Service manual

Service Manual Technical Overview 111
The SpO
2
board consists of the following subsystems:
• 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
PULSE OXIMETRY BLOCK DIAGRAM
INTERNAL
PREAMP
AC COUPLED AMP
PEAK DETECTOR
INVERTING AMP
(VAR. GAIN STAGE)
PEAK DETECTOR
COMPARE 1
COMPARE 2
ADC
FILTERS
SPLITTERS
SYNCHRONOUS
DETECTOR
CAL
SIGNAL
CPU
LED
DRIVERS
TO
LEDS
CPU
CONTROL
TO SCP
BOARD
IR SIGNAL
RED SIGNAL
DATA/CONTROLS
TO/FROM
CPU
PHOTODIODE
SIGNAL FROM
SENSOR
SWITCH