Aspect BIS VIEW, Руководство по эксплуатации

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SECTION 3 PRINCIPLES OF OPERATION
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3-5
A/D Conversion
There are two independent sigma-delta modulators for the two channels. These run at 16384
samples per second.
Output from the two channels is multiplexed in a field programmable gate array (FPGA).
Multiplexed with the EEG data is status information such as BISx identification,
“lead off” indication, and power supply faults.
Ground Fault Monitor
Patient auxiliary current from a single electrical fault must be limited to 50 uA (as required by
the safety standard, IEC 60601-1). A single fault current will always return through the
patient ground lead because all other inputs are high impedance. The ground fault monitor
therefore monitors the current in the ground lead.
The circuit is designed to measure either positive or negative currents in excess of 50 uA.
Faults lasting for a period of 2 seconds are flagged and cause the isolated analog supply to
shut down. Additional circuitry prevents false triggering due to excessive noise such as those
generated by electro-surgery.
Smart Chip Functions
The smart chip is an integrated circuit embedded in the sensor. It is used for sensor
authentication and to keep track of the number of times the sensor is used. The
communication between the BISx and the smart chip is initiated when the BISx detects the
presence of the smart chip. After the data is read, it is discontinued during the monitoring
process to prevent interference with the EEG signal. If the current exceeds 25 mA, it is
assumed there is something wrong with the smart chip and power to the chip is switched
off.
Cautery Detector
The cautery detector detects the common mode current passing through the sensor and out
through the transformer capacitance. It can detect large differential lead voltages also. The
electro-cautery detection circuit is connected to the Channel 1 – lead, as this is the farthest
lead form the ground lead. As cautery energy has frequency content in the 100 - 500 KHz
range, the detector senses only signals above 20 KHz and will send out a cautery detect
signal when these signals exceed 20 uA.
Test Signal
A calibrated test signal is generated during the DSC Self Test. The signal is a 2 Hz square
wave of approxi/-50µV. It is applied to the inputs of the differential amplifiers,
resulting in a test of the entire signal path except for the input connections and protection
circuits. During the Self Test, noise, gain and frequency response are checked.
Interface to the Monitor
The BISx uses an RS-232 interface to communicate with the VIEW monitor. Control
commands are received from the monitor and acknowledged by BISx and BISx transmits
data and status information to the monitor. The Monitor Interface Cable also supplies power
to the BISx and includes a circuit that allows the monitor to detect connection of the BISx.