SLAA274B – November 2005 – Revised February 2012
A Single-Chip Pulsoximeter Design Using the MSP430
Vincent Chan, Steve Underwood ................................................................................. MSP430 Products ABSTRACT This application report discusses the design of non-invasive optical plethysmography also called as pulsoximeter using the MSP430FG437microcontroller (MCU). The pulsoximeter consists of a peripheral probe combined with the MCU displaying the oxygen saturation and pulse rate on a LCD glass. The same sensor is used for both heart-rate detection and pulsoximetering in this application. The probe is placed on a peripheral point of the body such as a finger tip, ear lobe or the nose. The probe includes two light emitting diodes (LEDs),one in the visible red spectrum (660 nm) and the other in the infrared spectrum (940 nm). The percentage of oxygen in the body is worked by measuring the intensity from each frequency of light after it transmits through the body and then calculating the ratio between these two intensities. This application report uses the MOD-PULSE Pulseoxmeter and Heart-Rate Monitor Using the MSP430FG439development board by OLIMEX Ltd (http://www.olimex.com/dev/mod-pulse.html). A revised version of this application is described in the application report Revised Pulsoximeter Design Using the MSP430 (SLAA458).
The Pulsoximeter is a medical instrument for monitoring the blood oxygenation of a patient. By measuring the oxygen level and heart rate, the instrument can sound an alarm ifthese drop below a pre-determined level. This type of monitoring is especially useful for new born infants and during surgery. This application report demonstrates the implementation of a single chip portable pulsoximeter using the ultra low power capability of the MSP430. Because of the high level of analog integration, the external components can be kept to a minimum. Furthermore, by keeping ONtime to a minimum and power cycling the two light sources, power consumption is reduced.
Theory of Operation
In a pulsoximeter, the calculation of the level of oxygenation of blood (SaO2) is based on measuring the intensity of light that has been attenuated by body tissue. SaO2 is defined as the ratio of the level oxygenated Hemoglobin over the total Hemoglobin level (oxygenated anddepleted): HbO 2 SaO 2 + Total Hemoglobin
Body tissue absorbs different amounts of light depending on the oxygenation level of blood that is passing through it. This characteristic is non-linear. Two different wavelengths of light are used, each is turned on and measured alternately. By using two different wavelengths, the mathematical complexity of measurement can be reduced. log(l ac)l1 R +SaO 2 a R log(l ac)l2 (2) Where λ1 and λ2 represents the two different wavelengths of light used. There are a DC and an AC component in the measurements. It is assumed that the DC component is a result of the absorption by the body tissue and veins. The AC component is the result of the absorption by the arteries.
SLAA274B – November 2005 – Revised February 2012 Submit Documentation Feedback ASingle-Chip Pulsoximeter Design Using the MSP430 1
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In practice, the relationship between SaO2 and R is not as linear as indicated by the above formula. For this reason a look up table is used to provide a correct reading.
RS232 Oxi Lvi Pulse Rate LoBatt Heart RateCalculation SaO2 = Fn [ RMS(ir)/ RSM(vr)] De− MUX Zero Crossing Infra Red Samples Only Infra Red/ Normal Red Band Pass Filter DC Tracking G2
Brightness Range Control Infra Red/ Normal Red DAC12_0
Probe Connector Red LED Gain InfraRed LED Gain Cable Red LED ON/OFF InfraRed LED ON/OFF PIN Diode PIN Diode InfraRed LED Red LED
Pseudo Analog Ground G2 MUX ADC12...