Aspect Medical Systems BISPECTRAL INDEX A-2000, Руководство по обслуживанию

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SECTION III PRINCIPLES OF OPERATION
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3.1.1.1 The DSC’s Preamplifier Board
The preamplifier board contains the input protection circuits, montage mode switches,
differential amplifiers, filter and gain amplifiers, impedance test circuits, self test circuits, and
parts of the sensor ID circuit.
3.1.1.2 DSC Signal Conditioning
The input protection circuits are designed to protect the inputs from destruction by electric
shock from sources such as electrostatic discharge (ESD) or defibrillation. The protection
circuits also reduce the effects of high frequency ambient noise from sources such as
electro-cautery and other devices.
The montage mode switches follow the protection circuits. These switches configure the
inputs appropriately for the sensor. The signals are amplified by instrumentation amplifiers
which have a fixed gain. The amplifiers have DC servos which remove the signals below
high pass cutoff frequency. In the event of amplifier overload, the servos are changed to a
higher frequency to facilitate fast recovery (blocking) under control of the host processor.
Each channel is further amplified to the level required by the A/D converters. The amplifiers
also serve as filters to prevent aliasing by the converters.
3.1.1.3 DSC Impedance Testing
In the default state of the DSC the combined channel electrodes’ impedance is continuously
checked. A small current (approximately 1 nanoampere) is injected into each electrode at
128 Hz, just above the EEG band. The resulting voltages are measured. Equal but opposite
currents are injected into the (+) and (-) electrodes simultaneously while the digital signal
processor measures the resulting voltage. BIS monitoring is performed while combined
impedance is checked.
The DSC can also measure the individual electrode impedance by injecting current into the
REF electrode only. Individual electrode impedance is derived by subtracting the resulting
value from the combined value. BIS monitoring is interrupted while individual impedance is
checked.
The ground electrode impedance is also measured while injecting current into the REF
electrode. BIS monitoring is interrupted while the ground impedance is checked.
The impedance check signal can occasionally interfere with other monitoring equipment
connected to the patient. Evoked potential monitors are particularly susceptible because
they use a wide bandwidth. The automatic impedance check feature can be turned off by
selecting “Impedance Checking – OFF” in the Diagnostics Menu (See Operating Manual for
specific instructions).