natus Grass S88X, User & Service Manual

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S88X
Dual Output Square Pulse Stimulator
8-51
s
timulus
i
solatioN
Warning:
The S88X output is not isolated. Do not use on humans.
Isolation of the signal from ground is most effective in the reduction of
those artifacts due to ground currents arising from the stimulating and
recording systems which are conductively joined by the preparation.
When stimulus isolation is used during cortical stimulation, and in
similar instances when a large volume of tissue surrounds closely spaced
stimulating electrodes, the stimulus current sets up a three-dimensional
field pattern, wherein the strength of the field usually decreases with
the cube of the distance from the stimulating electrode. Isolation of the
stimulus from ground in this instance is particularly effective because
it reduces ground currents. Stimulus isolation is particularly necessary
with multi-channel recording. Is is also valuable from the standpoint of
safety, because it isolates the stimulating electrodes from ground. It has
the further advantage of permitting direct addition of stimuli of either
algebraic sign.
In many applications, a very low output impedance of high power is
required and is featured in the S88X. Such an output will stimulate in
solutions, will drive a long lead line or other capacitive loads without
degrading the stimulus pulse. This is not possible with a high impedance
source (constant current). It is possible and economical to drive isolated
and high impedance circuits, but it is not economical to build low
impedance outputs from high impedance sources. Furthermore, it is most
often desirable to have the isolating and constant current circuits as
close to the preparation as possible to preserve the isolation and fidelity
of the stimulus. Thus, separate cabinets for these circuits are preferred.
Furthermore, not all applications require SIUs. The argument for this
system is like that for high impedance probes for amplifiers.
The characteristics of the Grass SIUs permit the shielding of stimulating and
recording electrode leads, thus reducing the capacitive coupling between
recording and stimulus leads. For maximum reduction of artifact, the SIU
should be placed as close to the preparation as is possible. Ideally, short
unshielded leads to the electrodes should be used and every attempt
should be made to reduce conduction and capacitance between the output
leads, recording leads and ground to a minimum. If it is not possible to use
short electrode leads, use 2-wire shielded low capacitance cable.
Additional information and operating details are further discussed in the
specific Stimulus Isolation Unit Instruction Manual.