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Digital temperature sensing versus analog temperature sensing

Author: Release time:2022-08-03 Source: Font: Big Middle Small View count:27

Temperature sensing is used in a wide variety of sensing applications. Previously, temperature sensing was mainly performed using analog components such as RTD, NTC, or thermocouples to measure temperature. Still, with the rise of emerging application scenarios such as the Internet of Things, digital temperature sensors have become popular in industrial control, consumer devices, and medical devices.


In these new applications, sensors are often required to provide excellent sensing, ease of use, and cost. Digital temperature sensors, which require no additional circuitry to bias the sensing components or determine the measured temperature, fit these needs. They do not require further calibration or linear tuning of the detection signal to produce repeatable and reliable results.


Traditional RTD, NTC, and Thermocouple Analog Temperature Sensing


The difficulty lies in need for external excitation, the RTD resistance temperature detector, arguably the most stable and accurate temperature measurement method. At the same time, the circuit is more complex and requires calibration. The medium temperature range (<500 ℃) is the first option. However, RTD can not measure high temperature like thermocouples but has high linearity and better repeatability.


NTC thermistors have been well used for temperature measurements where durability, reliability, and stability are important due to their high sensitivity and high accuracy. Although there are many materials for thermistors, semiconductor resistors such as NTCs are easier to process, smaller, and lighter than conductors such as metals. In addition, because of its fast response, it is also suitable for small diameter precision devices. Although linearization is very low, other advantages are also obvious, and it is undoubtedly a good choice in applications with low cost and low-temperature range.

NTC,TDK.png


For example, type K thermocouples (made of nickel-chromium and nickel-aluminum-gold alloys) can be used to measure temperatures over 1000°C. The thermocouple is robust and self-powered, which, combined with the low cost, makes it very suitable for applications with different measurement ranges. However, a complete thermocouple temperature measurement system requires cold-end compensation.


High accuracy for digital temperature sensing


The electronics industry demands more accuracy, and temperature sensing is no exception. There are many temperature detection solutions on the market today, and each can seen to have its advantages and disadvantages. With digital temperature sensors, the linearity is relatively high, and the accuracy is far superior to other solutions. In digital temperature detection, high resolution and high accuracy have been achieved without problems.


Digital temperature sensors do not require cold-end temperature compensation or linearization, can provide both analog and digital outputs, and are pre-calibrated, which is certainly more convenient in terms of ease of use than other analog sensing means. With analog temperature sensors, the ADC gain and detuning 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 data-sheet is not guaranteed. Digital sensors need not be calibrated to obtain the accuracy guaranteed in the datasheet. Although the limited temperature range is a shortcoming that cannot be avoided with digital temperature sensors, this shortcoming is acceptable with the development of increasing accuracy and resolution.

TMP117,TI.png


±0.1°C is often required in industrial control, medical and health applications. Initially, digital temperature sensors generally provided only a moderate measurement accuracy. But with the development of electronic technology, the leading digital temperature sensing IC manufacturers have improved the device's accuracy to ± 0.1 ℃, such as TI's TMP117, ADI's ADT7422, TEs TSYSO1, and so on.

ADT7422,ADI.png


The accuracy of 0.1℃ can only be maintained within a certain range, such as the ADR7422 in the temperature range of 25℃ to 50℃, to ensure the accuracy of ±0.1℃. Suppose it needs to be used in industrial applications. In that case, the manufacturer will make some adjustments and will slightly reduce the accuracy to make it available to expand the temperature range generally. Industrial applications will achieve ± 0.2 ℃ accuracies to expand the temperature range from -10 ℃ to 85 ℃.


The accuracy of the digital temperature sensing IC is easily affected by the pressure on the die when using an extremely accurate reference voltage, which can destroy the sensor's accuracy, as well as the effects of PCB thermal expansion, soldering, etc. After welding, the device can still maintain the accuracy of 0.1 ℃ to be called high precision.


Summary


Compared with traditional analog temperature sensors, digital temperature sensors are unique advantages of low cost and direct digital output. Previously relatively poor absolute accuracy has also caught up with the upgrade of the process, allowing digital temperature sensors directly through the digital interface to provide calibrated high precision temperature data. This accurate, low-cost temperature measurement meets the growing number of applications and market demands.


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