Multichannel Systems STG4004+STG4008 Manual - Manual (Page 81)

Analog Output Signals
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11.4 Measuring Output Signals with an Oscilloscope
For test purposes, you can measure the output signals with an oscilloscope. Please see
the appendix for the pin layout of the sockets on the front panel of the STG4000 series.
Most standard oscilloscopes have a probe with a ground cable. This probe is usually connected
to the oscilloscope via a BNC socket. What you do for measuring the voltage output is the
following: Choose voltage mode in the main menu of the MC_Stimulus II software. You plug in
the standard STG cables into the +U output and the corresponding GND output. You then touch
the open end of the cable from the +U output with the probe from the oscilloscope, and the open
end of the cable from the GND output with the ground cable from the probe. This means you
measure the potential difference between the +U output and ground.
Please note that you need a resistor in parallel to measure the current output properly because
oscilloscopes measure voltages. If the input resistance of the oscilloscope is too low, the voltage
output of the STG will be small as well, and you will not see any signals on the oscilloscope.
MCS recommends a resistor of 10 k
 because the output signal of the current output should
then be identical to the output signal of the voltage output. You can use the same test protocol
(regardless of the output mode) for the current and voltage outputs, and compare the signals.
As an alternative, you can also use a resistor that mimics the electrode resistance for testing your
experiments.
Choose current mode in the main menu of the MC_Stimulus II software. What you do is, you plug
in cables into the +I output and into the GND output of the STG. You then use alligator crimps
for connecting a 10 k
 resistor between the two open ends of the cables. You then touch the
cable on one side of the resistor with the probe and on the other side with the ground of the
probe. (Alternatively use the provided cable with integrated 10 k
 resistor). That is, you measure
the potential difference over the resistor. You can then calculate the current output according
to Ohm's law (U = R * I). For example, if you stimulate with 100 μA, the voltage over the 10 k

resistor has to be U = 10000
 * 0.1 mA = 1000 mV = 1 V.
The following illustration shows four possible combinations of connecting the current outputs on
the front panel of the stimulus generator to the oscilloscope. Again, it is important that unused
current outputs are connected to ground.
Measuring current output signals with an oscilloscope
11.5 Rise Time
The rise time of the voltage output is constant and independent from the stimulus protocol.
The rise time of the current output is no fixed hardware property. The kinetics depend on
the amplitude of the current pulse and the load resistance, that is, generally the electrode
impedance. The resistance affects the kinetics much stronger than the amplitude.
Generally, this does not make much difference for biological applications. If you have very time-
critical applications, or if you use very short pulses, you should check the output of the STG
with an oscilloscope before starting the experiment.