Welch Allyn PROPAQ CS - Manual (Page 128)

124 Technical overview Welch Allyn Propaq CS Vital Signs Monitor
• 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
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
2
saturation value. The synchronous detector inverts the ambient
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
2
signal level is controlled through the variable gain stage of the inverting amplifier
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
2
signal levels at two points, directly sending their outputs to the
CPU. The CPU monitors these levels as it sets the gain stages of the inverting amplifier
and LED current driver.