Service manual

Neurodiagnostic Instruments Service Manual
2 - 8 Revision 2/18/05
Signal Conditioning
The energy of each type of evoked response lies within
a well-defined band of frequencies. The signal
conditioning block reduces frequencies outside of this
window of interest in an effort to “clean up” the
waveform before averaging takes place. The signal
condition block also adjusts the signal amplitude for
optimum results. The component blocks of this stage
are:
Low Frequency Filter (LFF)
The low frequency filter reduces the amplitude of the
signals below the frequency band of interest. This filter
is also referred to as a high pass filter (HPF).
High Frequency Filter (HFF)
The high frequency filter reduces amplitude of signals
above the frequency band of interest. This filter is also
referred to as a low pass filter (LPF)
Notch Filter (NCH)
This is a narrow band filter that reduces amplitude of
50Hz or 60Hz line interference. The use of this filter is
discouraged as it can mask poor electrode application
techniques and is in the middle of the VEP and SEP
waveform bands. The notch filter may be used when
there is no other means of reducing line interference.
This can allow patient signals to be recorded in an
environment that would otherwise result in
unacceptable patient signals.
Sensitivity Stage (SNS)
The sensitivity stage sets the overall voltage gain of the
conditioned patient input signal. The sensitivity range is
typically 10uV full screen (a voltage gain of 500K) to
100mV full screen (a voltage gain of 10).
Stimulators
The stimulators provide a single or repetitive stimulus to
the patient. The are three typical types of stimulators
that are used on a NCS/EMG/EP system. They are the
Electrical Stimulator, the Auditory Stimulator, and the
Visual Stimulator.
Electrical Stimulator
The electrical stimulator is used to provide a small
electrical signal to the patient. VIASYS instruments
support two types of electrical stimulation: constant
current and constant voltage.
The constant current range for VIASYS instruments is
0-100mA. When using the electrical stimulator is this
mode, the stimulator will try to maintain the selected
current level, regardless of how much voltage it takes to
overcome the skin impedance. If the stimulator is
unsuccessful, the system displays an “Impedance
Limited” message.
The constant voltage mode delivers a selected voltage
level to the patient. The constant voltage range for
VIASYS instruments is 0-400V.