Banner Sensors – the development trend of smart temperature sensors

The three foundations of modern information technology are information collection (i.e., sensor technology), information transmission (communication technology), and information processing (computer technology). Sensors are cutting-edge products of information technology, especially temperature sensors are widely used in industrial and agricultural production, scientific research and life, and the number ranks first among all kinds of sensors. In the past 100 years, the development of temperature sensors has roughly gone through the following three stages; (1) Traditional discrete temperature sensor (including sensitive components); (2) Analog integrated temperature sensor/controller; (3) Intelligent temperature sensor. At present, the new temperature sensor in the world is developing from analog to digital, from integrated to intelligent and networked

Banner Sensors – the development trend of smart temperature sensors

1 Product classification with integrated temperature sensor

1.1 Analog integrated temperature sensor

Integrated sensors are made using a silicon semiconductor integration process, so they are also called silicon sensors or monolithic integrated temperature sensors. The analog integrated temperature sensor was introduced in the 80s of the 20th century, which is a special IC that integrates the temperature sensor on a chip and can complete the function of temperature measurement and analog signal output. The main characteristics of the analog integrated temperature sensor are single function (only measuring temperature), small temperature measurement error, low price, fast response speed, long transmission distance, small size, micro power consumption, etc., suitable for long-distance temperature measurement and temperature control, no need for nonlinear calibration, and simple peripheral circuit. It is currently the most widely used integrated sensor at home and abroad, and typical products include AD590, AD592, TMP17, LM135, etc.

1.2 Analog integrated temperature controller

The analog integrated temperature controller mainly includes temperature control switches, programmable temperature controllers, and typical products are LM56, AD22105 and MAX6509. Some enhanced integrated temperature controllers, such as the TC652/653, also include an A/D converter as well as a cured program, which has some similarities with smart temperature sensors. However, it is a system of its own and does not operate under the control of a microprocessor, which is the main difference between the two.

1.3 Intelligent temperature sensor

Smart temperature sensors (also known as digital temperature sensors) came out in the mid-90s of the 20th century. It is the crystallization of microelectronics, computer technology and automatic testing technology (ATE). At present, a variety of intelligent temperature sensor series products have been developed internationally. Smart temperature sensors all contain a temperature sensor, an A/D converter, a signal processor, memory (or registers), and interface circuitry. Some products also include multiple selectors, central controller (CPU), random access memory (RAM), and read-only memory (ROM). The intelligent temperature sensor is characterized by the ability to output temperature data and related temperature control quantities, and is suitable for various microcontrollers (MCUs); And it is based on the hardware through the software to achieve the test function, its degree of intelligence also depends on the level of software development.

2. The new trend in the development of intelligent temperature sensors

After entering the 21st century, intelligent temperature sensors are developing rapidly in the direction of high precision, multi-function, bus standardization, high reliability and security, the development of virtual sensors and network sensors, and the development of monolithic temperature measurement systems.

2.1 Improve the accuracy and resolution of temperature measurement

In the mid-90s of the 20th century, the first intelligent temperature sensor was introduced, using an 8-bit A/D converter, which has a low temperature measurement accuracy and a resolution of only 1°C. At present, a variety of high-precision, high-resolution intelligent temperature sensors have been launched abroad, using 9-12-bit A/D converters, and the resolution can generally reach 0.5-°C. The DS1624 high-resolution intelligent temperature sensor, newly developed by United States DALLAS Semiconductor, can output 13-bit binary data with a resolution of up to °C and a temperature measurement accuracy of ±0.2°C. In order to improve the conversion rate of multi-channel intelligent temperature sensors, some chips use high-speed successive approximation A/D converters. Taking the AD7817 5-channel intelligent temperature sensor as an example, the conversion time for the local sensor and each remote sensor is only 27 μs and 9 μs, respectively.

2.2 Add test function

The testing capabilities of the new smart temperature sensors are also being continuously enhanced. For example, the DS1629 single-wire smart temperature sensor adds a real-time calendar clock (RTC) to complete the functionality. The DS1624 also adds a memory function that utilizes 256 bytes of E2PROM memory inside the chip to store the user's short message. In addition, intelligent temperature sensors are developing from single-channel to multi-channel, which creates good conditions for the research and development of multi-channel temperature measurement and control systems.

The intelligent temperature sensors have a variety of working modes to choose from, mainly including single conversion mode, continuous conversion mode, standby mode, and some also add low temperature limit extension mode, which is very easy to operate. For some smart temperature sensors, the host computer (external microprocessor or microcontroller) can also set its A/D slew rate (typical MAX6654), resolution, and maximum slew time (typical DS1624) via the corresponding registers.

The intelligent temperature controller is developed on the basis of the intelligent temperature sensor. Typical products are DS1620, DS1623, TCN75, LM76, MAX6625. The intelligent temperature controller is adapted to various microcontrollers to form an intelligent temperature control system; They can also work separately from the microcontroller to form a thermostat on their own.

2.3 Standardization and normalization of bus technology

At present, the bus technology of intelligent temperature sensors has also been standardized and standardized, and the buses used mainly include single-wire (-Wire) bus, I2C bus, SMBus bus and spI bus.

