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SECTION 3 PRINCIPLES OF OPERATION

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3-4

3.2.1.1 BISx Signal Conditioning

The input protection circuits are designed to protect the input 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.

Input 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.

3.2.1.2 BISx Impedance Testing

In the default state of the BISx 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 BISx measures the individual electrode impedance during a sensor check 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. Ground

impedance checking occurs when a sensor check is performed and thereafter on a 10-minute

schedule during patient monitoring.

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.” (See Operating Manual for specific instructions).

3.2.1.3 BISx Processor and Communications Circuits

The BISx contains: an analog to digital (A/D) converter for each channel, the monitor

interface, the sensor interface and the power supply circuits. A crystal controlled BISx

master clock is on this board. This clock is the system’s BIS processing clock.

A/D Conversion

There are two independent sigma-delta modulators for the two channels. These run at 16384

samples per second.

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Aspect BIS VISTA Spécifications Page 26