Mallinckrodt NELLCOR NPB-4000 Service Manual page 99

Section 11: Isolated Front End Functions -- Theory of Operation
• Isolation is provided by optocouplers. A total of four optocouplers are used
for digital signals and a linear optocoupler with a multiplexer is used for all
analog signals. The isolated side has its own power supply which generates
+5 volts and -5 volts for analog circuitry and +5 volts for digital circuitry.
A switching driver and an isolated transformer provide 100 kHz AC-power
to the isolated side.
• An A/D converter on the non-isolated side digitizes signals coming from the
linear optocoupler line as well as three signals from the NIBP circuit and
other analog signals, such as samples of power supply voltages on the non-
isolated part.
An expanded block diagram of the Front End is presented in the Figure 11-3.
Figure 11-3: Front End Block Diagram, Expanded
Circuit descriptions that follow include reference designations of components
identified in the Main PCB Schematic Drawing, Figure 17-7.
11-3