GE Corometrics 126, Руководство по обслуживанию

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Revision B 120 Series Maternal/Fetal Monitor 4-71
2015590-001
Theory of Operation: MSpO
2
Board
MSpO
2
Board
Nellcor Module
Functional Overview
This internal module is manufactured by Nellcor Puritan Bennett, Inc. The module
handles all circuitry involving MSpO
2
with the exception of the display circuitry.
The module measures the maternal oxygen saturation and pulse rate using the
principles of spectrophotometry and plethysmography. The MSpO
2
sensor is
comprised of two light-emitting diodes (LEDs), which are the emitters, and one
photodiode which is the detector. One LED emits red light and the other infrared
into the patient’s skin. The detector receives the amount of light that is not absorbed
at the sensor site and the module uses the relative absorption of red and infrared to
compute the percentage of functional hemoglobin that is saturated with oxygen.
The following technical information is a summary of the module’s circuitry. This
information is provided to give the hospital biomedical engineer an overall summary
of the module’s functions in relation to the host unit — to assist in isolating
troubleshooting problems to the board level. This information is not intended to
provide a complete circuit theory and should not be used to repair any problems on
the module itself.
When a problem does exist with the module, contact your Corometrics Service
Representative for information about repair/exchange of the module.
LED Driver Circuitry
This circuitry controls the on and off pulsing of the infrared and red LEDs at the
measurement site. Signals REDLED/ and IRLED/ alternately close FET switches
U4A and U4B, respectively. The LED drive voltages are developed by dual DAC
U1. The LEDs operate in a four-phase sequence as follows:
‹ 184.5 µs: infrared LED on
‹
184.5 µs: both LEDs off
‹ 184.5 µs: red LED on
‹
184.5 µs: both LEDs off
Transistors Q1, Q2, Q3, Q4, Q5, and Q6 control the drive current switching. The
four-phase LED drive signal is routed to LED driver U3A. The resulting drive
potential is coupled equally to both sides of the bridge circuit via R31 to Q4 and R36
to Q3. Signals IRLED/ and REDLED/ are pulsed low to cause their respective
LEDs to illuminate. When both lines are a logic high, all bridge transistors are
turned off. The infrared LED is illuminated when signal IRLED/ is pulsed low. The
red LED is illuminated when the signal REDLED/ is pulsed low.
Input Source Selection Circuits
The sensor is connected directly to the Nellcor module; the 120 Series Monitor does
not provide an external pre-amplifier. Two inputs are handled by this circuitry. The
SAT signal, developed by the photodiode in the sensor, enters through JP1 (pins 1
and 4). Pre-amplifier U8 is used as a current-to-voltage converter for the photodiode
current. The signal is then coupled through U10A to the input amplifier and
synchronous detector circuits. The wavelength of the red LED is derived from the