Mallinckrodt NELLCOR NPB-4000, Service Manual

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Section 11: Isolated Front End Functions -- Theory of Operation
11-8
11.7 OPTOCOUPLERS
Figure 11-7: Optocouplers Block Diagram
See Figure 11-7. The optical couplers that isolate the Front End signals from the
non-isolated digital hardware are of two types: Linear and Digital.
11.7.1 Linear optocoupler
All analog signals, ECG, Respiration, Temperature, VREF and AGND, as well
as voltage levels of binary signals PM( pacemaker), QRS (R-wave), and
LOFF(leads off) are fed to the inputs of analog multiplexer U67 and to the linear
optocoupler U57. The optocoupler has an LED and two matched sensing
photodiodes and works in a servo mode with two operational amplifiers, U60
and U51. Input current set by multiplexer output and resistor R166 is compared
by U60 with the current of input photodiode. The LED is controlled by U60
until two currents are equal. The current of the output diode mirrors the input
diode’s current, thus producing an image of the multiplexer voltage at the output
of U57. Operational amplifiers maintain very close to zero impedance at diode
outputs, which provides for very linear diode currents and a very linear and
stable current ratio. A small positive DC offset is provided from divider R168,
R169 to compensate for possible amplifier negative voltage offset.
11.7.2 Digital Optocouplers.
Digital optocouplers are inverting, high-gain, low-power types. The baud rate is
set for 50 kHz operation. Inverter U61 provides inversion of optocoupler
signals.