Aspect Medical Systems BISPECTRAL INDEX A-2000, Service Manual

Page: 22 / 92

SECTION III PRINCIPLES OF OPERATION
- 3-5 -
3.1.1.4 The DSC Communications Board
The communications board contains the analog to digital (A/D) converter for each channel,
the monitor interface, the sensor interface and the power supply circuits. A crystal controlled
DSC master clock is on this board. This clock is the system’s BIS processing clock.
A/D Conversion (Patented technology)
The communications board contains sigma-delta modulators for the two channels. These
run at 16384 samples per second.
Test Signal
A calibrated test signal is generated on the communications board during 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 self test noise, gain and frequency response are
checked.
Interface to the Monitor (Patented technology)
The outputs from the two channels are multiplexed in a field programmable gate array
(FPGA). Multiplexed with the EEG data is status information such as DSC identification,
mode (bipolar or referential), lead off indication, and power supply faults. The status
information is only transmitted on command from the host. The output is transformer
coupled onto a balanced twisted pair line. The transformer provides the required patient
isolation.
The communications board decodes the control information coming from the host via a
command line. Commands such as “block” amplifier saturation and conduct impedance
tests are transmitted.
DSC Power Supply (Patented technology)
The DSC derives power from the command line. The line is balanced twisted pair coupled
via a transformer on the communications board. The isolated signal is rectified and linearly
regulated to provide DSC power.
3.1.1.5 The DSC Mechanicals
The DSC is contained in a small custom designed plastic case about the size of a computer
mouse (see Figure 3-3). It is connected to the monitor via a narrow, highly flexible cable.
The case has one large eyelet at the corner to allow the user to use a strap to hang the DSC
or pin it to the bed sheets. The patient connection is accomplished with an Aspect BIS
Sensor for low and balanced impedance. The Patient Interface Cable pigtail and the monitor
cables are strain relieved and permanently attached to the case. A proprietary connector is
used for the patient interface cable connection and a high quality plastic connector is used
for connection to the monitor.
There are no ventilation holes in the DSC case. It will not leak when splashed with liquids.
The case is electrically shielded both to prevent spurious emissions from the DSC and to
prevent externally caused interference with the DSC circuits.