Nellcor NPB-40 Service Manual page 47

Technical Supplement
Before the current from the photodetector is converted to voltage, any high
frequency noise is filtered by C9 and R21. The op-amp U5A is used in parallel
with the current-to-voltage converter U5B to cancel any DC voltage,
effectively AC coupling the output of U5B. The average value of the SpO
2
analog reference voltage (VREF) of U5B, 5 V, is measured at test point 6.
U3B, a single-pole-single-throw (SPST) analog switch, is controlled by the
same line that controls the on/off pulsing of the LEDs. When either of the
LEDs are on (the line is low and the switch is closed) U4B is used as a
noninverting amplifier. When the LEDs are both off, U4B is used an
inverting amplifier. The signal at the output of amplifier U4B is then
demultiplexed.
The CPU HSO lines SAMPRED and SAMPIR, which are both active low,
control SPST analog switches in U3A and U3C respectively. Switch U3A is
closed to sample the red signal; switch U3C is closed to sample the IR signal.
The sampling rate for both switches is 10 kHz. Switching is coordinated with
the LED transmission so that the IR and red signals are each sampled twice
per cycle; that is, once when the LED is off (signal inverted), and once when
the LED is on (signal not inverted). The filtering circuit that follows has a
long time constant, thereby acting as an averaging circuit.
If the instantaneous average photocurrent (DC offset) is excessive and U5B
cannot bring it to VREF, the PHOTOI line to the CPU (HSI0) is activated.
This action is an indication of excess ambient light into the photosensor, or
the occurrence of excess noise in the input circuit. It also serves as a warning
to the instrument that the sensor signal may be contaminated and causes the
software to send an error message.
S3.3.3 Signal Gain
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.
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.
S3.3.3.1 Variable Gain Circuits
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 U9.
The gain in each of the circuits is accomplished by means of a feedback loop,
which includes one of the SPDT switches in U9. The PWMs control whether
the feedback loop is connected to ground or to the amplifier output. The
feedback is then averaged by C17/R27 (red), and C20/R26 (IR). The higher the
value of PWM2, the greater the IR gain; the higher the value of PWM1, the
greater the red gain.
S-15