Insufficient analysis of 12bit ADC MCU + op amp forehead temperature gun solution
“Due to the impact of the epidemic, additional guns using infrared temperature measurement technology are in short supply and are also a hot topic that attracts the attention of the majority of Electronic engineers. Around the forehead thermometer, there was basically only one mainstream solution before the epidemic, which generally used the analog front end of the Sigma-delta ADC of 16bits and above for measurement.
Due to the impact of the epidemic, additional guns using infrared temperature measurement technology are in short supply and are also a hot topic that attracts the attention of the majority of electronic engineers. Around the forehead thermometer, there was basically only one mainstream solution before the epidemic, which generally used the analog front end of the Sigma-delta ADC of 16bits and above for measurement. However, during the epidemic, due to the overwhelming demand and the early high-precision Sigma-delta ADC analog front-end solutions, there was a certain gap, so that general-purpose MCU manufacturers and solution providers who did not pay attention to this field also intervened in this field and introduced Sigma-delta not adopted. Another solution for the ADC analog front end is typically a solution that uses a general-purpose 32-bit MCU with a 12-bits ADC and an operational amplifier. So what are the advantages and disadvantages of these two solutions? The author tries to analyze several aspects.
1. Comparison of accuracy and dynamic range
Due to the small signal of the sensor probe and the limited resolution of ADC, in order to meet the temperature measurement Display resolution of 0.1℃, the sensor probe signal of the infrared thermopile and ADC are directly added to the operational amplifier for signal amplification. In fact, to achieve a display resolution of 0.1°C, the bottom layer resolution should be at least 0.05°C. In order for the subsequent algorithm (filtering and denoising) processing to not bring distortion to the accuracy of the measurement, the best bottom layer resolution is 10 of the display resolution. Times above, and 0.01℃. Simply to solve the problem of resolution, you can start by increasing the magnification of the op amp, but the magnification cannot be increased arbitrarily, because another indicator has constraints on it, which is the dynamic range. For example, the minimum dynamic range that the forehead temperature gun must meet is 15~35℃ when the ambient temperature changes, the target temperature needs to be measured in the range of 32~42℃, that is, 42-15=+27℃, 32-25=-3℃. Dynamic Range. This is the minimum requirement. In fact, considering the use scenario, the ambient temperature range may exceed 15~35°C. For example, the ambient temperature measured outdoors in winter may reach 10°C, and in summer, the ambient temperature in tropical areas may reach 40°C. In addition, in order to increase the usage scenarios of the forehead thermometer and increase the added value, the object temperature mode is generally set, such as measuring water temperature and milk temperature, which is especially practical for families with breastfeeding children. At this time, the dynamic range is required to be wider, and it will reach +50°C or more. Since the signal is amplified in order to improve the resolution, and the dynamic range is reduced after the signal is amplified, these two indicators need to be considered and carefully designed. In addition to considering the amplification factor that will affect the measurement performance, the op amp itself must also consider its offset voltage and its drift, noise, common-mode rejection ratio, input impedance current and other parameters, otherwise it will significantly affect the final measurement effect.
In addition, infrared temperature measuring probes are still scarce on the market, and there are no less than 15 types of sensors. There are certain differences in the signal response of each company. Combined with the difference in probe structure, the signal amount of different sensor probes is quite different. . The semaphore significantly affects the measurement resolution and dynamic range indicators, which in turn brings challenges to the design. Table 1 compares and analyzes the difference in measurement resolution and dynamic range of different sensor probes at the same magnification.
Table 1 Comparative analysis of measurement resolution and dynamic range with different sensor probes
In Table 1, let’s first look at the solution using 12bits ADC in column 2 and column 4 (considering nonlinearity, noise, etc., usually effective bits are only 11 bits, for example, a domestic manufacturer’s 12bit ADC has only 10.3 bits effective bits). The signal of the sensor probe 1 in the second column is about 30uV/℃. In order to achieve a minimum of 0.1℃, there is a resolution of 2 LSBs (the second to last row), and the magnification needs to reach 800 times, and the dynamic range at this time (the first to last row) ) Is +/-46.88℃, it can be said that the dynamic range barely meets the minimum requirements. The signal of the sensor probe 2 in the fourth column is about 80uV/℃. If the magnification is still 800, the resolution can be increased to 5.2 LSB, but the dynamic range is only +/-17.6℃, which does not meet the requirements. In order for the sensor probe 2 to meet the requirements, the magnification must be reduced to about 400. Therefore, it can be said that in order to adapt to different sensor probes, the 12bits ADC solution needs to change the magnification to adapt, which increases the debugging time. And this is only the minimum requirement. The above analysis said that a resolution of 2 LSBs will actually bring distortion errors (non-linear folding of noise) to the measurement in the subsequent filtering and denoising, which will affect the accuracy. If, according to a more ideal situation, the resolution reaches 0.1°C, there is a resolution calculation of 10 LSBs, then the 12bits ADC+ op amp is not designed to meet the requirements anyway.
