Temperature sensing is widely used in various sensing sensing. Previously, temperature sensing mainly performed temperature measurement by sensing analog components such as RTDs, NTCs or thermocouples. With the rise of emerging application scenarios such as the Internet of Things , In industrial control, consumer equipment, medical equipment, digital temperature sensors have become popular.
In these new application scenarios, sensors are often required to be considered for ease of use and cost in addition to excellent sensing, and digital temperature sensors that do not require additional circuitry to bias the sensing components or determine the measured temperature fit these needs, and digital The temperature sensor does not require further calibration or linear tuning of the detection signal to give repeatable and reliable results.
Traditional RTD, NTC and Thermocouple Analog Temperature Sensing
RTD resistance temperature detector can be said to be the most stable and accurate temperature measurement method. The difficulty lies in the need for external excitation, and the circuit is complex and requires calibration. Basically, it is the first choice in the medium temperature range (<500°C). . Although RTDs cannot measure high temperatures like thermocouples, they have high linearity and good repeatability.
Due to their high sensitivity and high precision, NTC thermistors have been well used for temperature measurements where durability, reliability, and stability are important. Although there are many materials for thermistors, semiconductor resistors such as NTC have the advantages of being easier to process, smaller and lighter than conductors such as metals. In addition, due to its fast response, it is also suitable for small diameter precision equipment. Although the degree of linearization is very low, other advantages are also obvious, and it is undoubtedly a good choice in low-cost and low-temperature applications.
Take type K thermocouples (made of nickel-chromium alloys and nickel-aluminum-gold alloys) for example, which can be used to measure temperatures in excess of 1000°C. Thermocouples are robust, self-powered, and inexpensive, making them ideal for applications with different measurement ranges. But a complete thermocouple temperature measurement system requires cold junction compensation.
High Accuracy of Digital Temperature Sensing
The electronics industry is demanding more and more precision, and temperature detection is no exception. There are many temperature sensing solutions on the market today, and you can see that each has its pros and cons. The digital temperature sensor has relatively high linearity and far more accuracy than other solutions. In the field of digital temperature detection, the realization of high resolution and high precision is no longer a problem.
Digital temperature sensors do not require cold junction temperature compensation or linearization, can provide analog and digital outputs, and are pre-calibrated, which is undoubtedly more convenient than other analog sensing methods in terms of ease of use. For analog temperature sensors, the gain and offset of the ADC need to be calibrated to achieve the desired system accuracy. Since the system temperature accuracy depends heavily on the ADC reference error, the accuracy in the datasheet is not guaranteed. Digital sensors do not require calibration to obtain the accuracy guaranteed in the datasheet. Although the limited temperature range is an unavoidable shortcoming of digital temperature sensors, this shortcoming is also acceptable with the development of increasing accuracy and resolution.
±0.1°C, which is usually required for devices to achieve this high precision in industrial control and medical and health applications. Initially, digital temperature sensors generally provided only a moderate level of measurement accuracy. However, with the development of electronic technology, the leading digital temperature sensing IC manufacturers have been able to improve the accuracy of the device to ±0.1℃, such as TI’s TMP117, ADI’s ADT7422, TE’s TSYSO1 and so on.
The accuracy of 0.1°C can only be maintained within a certain range. For example, the ADR7422 can ensure the accuracy of ±0.1°C within the temperature range of 25°C to 50°C. If it needs to be used in industrial applications, the manufacturer will make some adjustments, which will slightly reduce the accuracy to expand the usable temperature range. Generally, in industrial applications, an accuracy of ±0.2°C will be achieved, so that the temperature range can be expanded to -10°C to 85°C. .
The accuracy of digital temperature sensing ICs can be easily affected. When using extremely accurate reference voltages, the pressure on the die can destroy the accuracy of the sensor, as well as the effects of PCB thermal expansion, soldering, etc. After soldering, the device can still maintain the accuracy of 0.1 ℃ to be called high precision.