LED Drive Circuit
The IR and red LEDs are separately controlled with their drives currents multiplexed over two shared wires.
Current to the IR LED is in the range of 4.3-50.0 mA; and, current to the red LED is in the range of 6.5-75.0
mA. Currents are limited to less than 100 mA for two reasons: (1) slight excess current can potentially change
the emission characteristics of the LEDs, and (2) large excess current could create excessive heat at the sensor
site.
The IR/red LED transmission signal (HSO1 of the CPU) is fed into the select inputs of the triple single-pol-
double-throw (PDT) analog multiplexing switch U10, causing either the IR or the red LED transmission to be
enabled.
PWM1, which is filtered by the network of R44, R52, and C38, is input to the LED drive circuit switch U10, and
controls the magnitude of the IR LED current supply.
PWM2, which is filtered by the network of R43, C36, R53, and C39, is also input to U10, and controls the red
LED current magnitude.
Two NPN transistors (Q1 and Q2) act as current sources for the IR and red LED outputs. Two PNP transistors
(Q3 and Q4) act as switches between the IR and red LED output lines. Transistor Q5 acts as an LED drive
current limiter; it clamps output of the current regulator circuit to the required level. If any resistor in the LED
drive circuit fails, current to the LED will still be limited to a safe level.
The RSENS line senses the RCal value and enables the CPU to make the proper calculations based on the type
of sensor being used.
Input Conditioning
Input to the SpO
2
analog circuit is the current output of the sensor photodiode. In order to condition the signal
current, it is necessary to convert the current to voltage.
A differential synchronous demodulation circuit is used to reduce the effects of other light sources and stray
frequency inputs to the system. Because the IR and red signals are absorbed differently by body tissue, their
received signal intensities are at different levels. Therefore, the IR and red signals must be demodulated and
then amplified separately in order to compare them to each other. Demultiplexing is accomplished by means of
two circuits that alternately select the IR and red signal. Selection of the circuits is controlled by two switches
that are coordinated with the IR and red transmissions. A folter with a large tiem constant follows to smooth the
signal and remove noise before amplification.
Differential Synchronous Demodulation Circuit
Before the current from the photodetector is converted to voltage, any high frequency noise is filtered by C40
and R17. The op-amp UIA is used in parallel with the current-to-voltage converter UID to cancel any DC
voltage, effectively AC coupling the output of UID. The average value of the SpO
2
analog reference voltage
(VREF) of UID, 5 V, is measured at pin 14 of test point 49.
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