脉搏血氧检测-Pulse Oximeter

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Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation. Its reading of SpO2 (peripheral oxygen saturation) is not always identical to the reading of SaO2 (arterial oxygen saturation) from arterial blood gas analysis, but the two are reliably enough correlated that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in clinical use.

In its most common (transmissive) application mode, a sensor device is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot. The device passes two wavelengths of light through the body part to a photodetector. It measures the changing absorbance at each of the wavelengths, allowing it to determine the absorbances due to the pulsing arterial blood alone, excluding venous blood, skin, bone, muscle, fat, and (in most cases) nail polish.

Reflectance pulse oximetry may be used as an alternative to transmissive pulse oximetery described above. This method does not require a thin section of the person's body and is therefore well suited to more universal application such as the feet, forehead and chest, but it also has some limitations. Vasodilation and pooling of venous blood in the head due to compromised venous return to the heart, as occurs with congenital cyanotic heart disease patients, or in patients in the Trendelenburg position, can cause a combination of arterial and venous pulsations in the forehead region and lead to spurious SpO2 results.

The (transmissive) pulse oximeter measures blood oxygenation by sensing the infrared and red-light absorption properties of deoxygenated and oxygenated hemoglobin . The oximeter is comprised of a sensing probe that attaches to a patient’s ear lobe, toe or finger and is connected to a data acquisition system for the calculation and display of oxygen saturation level, heart rate and blood flow.
Light sources, typically light-emitting diodes (LEDs), shine visible red and infrared light . Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light . Highly oxygenated hemoglobin allows more red light to pass through and absorbs more infrared light.
The oximeter senses and calculates the amount of light at those wavelengths proportional to the oxygen saturation (or desaturation) of the hemoglobin . The use of light in the absorbency measurement requires the designer to have a true “light-tovoltage” conversion using current as the input signal .
TI方案:
Amplifiers and Processors
The classic resistor-feedback transimpedance amplifier and capacitorfeedback switched integrator are suitable for pulse oximetry applications . In either amplifier configuration, the resulting output voltage is read by an analog-to-digital converter and serialized for the MSP430™ microcontroller or TMS320™ DSP for processing.
Processor selection should be based on signal-processing needs . TI has a wide variety of MSP430 products offering up to 25MIPS performance and extensive mixed-signal integration . For mid-range to high-end systems requiring much higher digital signal performance for enhanced signal conditioning and processing, low-power DSP processors such as C55x™ can be used . These processors offer higher than 100MIPS at very low power .

Low-End Portable Pulse Oximeter
For low-end designs, TI’s highly integrated MSP430FG437 reduces the number of external components . The design of a non-invasive optical pulse oximeter using the MSP430FG437 microcontroller (MCU) consists of a peripheral probe combined with the MCU displaying the oxygen saturation and pulse rate on an LCD glass . In this application, the same sensor is used for heart-rate detection and pulse oximetry.
The probe is placed on a peripheral point of the body, such as a fingertip,an ear lobe or the nose . The probe includes two LEDs — one in the visible red spectrum (660nm) and the other in the infrared spectrum (940nm) . The percentage of oxygen in the body is determined by measuring the intensity from each frequency of light after it is transmitted through the body . Then, the ratio between these two intensities is calculated.
The diagram below demonstrates the implementation of a single-chip, portable pulse oximeter using the ultra-lowpower capability of the MSP430 MCU.


Mid-Range and High-End Applications
For mid-range and high-end applications where higher performance and higher measurement accuracy are necessary, there is a need for higherperformance processors and highprecision analog components that provide lower system power .
For example, several sources of interference such as neon lamps, UV lamps and other light emitters may influence the optical path between LEDs and the photoreceiver, affecting measurement accuracy . There could also be signal distortion caused by motion that occurs while the reading is taken . Sophisticated DSP technology can be applied to eliminate or reduce these effects and extract the vital signal of interest . Often, these DSP technologies require high-sample-rate signalprocessing operations such as demodulation, digital filtering, decimation, and frequency-domain analysis, which can be efficiently mapped to a C55x™ low-power digital signal processor.

Analog front-end (AFE): AFE4490 用于脉冲血氧仪的集成模拟前端
The AFE4490 is a fully-integrated analog front-end (AFE) that is ideally suited
for pulse-oximeter applications . The device consists of a low-noise receiver
channel with a 22-bit analog-to-digital converter (ADC), an LED transmit section,
and diagnostics for sensor and LED fault detection . The AFE4490 is a very
configurable timing controller . This flexibility enables the user to have complete
control of the device timing characteristics . To ease clocking requirements and
provide a low-jitter clock to the AFE4490, an oscillator is also integrated that
functions from an external crystal . The device communicates to an external
microcontroller or host processor using an SPI™ interface .

This AFE4490 is a complete AFE solution packaged in a single, compact QFN-40
package (6mm × 6mm) and is specified over the operating temperature range
of –40°C to +85°C .官方资料：链接

智能手表检测案例：
1.硬件电路
    主要包括反射式光电传感器(包括LED发射器和接收器)，AFE前端，MCU等。
    LED反射式光电传感器，由于手表的小型化要求，作者选择了日本JRC的光反射器NJL5303R，该反射器使用了570nm发光波长的绿光，把绿色LED和光电晶体管组合封装入小型的COB封装，尺寸:1.9*2.6*0.8mm。其中选择570nm发光波长的绿光，与红外光相比其反射率，测量感度更高，大大提高了S/N比。
    AFE前端，TI的AFE4400。
    MCU，由于涉及软件算法，选择美国德州仪器（TI）的MSP430系列MSP430F5528来搭配，MSP430系列单片机是TI1996年开始推向市场的一种16位超低功耗、具有精简指令集（RISC）的混合信号处理器，在水表，电表，燃气表等低功耗产品和医疗仪器有广泛的运用。

2.实验测试结果
    理论上可以通过光电容积法测量获得血氧饱和度值，但是由于该检测方法在手表上运用准确度不够，目前比较少产品有做血氧检测，此处只做心率测试。
    静态测试，人佩带手表静止不动测试,结果如下图所示，测试值比较准确。


    动态测试，用绷带包紧，骑单车测试，和Polar 胸带，MIO迈欧手表做对比测试,结果如下图所示(横轴代表:脚踏车圈数，纵轴代表:心率次数):


厂商：ADI
产品：ADPD142
应用：三星手表