Service manual

Neurodiagnostic Instruments Service Manual
2 - 6 Revision 2/18/05
This noise appears as a spike in the waveform with a
frequency the same as the monitor sync frequency.
Environmental Noise
The sensitive amplifiers can pick up all kinds of
electromagnetic radiation through the electrode leads or
power lines. Some typical sources of environmental
noise are:
• 50/60 Hz power radiation
• switching noise from light dimmers
• fluorescent lights
• electrocautery instruments
• other electrically powered instruments in vicinity
• nearby radio stations
Typically, these effects are magnified by a poor ground
connection at the power receptacle.
Physiological Noise
Muscular artifact is an example of physiological noise.
For an ABR, this noise typically comes from tense neck
or jaw muscles. Relax the patient.
Reducing Noise Pickup
Try the following suggestions to reduce the pickup of
environmental noise:
Electrode Considerations
• Achieve low electrode impedances (<5Kohm).
• Balance electrode impedance (<1.5Kohms
difference).
• If using needle electrodes, are they fully inserted?
They can act as an antenna if not fully inserted.
• Dress (Braid or Tape) the electrode lead wires
together or bundle them up to reduce the effective
length.
• Bundle all stimulating electrodes and separate as
much as possible from recording electrodes.
• It is recommended that use of all the same type of
electrodes. Do not mix metals or types of electrodes.
• Try a different brand of electrodes to eliminate the
possibility of common flaw in the type, batch or style
of electrodes.
• Use of EEG grounding mats are discouraged as they
may act as an antenna for noise.
• Do not place ground in middle of patient chest as you
may get EKG residual waves on your waveforms.
These residuals look much like 60Hz.
• Use a ground electrode with a large surface area. Do
not use a needle electrode as a ground.
Instrument Consideration
• Change the stimulator rate and always use an
irregular number. Avoid numbers that are a harmonic
of the line voltage and the frequency of the noise you
are trying to eliminate.
• Do not let the stim rate exceed the window of
acquisition.
• Change to “View Input” mode to view the live signal.
Notice if the noise is locked or seemingly triggered.
If the noise appears time locked or triggered, adjust
the stim rate until it rolls or appears randomly
throughout the trace. This should make the noise
average out in the “View Averaged” mode.
• Run a base line in the room with all other equipment
turned off and unplugged.
• Run a base line in another room or location. The more
distant the location the better.
• Put a loose knot in the power cord to disrupt the
antenna effect of the cord.
Power Considerations
• Typically, hospital “Isolated” receptacles are
extremely noisy. Have them checked.
• Plug the instrument into another wall receptacle.
• Check receptacle for proper ground and proper
negative and neutral using an ECOS meter.
• Check to see what other equipment is powered from
the same electrical circuit. The other equipment may
be the source of the noise.