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  • dsPIC33FJ128GP802-Based Pulse Oximeter

  • Created: Aug 24, 2016

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A pulse oximeter is a non-invasive medical device that is used to monitor the blood oxygen saturation (SpO2). A pulse oximeter monitors the oxygen saturation (SpO2) of a human’s blood based on the red light (600-750nm wavelength) and infrared light (850-1000nm wavelength) absorption characteristics of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb). The pulse oximeter flashes the red and infrared lights alternately through a finger to a photodiode. HbO2 absorbs more infrared light and allows more red lights to pass through. On the other hand, Hb absorbs more red lights and allows more infrared light to pass through. The photodiode receives the non-absorbed light from each LED.

The SpO2 probe in this design is a finger-clip type of probe, which integrates a red LED, IR LED, and a photodiode. The LEDs are controlled by the LED driver circuit. The red light and IR light passing through the finger are detected by the signal conditioning circuit and are then fed to a 12-bit ADC module of the microcontroller where %SpO2 can be calculated. A dual SPDT analog switch driven by two PWM signals from the microcontroller turns the red and infrared LEDs ON and OFF alternately. In order to acquire the proper number of ADC samples and have enough time to process the data before the next LED turns on, the LEDs are switched ON/OFF. The signal conditioning circuit is composed of two stages: a transimpedance amplifier and a gain amplifier. The transimpedance amplifier converts the current, generated by the photodiode, to voltage. The signal received from the first stage amplifier passes through a highpass filter, which is designed to reduce the background light interference. The output of the highpass filter is sent to a second stage amplifier with a gain of 22 and a DC offset of 220mV. The values for the amplifier’s gain and DC offset are set to properly place the output signal level of the gain amplifier into the microcontroller’s ADC range.

The output of the analog signal conditioning circuit is connected to the ADC module of the dsPIC33FJ128GP802 DSCs. One ADC sample is taken during each LED’s on-time period, and one ADC sample is taken during both LED’s off-time period. The SpO2 and pulse rate data can be sent to a computer through a UART port with the PICkit serial analyzer.



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