3 current mode – Multichannel Systems STG4004+STG4008 Manual User Manual
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Stimulus Generator 4000 Series Manual
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11.3 Current Mode
If you want to operate the STG in current mode, you have to select "Current" in Output Mode of
the main window of MC_Stimulus II. You will usually use the + and GND outputs. See illustration
"Standard setup".
The output signals are inverted (multiplied by -1) if you use the - outputs.
If the load resistance, that is, the electrode impedance is very high, the output voltage
can get very high, too. The stimulus generator limits the output voltage to 75 V between + or -
and GND. The guaranteed compliance voltage is +/- 60 V, that means 120 V between + and -.
The current output of the STG is limited by the compliance voltage and the electrode
impedance. If the maximum load resistance for a given current output is exceeded, the
compliance voltage (60 V between +I and GND) of the STG will not be sufficient for delivering
enough current, and the output will be clipped. You can calculate the maximum load resistance
by Ohm's law. For example, if you want to program a 100 μA pulse, the maximum load resistance
will be 600 k
. According to Ohm's law: R = U / I = 60 V / 100 μA = 600 k, that is, 60 V are
sufficient for delivering a current of 100 μA at a load resistance of 600 k
Therefore, if you use
current driven stimulation, you should always take the impedance of the stimulating electrodes
into account when planning your stimulus protocol. You should also keep in mind that most
electrodes or amplifiers support only limited voltages.
You can increase the compliance voltage to 120 V by using the + and – outputs. The output
current even at the maximum output voltage of 150 V should not be dangerous to human health
because the maximum output current in current mode is only +/-1.6 mA. See illustration
"Doubled maximal output voltage".
A bipolar stimulation is recommended for extracellular recordings from microelectrode arrays
(MEAs) if a very high output signal density in a defined area between two stimulating electrodes
is needed. The current that is delivered by the first electrode flows directly back to the second
electrode so that the surrounding tissue is not affected. See illustration "Dipolar stimulation".
Screen shot from a standard oscilloscope triggered on the Sync Out pulse (see "Measuring output
signals with an oscilloscope"). Green trace = current output (+I), red trace = inverted current
output (I), pink trace = TTL output from the Sync Out.