User`s guide

M3000A/M3046A/M3015A/M3016A Service Guide
Introduction to the Instrument 1-31
This illustration contains an example of a typical wave in SpO
2
.
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The signals progress through the circuit as follows:
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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
2
CPU.
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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.
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A comparator detects clipping of the photo-amplifier signal caused by, for example, ambient
light. The clipping detection is connected directly to the SpO2 CPU to generate an INOP if
necessary.
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photo
LED
Rtype/
Rlambda
Self-Test Signal
Generator
Photo
Clipping
Detector
LED Current
Source
RCode
Measurement
Var iabl e
Gain
ADC
Digital
Signal
Processor
CPU
ROM/RAM
ASIC
current
current
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Bandpass
Amplifier