7
Signal Acquisition with the MED64 system
The MED probe has 64 recording electrodes and 4 reference electrodes. The difference between potentials at the
recording electrodes and the reference electrodes are measured with the MED64 system. Fig.1 shows a
schematic diagram of the circuit between the recording and reference electrodes with the MED64 system.
Amplifier Input-Impedance Requirements for avoiding
Signal Attenuation
One of the planar microelectrode's unique characteristics is that it forms an “electric double layer capacitor”
between the electrode and solution. This “electric double layer capacitor” results in a rise in the electrode
impedance at low frequencies. (See Fig.2) This detail should be noted because acquired signals are attenuated if
the electrode impedance becomes higher than the input impedance of the amplifier.
The platinum-black microelectrodes on the MED probe intrinsically have very low-impedances (at 1kHz, 10 k
Ω
for 50
m electrodes, 40 k
Ω
for 20
m electrodes). The impedances are small even at low frequencies (as seen
in Fig.2). Thus, the input-impedance of 100 M
Ω
for the MED64 Head Amplifier is high enough for the MED64
system. Low-frequency signals such as fEPSPs or even slower signals such as from smooth muscle (0.1 Hz) can
be acquired without attenuation.
If 1Hz signal is acquired with flat ITO electrodes, signals are attenuated because the electrode impedances are
larger than 100 M
Ω
(blue line in Fig. 2.). However, when 1Hz signals are acquired with the platinum-black
electrode on the MED probe, signals are acquired without attenuation because the electrode impedances are
smaller than 100 M
Ω
(green and red lines in Fig.2).
Fig.1.
Electrodes embedded on the MED probe (left) and the mechanism of recording of field potentials with the MED64
Reference electrodes
system (right).
(100 M
Ω
)
MED64 electrode (Pt. black. 50
m)
ITO electrode (50
m)
MED64 electrode (Pt. black. 20
m)
Fig.2.
Electrode impedance and input impedance of the MED64 Head Amplifier.
Input impedance of MED64 Head Amplifier
Attenuation
