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Principle and comprehensive solution of pulse oximeter
The pulse oximeter uses non-invasive techniques to measure the oxygen content in blood oxygen. The more accurate measurement object is called blood oxygen saturation, SpO2.
Today, I will mainly introduce the pulse oximeter to you. One is to introduce the working principle of the pulse oximeter; in addition, the challenges faced in accurate measurement also include device selection.
I have done a monitoring oximeter and a nail oximeter. It is clear that the monitor has a module with blood oxygen, and there are smaller nail-type ones, which are biased towards home and personal use. The model is different. What is a pulse oximeter? Let me talk about oxygen first. Everyone knows that people need oxygen to live. How can oxygen survive? The red blood cells in the blood are supplied to the capillaries by the red blood cells through the arteries. The pulse oximeter measures the saturation of blood oxygen. How do we measure it with SpO2? Is it the ratio of actual oxygen content to total oxygen saturation? Just now I introduced how oxygen is delivered to various organs and capillaries of the human body, which rely on red blood cells. The red blood cells are actually very small, very, very small. We have listed the size here, which is 6-8 microns in diameter and 2 microns in thickness. The life span of each cell is 100-120 days, and it will be recovered and regenerated. These cells are produced by the bone marrow, so it takes 7 days for each cell to be produced. It is not directly related to the electron, but it is still a bit interesting to talk about it if you don't understand it. The production of cells in adults is 4 million per second. We are stimulated by hormone EPO. Athletes will be injected with these hormones. The more red blood cells, the stronger the ability to carry oxygen. The process of competition is unfair to those who do not have injections. Yes, so the story in sports hormones will be heard. Every adult has 20-30 trillion red blood cells, and men have 20% more than women. The main function is to send oxygen from the lungs to the organs of the human body to ensure the work of each organ. The circulation of each red blood cell in the body goes out of the lungs and then back. It takes about 20 seconds.
What is a pulse oximeter? In this, we must first understand the definition of HbO2. In the lungs, red blood cells are like this, with hemoglobin attached to them, the symbol is Hb, one is oxyhemoglobin, which is HbO2, and reduced hemoglobin. When the arteries pass through the capillaries and return to the cervical veins, the oxygen molecules fall off. This is my red blood cell, with hemoglobin, which contains four oxygen molecules, which are saturated hemoglobin molecules. The typical value of blood oxygen saturation is healthy people. This value is 90% -100% relatively normal, but in many cases it drops to 60%, which depends on many factors. The most important factor is the poor blood supply of the patient ’s body. The reading of HbO2 will drop.
The principle of measurement, we took the simplest example, which is also the most common situation. Most oximeters in the world are based on this principle, only through red light and infrared light, the two wavelength values are the most Commonly, the blue arrow refers to this line, reducing hemoglobin, that is, when hemoglobin does not carry oxygen molecules, the absorption of red light is relatively long, and the higher the vertical axis, the stronger the absorption intensity. The absorption length of infrared light is relatively weak, which is the wavelength, so the horizontal axis is the wavelength, and the vertical axis is the absorption length. Conversely, in addition to reducing hemoglobin, we also have oxyhemoglobin. This is indicated by the red arrow, that is, those with hemoglobin and oxygen molecules, the absorption of red light is weaker, and the absorption of infrared light is more Strong, we say that red light is 660 nanometers and infrared light is 610 nanometers, which is used in blood oxygen measurement. The difference between reduced hemoglobin and oxyhemoglobin for different light is the most basic data for measuring blood oxygen saturation. Here I emphasize that the most common is the two wavelengths. In fact, to achieve higher accuracy, in addition to the two wavelengths, it needs to be increased, even up to 8 wavelengths. The main reason is that the human hemoglobin in addition to reducing hemoglobin and oxygen In addition to hemoglobin, there are other hemoglobins. We often see carboxyhemoglobin. More wavelengths are conducive to better accuracy. 8 wavelengths already have many products in the world, of course it is high-end.
The basic measurement principle is as follows. This is a finger. The measurement of blood oxygen content and saturation is the most measured in the finger, and it can also be on the toes and ears. This is the most common place to measure blood oxygen. The principle is to emit with red light and infrared light, these two should be very close, to ensure that they are basically very close to the finger, can ensure the accuracy of detection. This itself is also a challenge to sensor technology and LED. The red light and infrared light work separately. When the red light works, the infrared light is turned off. When the photodiode is the same, it can ensure that the work between the red light and infrared light is very interfering. The distance was just very close. Ensure the information obtained in the same body tissue structure.
Here, the measurement points mainly include fingers, toes, and earlobes. We are talking about the working mode. The luminous body is here, and the photoelectric conversion is on the other side, which is penetrating. When designing a pattern, not only the sensor, but also the challenge to the pattern. Just now I talked about capillaries, this is arteries, this is venous return, just put the work of the circuit sensor and blood together, so why is it called energy when your blood is flowing? Because of power, when you have pulse, go When passing the capillaries, the oxygen molecules are dropped, and the returned hemoglobin is here. Therefore, when detecting, the red light and infrared light are the same light. On the one hand, it will be attenuated by the tissue structure, as well as the venous blood, and the arteries. The arteries have two parts, one part is in stock, It is inside. Due to the pulsation of the heart, there is a part of pulsating hemoglobin, which will increase.
