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Technical Supplement
9.5.2
Input Conditioning
9.5.1.2 Differential Synchronous Demodulation Circuit
9-8
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 regulators for the IR and red
LED return lines. 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 to the SpO
analog circuit is the current output of the sensor photodiode.
2
In order to condition the signal current, it is necessary to convert the current to
voltage.
A 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
signals. Selection of the circuits is controlled by two switches that are
coordinated with the IR and red transmissions. A filter with a large time
constant follows to smooth the signal and remove noise before amplification.
The differential synchronous demodulation circuit is illustrated in Figure 9-9.
Before the current from the photodetector is converted to voltage, C40 and R17
filter any high frequency noise. The op-amp U1A is used in parallel with the
current-to-voltage converter U1D to cancel any DC voltage, effectively AC
coupling the output of U1D. The average value of the SpO
voltage (VREF) of U1D, 5 V, is measured at test point 49.
analog reference
2
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