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Power Supplies
The SpO
2
PCBA generates several power supplies for its internal
use. These DC voltages are generated by two DC/DC converters,
which operate off the +9 V battery supply from the transmitter
PCBA. The DC/DC converters operate from 5.5 to 10 Volts and
p3 V (±0.15 V) and +5 V (±0.15 V). From these two voltages
a +3 VA (analog supply), +3 VL (LED drive supply), +5 VA (analog
supply), and +2.5 VR (reference supply) are produced.
•
The +3 is used for the CPU, RAM, FLASH ROM, and other logic.
•
The +5 V is used for the LED drive and switching logic.
•
The +3 VA is used for the 3 volt analog parts the LED drive DAC
and the CPU ADC.
•
The +5 VA is used for most of the analog including all op amps
and the red and IR ADCs.
•
The +3 VL is used for driving the red and IR LEDs in the SpO2
sensor.
•
The +2.5 VR reference is used by the CPU ADC, the red and IR
ADCs, and the LED drive DAC.
•
The transmit DAC is powered directly from the transmitter PCBA
+5 volts (+5 IN).
There are four grounds on the PCBA:
•
Power ground (PGND)
•
Digital ground (DGND)
•
Analog ground (AGND)
•
LED ground (LGND)
All grounds are connected at one point on the PCBA at the power
supply. PGND is used mainly in the power supply. DGND is used for
the CPU, RAM, FLASH ROM, and logic. AGND is used for the analog
circuitry. LGND is used for the Red and IR LEDs in the SpO
2
sensor.
Pulse Oximetry (SpO
2
)
Measurement
The pulse oximetry measurement uses a pulse oximetry sensor that
is connected to the patient. A disposable, reusable tape on band, or
clip-on finger sensor, can be used. The measurement function is
performed by controlling the flashing of the red and IR LEDs. The
LEDs are located within the patient sensor and illuminate the
patient’s finger. They operate in conjunction with a photodetector
located within the patient sensor that detects the varying levels of
transmissivity between the two different colors of light. Software
controls the LED drive level.
During each subsequent flashing of the red and IR LEDs, the analog
current output from the photodetector is converted to an analog
voltage and amplified. It is further amplified through a
programmable gain amplifier, conditioned to have the ambient light
portion of the signal subtracted, and then finally sampled and held
on separate red and IR output pins. Software controls the gain
setting. The separate red and IR analog signals are then converted
to a digital format by two high precision analog-to-digital
converters. Data averaging and error processing is performed.
