The integration of temperature sensor (TS) and UHF RFID technology has attracted wide attention theoretically and experimentally. The architecture, power consumption, temperature measurement range, accuracy, and communication distance are key indicators of the performance of UHF RFID temperature sensor chip (RID-TSC). This work aims to provide a clearer view of the development of UHF RFID-TSC integration technology. After a systematic analysis of the characteristics of ADC, TDC, and FDC used in an integrated TS, the key low-power technologies under different architectures are summarized. Through the observation of the latest researches and commercial products, the development trend of UHF RFID-TSC technology is obtained, including on-chip and off-chip coordination, multiprotocol and multifrequency support, passive wireless sensor intelligence, miniaturization, and concealment.
In recent years, the development of Internet of Things (IoT) technology has greatly expanded the function of RFID. The new sensor integrated with RFID chip is the bridge of integrating RFID technology into wireless sensor network [
There are many different ways to transform RFID tags into RFID sensor tags. For example, the traditional tag antenna is replaced by a sensitive material, which is sensitive to the physical characteristics of its surrounding environment [
Due to the lack of clear information, it is difficult for chip designers to make correct choices in different architectures and key technologies. Therefore, in this paper, the existing scientific research achievements and commercial products are analyzed. Furthermore, the typical technical architectures and key technologies are summarized. At last, the technical development trend of RFID-TSC is prospected.
The organization of this paper is as follows. In Section
The integrated TS architecture based on CMOS process can be divided into three categories [
Typical technology architectures for CMOS-based RFID TS. (a) ADC architecture. (b) TDC architecture. (c) FDC architecture.
The ADC architecture includes sigma-delta ADC, successive approximation ADC (SAR ADC), and zoom ADC composed of SAR ADC and sigma-delta ADC. This architecture has the advantages of high measurement accuracy and wide measurement range, but it also has the disadvantages of complex structure, high power consumption [
The TDC architecture usually produces two voltage signals: proportional-to-absolute-temperature (PTAT) and complementary-to-absolute-temperature (CTAT) [
The FDC architecture converts the temperature-dependent signal into a frequency through a ring oscillator, which converts the frequency into a temperature-dependent digital signal by the FDC [
Generally speaking, although the ADC architecture has high precision, it is not suitable for RFID applications because of its high power consumption and large chip area [
Regarding the integration technology of RFID and TS, many works have proposed different technical architectures [
RFID-TSC typical architecture.
The antenna has an important influence on the performance of the RFID-TSC. According to the material and production process, antennas include two types:
OCA performance is affected by area, CMOS process and materials, signal interference, silicon interconnect metal size, high dielectric constant, and low resistance silicon substrate. A multiport microstrip patch antenna was proposed [
The RF analog front-end realizes the RF carrier into DC power and generates reference voltages and signals for other modules. It consists of several rectifier circuits, demodulation circuits, backscatter modulation circuits, reference circuits, regulator circuits, clock circuits, and reset circuits [
The digital baseband mainly implements communication protocols, encryption and decryption, encoding, decoding, anticollision algorithms, and operation control [
Usually, the NVM for UHF RFID includes E2PROM [
The RFID chip is mainly extended by the sensor interface and can integrate various sensors such as temperature, humidity, pressure, and acceleration [
For UHF passive RFID tags, the power of the reader determines the communication distance, but it is strictly limited in different countries and regions. For example, the United States stipulates that the effective isotropic radiated power (EIRP) of UHF RFID reader cannot exceed 4W (36dBm), the European standard is 500 mW (27dBm), and China standard is 2W (33dBm). Because the integrated TS adds additional power consumption, which will further shorten the communication distance of the RFID tag, so low-power consumption is also a key technology in the RFID-TSC. The technical architecture and low-power technology used by TS together determine the overall power consumption of the RFID-TSC.
