Block Diagram Of The Spo 2 /Pleth Circuit; Theory Of Operation - Philips M3001A Service Manual

Arterial Oxygen Saturation and Pleth (SpO
This illustration contains an example of a typical wave in SpO
Block Diagram of the SpO
current
Rtype/
Rlambda

Theory of Operation

The signals progress through the circuit as follows:
LED Current Source
This generates the LED current from a constant voltage provided by the power supply. A
bridge consisting of four transistors switches the LED current for driving the red and infrared
LEDs. These switching transistors are controlled by the SpO
Photo Amplifier
The photo amplifier is an active input current to voltage converter. The input signal is filtered
by a low pass filter to eliminate higher frequencies generated, for example, by electro-surgery
units. Then the input current from the photo diode of the sensor is converted to a voltage.
Clipping Detector
A comparator detects clipping of the photo-amplifier signal caused by, for example, ambient
light. The clipping detection is connected directly to the SpO
necessary.
34
/PLETH) Measurement
2
Pleth
Self-Test Signal
Generator
Clipping
Detector
photo
Photo
Bandpass
Amplifier
LED Current
LED
Source
current
RCode
Measurement
2
/PLETH Circuit
2
Digital
Variable
Signal
ADC
Gain
Processor
CPU.
2
2
Introduction to the Instrument
.
CPU
ROM/RAM
ASIC
CPU to generate an INOP if