Welch Allyn PROPAQ CS Service Manual page 128

124 Technical overview
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The CPU detects the presence of a transducer plugged into the D-type connector. Switch
S1 routes the signal from the preamplifier to the ac-coupled amplifier. The input signal is
ac-coupled to remove any dc offset present. The result is a pulse waveform representing
light levels from three sources: red LED, infrared (IR) LED, and ambient. An inverting
amplifier with a variable gain stage scales the signal to allow processing the best available
pulse oximetry information.
Because the CPU controls the LEDS in the sensor, it knows which pulse level of the input
waveform represents which wavelength of light. When the red LED is activated, the
waveform level indicates red light plus ambient light. When the IR LED is activated, the
waveform level indicates IR light plus ambient light. When neither LED is activated, the
waveform level indicates ambient light. Only the detected red and IR light levels are used
to determine the SpO
light signal so that the ambient light signal can be subtracted from the red plus ambient
light signal and the IR plus ambient light signal.
The pulses are filtered through 5-pole filter networks to remove the signal components
above about 20Hz. The resulting signals indicate the detected RED and IR light levels and
can then be digitized. The data from the ADC is processed by the CPU and then sent to
the SCP Board for transmission to the monitor's CPU.
SpO signal level is controlled through the variable gain stage of the inverting amplifier
2
and the output current of the LED current driver. By controlling both signal gain and
current drive, the CPU can maintain the best possible signal level for processing.
Amplifiers sense the SpO
CPU. The CPU monitors these levels as it sets the gain stages of the inverting amplifier
and LED current driver.
internal preamplifier and switch
ac-coupled amplifier
inverting amplifier with variable gain stage
synchronous detector
splitter
5-pole low pass filters
16-bit ADC
internal calibration amplifier
CPU/Control
LED current drivers
compare amplifiers
saturation value. The synchronous detector inverts the ambient
2
signal levels at two points, directly sending their outputs to the
2
Welch Allyn Propaq CS Vital Signs Monitor