So what about the use of 24bits Sigma-delta ADC? The analysis is given in columns 3 and 5 in Table 1. In order to facilitate comparison, we assume that the reference voltage and sensor probe signal are the same as the above 12bits ADC. At this time, the effective bit of the 24bits Sigma-delta ADC is 18bits. Then when the op amp magnification is 32, the signal volume of different sensor probes , The measurement resolution (second line from the bottom) and dynamic range (the first line from the bottom) can easily meet the requirements, which can be said to be easy. Moreover, the analog front-end chip that generally adopts 24bits Sigma-delta ADC generally integrates an operational amplifier with 32 times magnification.
The 24bits Sigma-delta ADC of the analog front end generally adopts differential mode, and its integrated operational amplifier is generally fully differential. In contrast, the MCU+ operational amplifier mode of 12 bit ADC adopts single-ended mode, which is anti-RS interference The performance is worse than the former.
In addition, another consideration for infrared temperature measurement is the accuracy of NTC’s measurement of ambient temperature. Because NTC has a large internal resistance (100Kohm level) and changes greatly with temperature (from 200K level to 10K level), higher requirements are put forward for the input impedance of the measurement circuit. If the ADC is used for measurement and sampling directly, the input impedance is generally below 1Mohm, so that the change of the NTC internal resistance will greatly reduce the accuracy of temperature measurement, so it is necessary to add a Buffer circuit for impedance conversion. The analog front end using 24bits Sigma-delta ADC will integrate this Buffer circuit to increase the input impedance to the 100Mohm level, so that the impact of the NTC internal resistance change on the temperature measurement accuracy is reduced to a negligible degree.
In addition, in order to adapt to a wide range of environmental temperature changes, the ADC reference voltage also requires a lower temperature drift coefficient. This low-temperature drift reference is generally not integrated in the above-mentioned general-purpose MCU and requires additional configuration; on the contrary, the analog front end of the 24bits Sigma-delta ADC will integrate a low-temperature drift reference that meets the requirements (within 50ppm/℃, preferably 30ppm/℃) about).
Figure 1: Circuit block diagram of the forehead temperature gun solution using MCU with 12bits ADC
Figure 2: Circuit block diagram of the forehead temperature gun solution with 24bits SDADC AFE
Therefore, if a general-purpose MCU with 12bits ADC is used to realize the forehead temperature gun solution, as shown in Figure 1, two operational amplifiers and a low-temperature drift reference need to be added to the periphery, which makes the PCB layout and wiring of the signal measurement part more complicated and the design complexity high. As mentioned earlier, the offset voltage and its drift, noise, common mode rejection ratio, input impedance current and other parameters of the op amp, as well as the amplified signal bandwidth, need to be carefully considered. Each indicator needs to meet the system requirements and the indicator requirements High, otherwise it will significantly affect the accuracy of the final measurement. The peripheral circuit of signal measurement in the forehead thermometer with 24bits Sigma-delta ADC AFE shown in Figure 2 is relatively simple. The built-in operational amplifier has basically taken into account the needs of small signal measurement, and there is no need for analysis and selection. The difficulty is low.
To sum up, the general MCU with 12bits ADC is used to realize the infrared temperature measurement forehead gun program, which exists in measurement accuracy, dynamic range, adaptability to sensor probes, anti-interference, peripheral components, and design complexity. Insufficient; and the use of analog front-end solutions with 24bits Sigma-delta ADC can avoid the above-mentioned problems. The detailed comparison of the two is shown in Table 2. With the development of domestic semiconductor technology and industry, the current analog front end with 24bits Sigma-delta ADC has been localized, and the performance is not inferior to international counterparts, so you can choose with confidence.