natus Grass S88X, User & Service Manual

Page: 52 / 64

8-52
Chapter 8
Artifacts, Stimulus Isolation & Constant Current
s
timulus
c
urreNt
m
oNitoriNg
It is often desired to measure the actual stimulus current. There are two
ways of measuring the stimulus current. A DC ammeter can be connected
in series with the stimulating electrodes or an oscilloscope can be utilized.
When using a milliammeter or microammeter, it should be placed in series
with the stimulating electrodes. Stimulus isolation can be maintained if the
meter is isolated with an SIU from ground. See Figure 8-3.
With long duration pulses and very long pulse intervals, the meter
will provide accurate measurements of peak current. When moderate
frequencies and durations are used, the meter will indicate average
current. Peak current of a simple repetitive pulse can be calculated by the
following:
Peak Current = ___________________
Where Duty Cycle is defined as:
the percent “ON TIME”, i.e.,
Percent Duty Cycle = _____________ x 100
then by substitution,
Peak Current = ________________________________
If the repetition rate of the stimulus is close to the resonant frequency of
the meter, however, it will be impossible to read the meter accurately. This
method cannot be used for biphasic stimuli.
For further discussion, see the a
ppenDix
included in this manual.
S88X
SIU
Ammeter
Stimulating
Electrodes
Figure 8-3: Measuring Current
Average Current x 100
Duty Cycle Percent
Pulse Duration
Pulse Interval
Average Current x Pulse Interval (ms)
Pulse Duration (ms)