Nellcor N-180 Service Manual page 23

N-180 Pulse Oximeter Service Manual
for discussion and comparison. The PT-2500 conditions the LED drive voltage
and simulates a saturation of 81% +1 digit (80% to 82%) and a pulse rate of
40 +1 bpm (39 to 41 bpm). Note that pocket tester pulse rate is dependent on
LED switching rate and may differ on other Nellcor pulse oximeter models.
The SAT signal from the pocket tester is coupled to the SAT conditioning
circuits via the sensor input to JP1 (pin 1 and pin 4) on the oximetry module
PCB. After conditioning by current-to-voltage converter U8, the signal has
the following characteristics:
DC offset: approximately +8.5 volt
Frequency: 2710.6 Hz
Modulation: maximum peak-to-peak amplitude, approximately 5 volt
The only additional oxygen saturation input requirement is the wavelength of
the red LED. This number is derived from the RCAL resistor value located in
the sensor or PT-2500. When a sensor is connected to the monitor, the RCAL
resistor connects between JP1 pin 6 and ground to become part of a voltage
divider with R16 on the PCB. Power for this divider is a 2.5 volt reference
developed in the module. This calibration voltage (VCALIN2) is sent to the
analog to digital converter by buffer U21B.
Input Amplifier and Synchronous Detector
Refer to figure 3-4 and figure FO10-1 (sheet 3 of 7 and 4 of 7), The SAT signal
is continuously monitored, and the LED drives controlled to prevent
excessively high LED intensities, or the combination of LED intensity and
background light or energy, from overloading the sensor photodiode.
However, LED intensity must remain as high as possible to ensure optimal
signal-to-noise figure readings. To compensate for excessive light, device U7
* and associated components sample the SAT signal, while the signal is still DC
coupled.
Initially LED intensity is set at maximum (56 milliamps) when power is
applied to the monitor by setting the ON/STDBY switch to ON. If the total
LED energy, or external light energy, or the combined LED energy and
external light energy, is excessive, the DC offset at the output of U8 could be
insufficient to prevent the SAT signal from going into the negative region of
the U8 operating range. To prevent the output of U8 from going negative, U7
monitors the negative excursion of the SAT signal.
Device UTA is a negative peak detector that produces a DC output
proportional to the maximum negative excursion of the SAT signal pulses. If
negative excursions of the saturation signal exceed ~10 volts (indicating that
the current-to-voltage converter stage output is approaching the negative
supply voltage of —15 volts), the microprocessor that is monitoring the output
of U7A (COMPARE }) initiates action to reduce the output of the dual DAC that
controls the LED intensity. The microprocessor may reduce current through
only one of the LEDs, if necessary.
The SAT signal is next coupled through C55 removing the DC offset. The
signal is then coupled to U13B, a band pass filter with a gain of 1, low-
frequency roll-off at 200 Hz, and high- frequency roll-off at 260 kHz. The filter
passes the SAT signal (2710.6 Hz) and effectively removes noise on either side
ofthe SAT signal frequency.