Putting the human blood structure and sensors together, we see how they work. Because of the heartbeat, it is pulsating, and the light intensity transmitted through the finger will continue to increase. We said the volume descriptor. I just talked about the algorithm, which is the organizational structure. The meat, bones and skin of the finger will cause it to absorb. The venous blood will absorb it, and the arteries will also absorb it, but the arteries are divided into stock and pulsatile. If the measured data is such a graph, all volume descriptors. I just introduced the basic principle of blood oxygen measurement and how the human blood carries oxygen atoms to power the human body.
Let's talk about the analysis of LEDs and rectifier diodes. Let's look at the circuit design and how to choose components. This picture is relatively simple, this is the working voltage of LED, this is the output current. Normally, a relatively large current is required to produce relatively strong light. When the VF is relatively small, that is, when the curve is steep, the efficiency is relatively high, and the power consumption is relatively low. The LED is easier to help you achieve the design energy of the system. This is just A concept to choose the basic considerations of LED. Here is an example of the manufacturer's model to see the characteristics of LED light. The single light emitted by the LED is used to characterize the characteristics of the LED. This model is here, and the other model is more biased. The peak transmission wavelength is specified under optimal conditions, and this value is used as the operating point. The half bandwidth of the spectrum is very narrow. Under normal circumstances, the current must be very stable. If is very stable, to avoid wavelength shift, and the temperature is relatively stable, otherwise the wavelength will cause a large shift. In this design, not only is the choice of LED, but also the system design considers heat dissipation.
Transimpedance amplifier on the receiving side
Speaking of LEDs just now, the photodiode will be involved later. The photodiode will have zero bias or negative pressure. To ensure a good TIA, it is a cross-group amplifier to ensure that it can meet the photodiode specifications. We will do it later. Some analysis to introduce how to choose TIA to ensure that the photodiode index is reflected in the system. This is the most common structure of a transimpedance amplifier.
Let me talk about the current and voltage characteristics of photodiodes. I just introduced that the general situation is negative pressure. After negative pressure is applied, P0, P1, and P2 are different luminous fluxes. The working curve is the horizontal axis is voltage, and the vertical axis is current. So under different bias voltages, the working current is different. This shows that there is a change in the amount of current under the negative pressure of P0-P1. What does this current have to do with it? The luminous flux, one is the luminous flux, and the other is the conversion rate of light to electricity, you can calculate the number. This is the simplest principle of the photodiode. We gave the photodiode circuit model. This model is like a triode we learned at the university. It was disassembled and turned into a resistive current. There are many parameters in it, IL It is the current generated by the incident light, ID is the dark current, CJ is the junction capacitance, RSH is the parallel resistance, and there are series resistance, parallel resistance current, the voltage on the diode, the output current, and the output voltage. Why do we divide into the equivalent circuit Model, you need to do the noise analysis later, so that everyone has the opportunity to analyze how the noise is contributed to the entire system, which noise is very important to be considered, in the selection and design. After the noise is added to TIA, this is the equivalent circuit of the photodiode. This is the equivalent resistance and electrofusion. The op amp itself brings noise. We analyze the three parts of noise more and calculate its contribution to the system. Can understand which part of the noise should be particularly careful. There are of course voltage and current noises. What are the commonalities that voltage and current noises bring to the system? We will introduce them on the graph. Which index can become very important.
We have listed the engineering of noise calculation, assuming that the op amp resistance is very large, assuming that the parallel resistance is much larger than the series resistance, which is generally the case. We divide the noise of the photodiode. The noise of the photodiode consists of two parts. One is shot noise, which is divided into two parts. This is the noise structure of the photodiode. This announcement looks tedious, and is listed here as a reference. The operational amplifier noise is composed of voltage noise and current noise. The noise composition is the energy superposition, so it is the square root. The last one is the thermal noise of the feedback resistor. The thermal noise of the feedback resistor is 4KbTBRF, which is already listed here. What are the components of these noises? In order to ensure a good cooperation between the photodiode and TIA, we set two conditions. The first condition is the noise to noise ratio, which is how much the total noise of the photodiode and TIA. The noise of the photodiode is currently 1.25uv, which we will use below to ensure that the NNR is large enough. We choose some amplifiers for analysis. The voltage noise and current noise of the amplifier, and finally the noise of the entire TIA are analyzed. You will see that this is an upward movement. The more the voltage noise is, the more the current noise is. Big. So the photodiode is getting bigger and bigger compared to NNR, but the bigger the better. What does this mean? The importance of current noise, relative voltage noise in this case, contributes more to the noise.
We say that photodiodes work well with TIA and choose a good TIA. The second is that the signal-to-noise ratio is large enough. You need to do the calculation. The noise of the TIA and the output voltage should be large enough. We have calculated the output voltage. The value, 14.95v, is also the model just now, voltage noise, current noise, here is all the noise, here is the SNR, the equivalent resolution is 16-18 bits, our calculation of the system is like this.
This is a test we give to a third party. Compared with the same program, Low perfusion is very low. Everyone knows the standard of blood oxygenation is 0.3, but the lower the number, the better. This indicator is very weak when the patient's blood oxygen sign is very weak. Very important. There are two points to ensure this index, one is the algorithm, and the other is to ensure that the noise of the front-end circuit is sufficiently suppressed, including the ADC has enough good analog performance. On the last page, we show a picture, which is an interference to exercise. Except for low perfusion, blood oxygen detection cannot guarantee that it will not move at all. Human movement will bring artificial movement interference, so in your In the program, it is very important to have a function. You must be able to detect motion interference, remove the interference, and output the blood oxygen detection value at a sufficient speed. This is a third-party program, which is the same type of product. compare with.
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