Philips M3001A Service Manual page 51

Arterial Oxygen Saturation and Pleth (SpO /PLETH) Measurement
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Bandpass
The bandpass stage contains a bandpass filter for the modulated signals coming in from the
photo-amplifier. This filters out noise outside a passband centred on the modulation
frequency.
Variable Gain
This section amplifies the incoming signals. The gain is set by a digital to analog converter
(DAC) which allows 512 gain settings.
Analog to Digital Converter (ADC)
This is a 12-bit converter. Oversampling is used to get the required resolution. To optimize
the ADC input voltage, the variable gain adapts accordingly to the signal quality.
Digital Signal Processor (DSP)
The DSP demodulates and filters the signal from the ADC, and passes it on to the SpO
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ASIC.
Self-Test Signal Generator
This generates a wave that is similar to a patient signal. It is processed through the complete
circuitry starting at the photo amplifier stage. Just before the processing of the patient signal
begins, the test signal is switched on to check correct functioning of the circuitry.
RCode Measurement Circuit
This circuit measures the coding resistor of the transducer, digitizes it, and sends it to the
SpO CPU.
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Each transducer has coding resistors in the connector, so that it can be identified by this
measurement.
CPU, ROM/RAM and ASIC
The ASIC is the interface between the digital signal processor and the SpO CPU.
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The ASIC also acts as an interface to the ADC and contains all the frequency generators for
the ADC clock, the sampling frequency and the modulation frequencyThe main clock for the
ASIC is provided by a Pierce oscillator circuit external to the chip.
The CPU gets the processed SpO signal from the ASIC, and controls the LED current
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source, the RCode measurement, the variable gain stage, the clipping detection, the power
supply, and the Self-Test circuit. The CPU also detects INOP and error information and
handles communication with the system CPU.
Introduction to the Instrument 35