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Technical Supplement
If the LEDs reach maximum output without the necessary signal strength, the PWMs will increase the
channel gain. The PWM lines will select either a change in the LED current or signal gain, but will
not do both simultaneously.
The LED circuit switches between red and IR transmission and disables both for a time between
transmissions in order to provide a no-transmission reference. To prevent excessive heat build-up and
prolong battery life, each LED is on for only a small portion of the duty cycle. Also, the frequency of
switching is well above that of motion artifact and not a harmonic of known AC transmissions. The
LED switching frequency is 1.485 kHz. The IR transmission alone, and the red transmission alone
will each be on for about one-fifth of the duty cycle; this cycle is controlled by the HSOs of the CPU.
9.9.5.1 LED Drive Circuit
The LED drive circuit is illustrated in Figure 9-8 (at the end of this section).
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-pole-double-throw (SPDT) 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, C37, 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.
9.9.6
Input Conditioning
Input to the SpO2 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. Two switches that
are coordinated with the IR and red transmissions control selection of the circuits. A filter with a large
time constant follows to smooth the signal and remove noise before amplification.
9.9.6.1 Differential Synchronous Demodulation Circuit
The differential synchronous demodulation circuit is illustrated in Figure 9-9 (at the end of this
section).
9-8
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