Novametrix Medical Systems MARS PO2 TECH 2001, Руководство по обслуживанию

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2775 Main Board
Section 4
Theory of Operation
Rev. 01 Model 2001 Service Manual
15
When RDLED* returns high (logic 1), Q14 is biased on, forcing IC36 pin 10 to ground potential:
Q11 is biased off, and as a result, the Red LED in the sensor is also off.
The Infrared LED drive circuit operates in the same manner as the Red LED drive discussed
above. The IRLED* signal at IC42 pin 17 activates Q13 the LED2S signal causes a positive
signal at IC36 pin 14, and current can flow from the LEDSRC supply through the sensor’s
Infrared LED, Q12, and the current limiting resistor R52 to ground.
Sensor Photodiode Return Path
See sheet 2 of 4 on schematic.
Light, from the sensor’s Red or Infrared LED, shines through the pulsating vascular bed (the
patient’s finger, toe, etc.) placed between the LEDs and the photodiode. Some of this light
emerges from the tissue and impinges on the photodiode, causing the photodiode to conduct
current. IC40 pins 1-3 is set up as a differential amplifier that converts this input current to a
voltage at the amplifier output. The sensors are wired such that photodiode current produces a
positive voltage at IC40 pin 1.
The voltage at IC40 pin 1 is presented to an analog switch IC41 pin 6. This switch is controlled
at pin8 by INSIG* (Input Signal) from IC42, and will be closed (IC41 pins 6 and 7 connected)
except if the monitor is in a Probe Off Patient condition or is undergoing its Self-Test at system
power up. The switch IC41 pins 9-11, controlled from SIGND* (Signal Ground) at IC42 will be
open (no connection between IC41 pins10 and 11) except as noted above for the switch at IC41
pins 6-8. As a result, the IC40 pin 1 voltage passes undisturbed to the high pass filter consisting
of R59 and C90.
As shown in Figure 1., the ASAMP* signal is active whenever either sensor LED is turned on.
This causes Q15 to turn off and the charge at C90 passes through the unity gain buffer to IC40
pin 5.
If the signal at IC40 pin 7 is the product of the Red LED being turned on, then RDSMP* from
IC42 pin 12 will go low and close the switch at IC41 pins 2-3, thereby presenting the signal to
a sample and hold circuit consisting of R54 and C100 (that maintains the signal until next
sample pulse arrives), a gain stage (IC38 pin 1), a filter/divider network (C87, R45 and R46),
and finally, to the Red channel Analog-to-Digital Converter (ADC) IC34.
If the signal at IC40 pin 7 is the product of the Infrared LED being turned on, then ISMP* from
IC42 pin 13 will go low and close the switch at IC41 pins 14-15, thereby presenting the signal
to a sample and hold circuit consisting of R55 and C96 (that maintains the signal until next
sample pulse arrives), a gain stage (IC38 pin 7), a filter/divider network (C88, R49 and R50),
and finally, to the Infrared channel Analog-to-Digital Converter IC37.
Again referencing Figure 1., the ASAMP* line returns to a logic high when neither LED is being
driven, causing Q15 to turn on. With Q15 conducting, any charge at C90 is discharged to
ground and the next pulse will charge C90 from a known level. If it were not for Q15, any charge
remaining on C90 from the previous pulse or from ambient light reaching the photodiode would
be added to the charge from a new pulse-creating measurement errors.
Calibrating the 20-Bit Analog-to-Digital Converters
See sheet 2 of 4 on schematic.
The 20-bit ADCs are calibrated as part of the system self-test which occurs each time the
monitor is turned on. At power up, the microprocessor sets the CAL line high. The System
Calibration input SC1 is set high and SC2 is reset to a logic low. The CS5503 ADC will not