Can the 12-bit ADC built in the MCU be directly used in the forehead thermometer solution?

Can the 12-bit ADC built in the MCU be directly used in the forehead thermometer solution? Answer: Yes, and it can fully meet the relevant national standards for infrared thermometers. During the epidemic, in addition to masks, forehead thermometers for rapid temperature measurement have also become popular, and various excellent forehead thermometer solutions have continued to emerge.

Can the 12-bit ADC built in the MCU be directly used in the forehead thermometer solution? Answer: Yes, and it can fully meet the relevant national standards for infrared thermometers. During the epidemic, in addition to masks, forehead thermometers for rapid temperature measurement have also become popular, and various excellent forehead thermometer solutions have continued to emerge.

We see that the current mainstream solutions are

Mainstream solution 1: 12-bit SAR ADC + external low offset op amp, external low temperature drift reference source.

Mainstream solution 2: 16/24-bit Sigma-delta ADC + built-in PGA, built-in reference source.

Among them, the 16/24-bit Sigma-delta ADC solution has been mass-produced before the epidemic. After the outbreak, its prices skyrocketed due to insufficient production capacity. Engineers began to pay attention to another excellent solution that had also been mass-produced before the epidemic – MCU built-in 12-bit ADC with external low-offset op amp. Since this solution completely adopts general-purpose components, the market supply is sufficient and the price is close to the people. How to choose between these two options? Is it necessary to choose a 16/24 bit ADC?

Engineer A: 16/24 bit is definitely better than 12 bit.

Engineer B: 12-bit can also meet the demand.

Today, the author will also do some technical analysis around this hot topic for your reference.

1. Measurement accuracy

Most of the conventional infrared temperature sensors are the basic structure of NTC + thermopile. The output impedance of this sensor is large, the amplitude is small, and the μV level cannot be directly measured by the ADC. It is usually necessary to add an op amp to amplify the signal and then send it to the ADC for conversion.

The author analyzed about 10 common infrared temperature sensors. When the temperature changes by 1°C, the output change of the sensor is about 70~140μV. After the circuit is amplified by 220 times, the change per 1°C is 15.4-30.8mV.

The effective bits of the built-in 12-bit ADC in general-purpose MCUs can reach 10.5 bits. When the reference voltage is 2.048V, its resolution is 0.5mV, the effective resolution is about 1.41 mV, and the temperature that can be effectively resolved is 0.046-0.092°C. (Please refer to the end of the article for the specific calculation method.) It meets the measurement accuracy requirement of ±0.2°C specified in the national standard “GB-T 21417.1-2008 Medical Infrared Thermometer”.

2. Measuring range

Through further calculation from the above data, when the reference voltage is 2.048V, the convertible voltage range is 0~2.048V, corresponding to the output signal range of the thermopile 0~9.31mV. After conversion, it can measure the minimum temperature range of 66°C and the maximum range of 132°C. If 37°C is set as the midpoint of the range, the measurable temperature range is at least 3~70°C and maximum at -30~103°C. It meets the requirements of the measurement range of 35℃~42℃ specified in the national standard “GB-T 21417.1-2008 Medical Infrared Thermometer”.

3. Summary

To sum up, the 12-bit ADC built in the general-purpose MCU meets the national standards for forehead thermometers in terms of measurement accuracy and measurement range.

Appendix: Effective Resolution Voltage Calculation Formula

Can the 12-bit ADC built in the MCU be directly used in the forehead thermometer solution?

The Links:   5M1270ZF324I5N C070FW02-V2

Author: Yoyokuo