Aspect Medical Systems BISPECTRAL INDEX A-2000, Service Manual

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SECTION III PRINCIPLES OF OPERATION
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SECTION III
3.
PRINCIPLES OF OPERATION
The A-2000 BIS Monitoring System consists of:
•
The BIS monitor with built-in battery backup and detachable power cord
•
The Digital Signal Converter (DSC)
•
Aspect’s BIS Sensor Patient Interface Cable (PIC) and BIS Sensor
This section describes the architecture of the A-2000 monitor and DSC and how their
constituent parts interact.
3.1
SYSTEM ARCHITECTURE
Hardware is divided into two main components: the monitor and the digital signal
converter (DSC). The A-2000 monitor contains the circuits for digitally processing the
EEG data, computing the processed parameters, and displaying the waveforms and
processed parameters. The circuits to acquire and digitize the EEG signals reside in the
Digital Signal Converter (DSC).
The monitor also contains the circuits for powering the monitor, DSC, and companion
printer. Integral to these circuits is a battery backup. Printer function is not available
while on battery backup.
A block diagram depicting the monitor subassemblies appears in Figure 3-1. A signal flow
diagram appears in Figure 3-2.
After passing through input protection circuits in the DSC, the EEG signals are differentially
amplified and filtered to remove DC and high frequency components. The signals are
digitized by separate one bit sigma-delta analog to digital converters. The outputs from the
converters are multiplexed onto the DSC communications line and de-multiplexed in the
monitor prior to being fed to the Digital Signal Processor (DSP). The DSP filters the signals
and computes the processed variables. The results are passed to the host CPU for display.
3.1.1
The Digital Signal Converter (DSC)
The DSC contains the inputs, amplifiers, and digitizers for two channels of EEG. It has a
single point connection that connects via a Patient Interface Cable (PIC) to a BIS Sensor.
The A-2000 is designed to process one channel using the BIS Sensor, the A-2000 and DSC
hardware is capable of two channels. The sensor and PIC contains circuits for identifying
them to the monitor. This permits the monitor to configure automatically.
The DSC constantly monitors the combined source impedance at its inputs, and also has
facility for measuring the individual impedance of the channel and ground electrodes. There
are circuits for injecting self-test voltages into the amplifier inputs.