Why does a forehead thermometer require a high-precision Sigma-Delta ADC?

Due to the impact of the epidemic, infrared temperature measurement guns are in short supply and a hot topic of concern to electronic engineers. Before the epidemic, there was basically only one mainstream solution for forehead temperature guns, which generally used an analog front end of Sigma-delta ADC with effective bits of 16 bits or more for measurement. However, during the epidemic, due to the overwhelming demand and the certain gap in the early high-precision Sigma-delta ADC analog front-end solutions, general MCU manufacturers and solution providers who did not pay attention to this field also intervened in this field and launched another solution that did not use the Sigma-delta ADC analog front end. The typical solution is a general 32-bit MCU with a 12-bit ADC and an op amp. So what are the advantages and disadvantages of these two solutions? The author tried to analyze them in several aspects.

1. Comparison of accuracy and dynamic range

Since the sensor probe signal is small and the ADC resolution is limited, in order to meet the temperature measurement display resolution of 0.1℃, the sensor probe signal of the infrared thermopile and the ADC are directly added with an op amp for signal amplification. In fact, to achieve a display resolution of 0.1℃, the underlying resolution must be at least 0.05℃, and in order for the subsequent algorithm (filtering and denoising) processing to not cause distortion and affect the accuracy of the measurement, it is best that the underlying resolution is more than 10 times the display resolution, or 0.01℃. Simply to solve the problem of resolution, we can start by increasing the amplifier gain, but the gain cannot be increased arbitrarily because another indicator constrains it, which is the dynamic range. For example, the minimum dynamic range that a forehead temperature gun must meet is 15~35℃. When the ambient temperature changes, the target temperature must be measured in the range of 32~42℃, that is, the dynamic range of 42-15=+27℃ and 32-25=-3℃ must be achieved. This is the minimum requirement. In fact, considering the usage scenario, the ambient temperature range may exceed 15~35℃. For example, the ambient temperature measured outdoors in winter may reach 10℃, and the ambient temperature in tropical areas in summer may reach 40℃. 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 very practical for families with breastfeeding children. At this time, the dynamic range is required to be wider, reaching more than +50℃. Since the signal is amplified 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 that the amplification factor 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 probes are still scarce in the market, with no less than 15 different types of sensors. The signal responses of different manufacturers vary to a certain extent, and the different probe structures lead to large differences in the signal quantities of different sensor probes. The signal quantity significantly affects the measurement resolution and dynamic range, which in turn brings challenges to the design. Table 1 compares and analyzes the differences in measurement resolution and dynamic range of different sensor probes at the same magnification in detail.

Table 1 Comparative analysis of measurement resolution and dynamic range of different sensor probes

So what about the use of high-precision Sigma-Delta ADC? The analysis is given in the 3rd and 5th columns of Table 1. For the sake of comparison, we assume that the reference voltage and sensor probe signal are consistent with the above 12-bit ADC, and assume that the effective bit of the Sigma-Delta ADC is 18 bits. Then when the op amp gain is 32, under different sensor probe signal conditions, the measurement resolution (the second to last row) and dynamic range (the first to last row) can easily meet the requirements, which can be said to be more than enough. Moreover, today's high-precision Sigma-Delta ADC generally integrates a programmable amplifier, and gain adjustment can be achieved through software, making it more flexible to use. In Table 1, let's first look at the solution using 12-bit ADC in columns 2 and 4 (considering nonlinearity, noise, etc., the effective bit is usually only 11 bits, for example, the effective bit of a 12-bit ADC of a domestic manufacturer is only 10.3 bits). The signal of sensor probe 1 in the second column is about 30uV/℃. In order to achieve a minimum resolution of 2 LSB of 0.1℃ (the second to last row), the amplification factor needs to reach 800 times, and the dynamic range (the first to last row) is +/-46.88℃ at this time. It can be said that the dynamic range barely meets the minimum requirement. The signal of sensor probe 2 in the fourth column is about 80uV/℃. If the amplification factor is still 800 at this time, 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 sensor probe 2 to meet the requirements, the amplification factor 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 amplification factor to adapt, which increases the debugging time. And this is only the minimum requirement. As analyzed above, a resolution of 2 LSB will actually bring distortion errors (nonlinear folding of noise) to the measurement in the subsequent filtering and denoising, affecting the accuracy. If we follow the ideal situation and calculate the resolution to 0.1℃ with 10 LSB, then the 12-bit ADC+op amp cannot meet the requirement no matter how it is designed.

2. Others

In addition, another consideration for infrared temperature measurement is the accuracy of NTC in measuring ambient temperature. Since NTC has a large internal resistance (100Kohm level) and changes greatly with temperature (from 200K level to 10K level), it places high requirements on the input impedance of the measurement circuit. If the SAR ADC built into the MCU is used for measurement and sampling directly, the input impedance is generally below 1Mohm, so that the change in the internal resistance of the NTC will greatly reduce the accuracy of the temperature measurement, so a buffer circuit is needed for impedance transformation. High-precision Sigma-delta ADCs generally integrate this buffer circuit, which increases the input impedance to 100Mohm level, reducing the impact of the change in the internal resistance of the NTC on the accuracy of temperature measurement to a negligible level.

In addition, in order to adapt to wide ambient temperature changes, the ADC reference voltage also needs a lower temperature drift coefficient. This low temperature drift reference is generally not integrated in the above-mentioned general MCU and requires additional configuration; on the contrary, high-precision Sigma-delta ADCs will also integrate a low temperature drift reference that meets the requirements (within 50ppm/℃, preferably around 30ppm/℃).

Figure 1: Circuit diagram of a forehead thermometer solution using an MCU with a 12-bit ADC

Figure 2: Circuit diagram of forehead thermometer with high-precision Sigma-delta ADC

Therefore, if you want to implement the forehead thermometer solution using a general MCU with a 12-bit ADC, as shown in Figure 1, you need to add two op amps and a low-temperature drift reference on the periphery, which makes the PCB layout of the signal measurement part more complicated and the design complexity is high. As mentioned earlier, the offset voltage and drift, noise, common-mode rejection ratio, input impedance current and other parameters of the op amp, as well as the bandwidth of the amplified signal need to be carefully considered. Each indicator needs to meet the system requirements, and the indicator requirements are high, otherwise it will significantly affect the accuracy of the final measurement. The peripheral circuit of the signal measurement in the forehead thermometer solution with a high-precision Sigma-delta ADC shown in Figure 2 is relatively simple, and the built-in op amps have basically taken into account the needs of small signal measurement, so there is no need to do analysis and selection, and the design difficulty is low.

3. Conclusion

In summary, the use of a general MCU with a 12-bit ADC to implement the infrared temperature measurement forehead thermometer solution has deficiencies in measurement accuracy, dynamic range, adaptability to sensor probes, anti-interference, peripheral devices, and design complexity. The use of an analog front-end solution with a high-precision Sigma-delta ADC can avoid the above problems. A detailed comparison of the two is shown in Table 2. With the development of domestic semiconductor technology and industry, the analog front end with a high-precision Sigma-delta ADC has been completely localized, and its performance is not inferior to that of its international counterparts, so it can be chosen with confidence.