2.4 Reliability and safety design

Most of the traditional A/D converters use integral or successive comparison conversion techniques, which have low noise tolerance and poor ability to suppress aliasing noise and quantize noise. New smart temperature sensors (e.g., TMP03/04, LM74, LM83) generally use high-performance sigma-delta A/D converters, which can convert analog signals into digital signals with high sampling rates and low sampling resolution, and then use oversampling, noise shaping and digital filtering techniques to improve effective resolution. The sigma-delta A/D converter not only filters out quantization noise, but also requires low accuracy of peripheral components. In order to avoid malfunction when the temperature control system is disturbed by noise, a programmable "fAultqueue" counter is set inside the AD7416/7417/7817, LM75/76, MAX6625/6626 and other intelligent temperature sensors, which is designed to set the number of times the temperature value to be measured exceeds the upper and lower limits. The interrupt terminal can only be triggered when the measured temperature continuously exceeds the upper limit or below the lower limit and reaches or exceeds the set number of times n (n=1-4). If the number of faults does not meet the above conditions or the faults do not occur continuously, the fault counter is reset without triggering the interrupt terminal. This means that if n=3 is assumed, the occasional noise disturbance of one or two times will not affect the normal operation of the temperature control system.

The LM76 smart temperature sensor adds a temperature window comparator, making it ideal for designing a temperature control system that complies with the ACPI (AdvAnced Configuration And Power InterfAce) specification. This system has complete overtemperature protection and can be used to monitor the temperature of the CPU and main circuitry in laptops and servers. The maximum operating temperature that microprocessors can withstand is tH, which is generally 75°C for desktop computers and 100°C for dedicated CPUs for high-end laptops. Once the temperature of the CPU or the main circuit exceeds the set upper and lower limits, the INT terminal immediately interrupts the host, and then sends a signal through the power controller to quickly shut down the main power supply to protect it. In addition, when the temperature exceeds the limit temperature of the CPU, the severe overtemperature alarm output (T_CRIT_A) can also directly shut down the main power supply, and the terminal can also cut off the main power supply through an independent hardware shutdown circuit to prevent the main power control failure. The above-mentioned triple security protection measures have become a new concept in the design of temperature control systems in the world.

To prevent damage to the chip due to electrostatic discharge (ESD) of the human body. Some smart temperature sensors also add ESD protection circuits, which can generally withstand electrostatic discharge voltages of 1000-4000V. Usually the human body is equivalent to a circuit model made of 100PF capacitors and ohmic resistors in series, when the human body is discharged, the serial interface terminal, interrupt/comparator signal output terminal, and address input terminal of the TCN75 intelligent temperature sensor can withstand an electrostatic discharge voltage of 1000V. The LM83 intelligent temperature sensor can withstand an electrostatic discharge voltage of 4000V.

Newly developed smart temperature sensors (e.g., MAX6654, LM83) also add sensor fault detection, which can automatically detect open or short circuit faults in external transistor temperature sensors (also known as remote sensors). MAX6654 also features a PArAsitic ResistAnce CAncellAtion (prc) mode that compensates for temperature measurement errors caused by the lead impedance of the remote sensor, even when the lead impedance reaches 100 ohms. Remote sensor leads are available in plain twisted pair or shielded twisted pair.

2.5 Virtual Temperature Sensors and Network Temperature Sensors

(1) Virtual sensors

Virtual sensors are developed based on sensor hardware and computer platforms and through software. The software can be used to calibrate and calibrate the sensor to achieve the best performance indicators. Recently, United States B&K has developed a software-based TEDS-type virtual sensor, the main feature of which is that each sensor has a unique product serial number and comes with a floppy disk that stores the relevant data for the calibration of the sensor. When in use, the sensor is connected to the computer through the data collector, and the product serial number of the sensor is first input from the computer, and then the relevant data is read out from the floppy disk, and then the inspection of the sensor, the reading of sensor parameters, the sensor setting and recording are automatically completed.

(2) Network temperature sensor

Network temperature sensors are a new generation of smart sensors that include digital sensors, network interfaces, and processing units. The digital sensor first converts the measured temperature into a digital quantity and then sends it to the microcontroller for data processing. Finally, the measurement results are transmitted to the network, so as to realize the data exchange and resource sharing between the sensors, between the sensors and the actuators, and between the sensors and the system, and there is no need for calibration and calibration when replacing the sensors, so that the "plug & PlAy" can be achieved, which is very convenient for the user.

2.6 Monolithic temperature measurement system

System On Chip is a high-tech product in the 21st century. It is the integration of a system or subsystem on the chip, and its integration level will be as high as 108-109 components/chip, which will bring epoch-making progress to the IC industry and IC applications. The Semiconductor Industry Association's (SIA) projections for monolithic system integration are shown in the table. At present, some well-known IC manufacturers in the world have begun to develop a single-chip temperature measurement system, which is believed to be available on the market in the near future.

Banner Sensors – the development trend of smart temperature sensors

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