Mallinckrodt Nellcor N-20 Service Manual page 47

Before the current from the photodetector is converted to voltage, any high frequency noise is filtered
by C40 and R17. 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
analog reference voltage (VREF) of U1D, 5 V, is measured at pin 14 of test point 49.
The same line that controls the on/off pulsing of the LEDs controls U6D, a single-pole-single-throw
(SPST) analog switch. When either of the LEDs are on (the line is low and the switch is closed), U35
is used as a non-inverting amplifier. When the LEDs are both off, U35 is used as an inverting
amplifier. The signal at the output of amplifier U35 is then demultiplexed.
The CPU HSO lines SAMPRED and SAMPIR, which are both active low, control SPST analog
switches U6A and U6B, respectively. Switch U6A is closed to sample the red signal; switch U6B 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.
A simplified N-20 HSO timing diagram is illustrated in Figure 9-10 (at the end of this section).
If the instantaneous average photocurrent (DC offset) is excessive and U1D 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. After about 3 seconds of continuous photocurrent signal, pulse search annunciation will
begin. After about 10 seconds of continuous photocurrent signal, zeros will be displayed.
9.9.7 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.
9.9.7.1 Variable Gain Circuits
The variable gain circuits are illustrated in Figure 9-11.
Technical Supplement
2
9-9