry
and closer spatial proximity of the electrodes reduces interference
from undesirable sources. Disadvantage: At least three electrodes are
required instead of two and the amplifier circuit is slightly more
Multipolar means there are more than two electrodes located on the
scalp and accordingly several signals channels are derived from those
electrode locations. In this case the reference is derived from (at
least) one additional electrode located at a "quiet" location just
like with the monopolar scheme. Advantage: Using several channels
simultaneously allows for more complex analysis like spatial distri-
bution of activity or correlations. Disadvantage: Cost, obviously.
With a sufficiently large number of electrodes (>10) the reference
potential for a multipolar EEG could also be derived from the average
potential of all contributing individual electrodes, thereby making a
dedicated reference unnecessary. An additional electrode is still re-
quired for providing a bias well within the limits of the amplifier's
inputs. Advantage: The spatial proximity of the relevant electrode
locations leads to reduced susceptibility to interference. Disadvan-
tage: Due to the arithmetic coherence, there are slightly negative
correlations between the signals of all channels even when there is
none in reality.
Regardless of the scheme employed it is in all cases necessary that
the body is somehow connected through a low impedance electrode to
1.) either the amplifier's circuit ground or 2.) even better, to a
driven virtual ground potential like with driven right leg.
> ... meaning of "driven shields" and "driven patient ground"?
A shield to a signal-carrying wire reduces the interference from
electric AC fields acting on the wire and thus helps maintain the
sinal quality. Normally a shield is connected to the circuit ground
of the circuit at the receiving side.
A driven shield is not connected to the circuit ground, but to the
low impedance output of a buffer stage. This helps reduce the other-
wise detrimental effect of the capacitance between the signal-
carrying conductor and the cable shield in a high impedance configu-
Driven patient ground means the body is not connected to the circuit
ground through the reference electrode. Instead the voltage between
the patient's body and the circuit ground is controlled by active
means through a virtual ground driver and a dedicated driving elec-
trode. The patient's body potential is derived from the average of
all sensitive electrode's potentials. The feedback gain must be ne-
gative so that the undesired common mode signal is cancelled.
> 3. What is "isolation amplifiers"? What it is used for?
An isolation amplifier acts largely just like a normal amplifier,
but the circuits of the input and the output ports are electrically
isolated from one another. With an EEG device it is used for safety
reasons by interrupting an otherwise possible current path along
the patient, the electrode, the amplifier inputs, and the circuitry
being connected to the amplifier's output or its power supply.