Multichannel Systems STG4002 Manual - Manual (Page 94)

Stimulus Generator 4002 Manual
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11.7
Capacitive Behavior of Stimulation Electrodes
Regarding the generally used stimulus pulses, stimulating electrodes behave as plate capacitors.
They need some time to discharge themselves after stimulation. As a result, artifacts interfere
with the recording, and electrodes deteriorate over time due to electrolysis. This effect takes
place especially in current mode because the current cannot flow back to the stimulus generator
due to the high output resistance in current mode and thus is kept in the electrode. A current
source can constantly produce small currents, even though the current is set to zero. This might
damage the connected electrodes over time. To avoid this problem, the STG4002 is equipped with
switches that automatically disconnect the current outputs from the current source 1 s after each
programmed current stimulus. For the next stimulus, the current source is automatically
reconnected.
To reduce the effect described above in current mode, you should use biphasic pulses for
stimulation. The stimulus signal should be immediately followed by an inverse signal of the
same area (product of current and time), which helps to discharge the electrode. The easiest
way is to use the same signal amplitude with an opposite polarity.
Multi Channel Systems recommends using voltage driven stimulation. In voltage mode, it is not
necessary to use biphasic stimulation for discharging the electrodes, because the electrode should
be discharged at a voltage of 0, that is, it would be sufficient to apply the negative phase only.
Warning: When using MEA electrodes of TiN material, stimulation with higher amplitudes
is possible if you apply the negative pulse first . Also, regard the safe charge-injection limit
as described in the MEA manual. Otherwise, electrodes can be damaged during stimulation.
The following illustration shows the effect of a biphasic current pulse on the discharge of
the stimulation electrode. As you can see, the first monophasic pulse is followed immediately
by a pulse of the opposite polarity and the same product of current and time.
Effect of a bipolar current pulse on the electrode voltage