Nellcor NPB-40 Service Manual page 46

Technical Supplement
S3.3.2 Input Conditioning
S-14
At initialization of transmission, the LEDs' intensity level is based on
previous running conditions, and the transmission intensity is adjusted until
the received signals match the range of the A/D converter. 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 drive 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.
The IR and red LEDs are separately controlled with their drive 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 4.3 to 50 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 U7, causing either the IR or the red LED transmission to be enabled.
PWM1, which is filtered by the network of R45, R35, C41, and C42, is input
to the LED drive circuit switch U7 and controls the magnitude of the IR LED
current supply.
PWM2, which is filtered by the network of R29, R39, C35, and C23, is also
input to U7 and controls the red LED current magnitude.
Two NPN transistors (Q8 and Q10) act as current sources for the IR and red
LED outputs. Two PNP transistors (Q9 and Q11) act as switches between the
IR and red LED output lines. Transistor Q12 acts as an LED drive current
limiter; it clamps the 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
2
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 filter with a large time constant follows to smooth the signal
and remove noise before amplification.