Novametrix Medical Systems OXYPLETH 520A, Service Manual

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Section 9
Electronic Theory of Operation
24
Model 520A Service Manual 5693-90-01
PRELIMINARY
D0-D7 (IC30 pins 14-7) now control the output voltage of IC30 pin 8 (VLED)
based on the VREFB voltage on IC29 pin 18.
When the RDLED* signal at IC42 pin 18 goes low (logic 0), Q14 turns off and the
Red LED signal (VLED from IC30 pin 8) at R37 is divided by R37 and R41, finally
causing IC36 pin8 to go high. This positive output turns Q11 on and current flows
from the LED source (LEDSRC), through the Red LED in the sensor (it turns on)
returning as LED1SK (LED1 sink) across Q11 and the current limiting resistor R53
to ground. (See sheet 3 on 2471 schematic, sheet 2 on 2775 schematic.)
When RDLED* returns high (logic 1), Q14 is biased on, forcing IC36 pin8 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
R52 to ground.
Sensor Photodiode Return Path
9.3.9
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
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
pins6 and7 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 3., 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 7.
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
1. The Model 520A uses SuperBright™ sensors. If a non-SuperBright™ sensor is connected, IC40
pin 1 will go negative—a condition that causes an “Incompatible Probe” message to be displayed.