Nellcor NPB-190 Service Manual page 71

S8.4 Input Conditioning
S8.5 Signal Gain
S8.6 Variable Gain Circuits
At initialization of transmission, the LED's 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 CPU.
Input to the SpO analog circuit is the current output of the sensor photodiode. In
2
order to condition the signal current, it is necessary to convert the current to
voltage.
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.
The separated IR and red signals are amplified so that their DC values are within
the range of the A/D converter. Because the received IR and red signals are
typically at different current levels, the signal gain circuits provide independent
amplification for each signal as needed. The gain in these circuits is adjusted by
means of the PWM lines from the CPU.
After the IR and red signals are amplified, they are filtered to improve the
signal-to-noise ratio and clamped to a reference voltage to prevent the combined
AC and DC signal from exceeding an acceptable input voltage from the A/D
converter.
The two variable gain circuits are functionally equivalent. The gain of each
circuit is contingent upon the signal's received level and is controlled to bring
each signal to approximately 3.5 V. Each circuit uses an amplifier and one
switch in the triple SPDT analog multiplexing unit.
Technical Supplement
S-7