Second-order sigma-delta ADC or zoom ADC, dynamic element match (DEM), and bias current are commonly used in low-power design. Dynamic threshold technique and second-order zoom ADC are used to realize temperature detection [
Usually, MOS elements, MOSFET and substrate parasitic NPN bipolar subthreshold are used as temperature sensing units. Clock or circuit reuse, module time-sharing, PSRR cascode current source mirror bias circuit, and subthreshold technology bias transistors can further reduce power consumption.
The subthreshold and clock multiplexing of MOS elements are adopted, and the power consumption is 119nW [
A ring oscillator is used to design the TS, and the power [
Through low-power research and analysis of the three architectures, subthreshold MOS elements, bias current, circuit multiplexing, time division technology, etc. commonly used low-power key technologies, which can achieve ultra-low-power consumption of only 0.1
Power consumption, chip area, temperature range, and accuracy are important indicators that affect TS performance. Table
Comparison of TS.
Work/ Architecture | Power Consumption | Chip Area (mm2) | Work | Temperature | Accuracy |
---|---|---|---|---|---|
[ | 0.119 | 0.0416 | 0.50~1.00 | -10~30/-0.80~1.00 | - - |
[ | 0.112 | 0.0125 | 0.50 | -10~20/-0.10~0.30 | 0.049 |
[ | 0.68 | - - | 0.60~1.00 | -20~30/-0.80~0.80 | - - |
[ | 0.90 | 0.20 | - - | +27~47/-1.00~1.00 | - - |
[ | 0.35 | 0.14 | 1.00 | -30~60/-1.50~1.50 | 0.30 |
[ | - - | - - | 1.80 | -10~100/- - | 0.50 |
[ | 0.10 | - - | 1.50 | -20~80/- - | 0.403 |
[ | 31.5 | - - | 1.80 | -37~91/-0.10~0.43 | - - |
[ | 7.4 | 0.12 | 1.60~2.00 | -30~125/-0.20~0.20 | - - |
[ | - - | 0.08 | 1.50~2.00 | -55~125/-0.15~0.15 | 0.25 |
[ | 13.20 | 1.44 | 1.20 | -20~50/-1.00~0.80 | 0.02 |
[ | - - | - - | 1.80 | -20~120/-0.65~0.65 | - - |
[ | 0.60 | 0.731 | 1.00 | -40~85/-1.50~1.50 | 0.50 |
[ | 0.22 | 0.05 | 1.00 | 0~100/-1.60~3.00 | 0.30 |
[ | - - | 0.008 | 1.00 | 0~110/-1.50~1.50 | 0.18 |
[ | 0.125 | 0.062 | 0.50 | -40~80/-0.70~1.20 | - - |
[ | 0.095 | 0.04 | - - | +8~85/- - | 0.40 |
[ | 0.20 | 0.585 | 1.50 | -75~125/- - | 0.45 |
In commercial applications, RFID-TSC products not only pursue performance indicators but also are compatible with mainstream protocols and standards, communication distances, working modes, etc., which together determine their market share and potential. Because different brands of UHF RFID-TSC products adopt different protocols and standards, Table
Comparison of UHF RFID-TSC.
Work / | RF Band(MHz) | Working Mode | Air Interface Protocol | Temperature | Communication distance(m) |
---|---|---|---|---|---|
[ | 860~960 | passive | - - | -40.00~85.00/ | 6.00(read)/ |
[ | - - | passive/ semi-passive | EPC C1G2 | -20.00~50.00/ | 9.50(read)/ |
[ | 860~960 | passive | EPC C1G2 | -20.00~30.00/ | - - |
[ | 915 | passive | - - | -40.00~55.00/ | 7.50 |
[ | 860~960 | passive/ | EPC C1/ | -20.00~60.00/ | 5.00~6.00 |
[ | 860~960 | passive | EPC C1G2/ | -30.00~85.00/ | 2.00 |
[ | 860~928 | semi-passive | EPC C1G2/ | -40.00~85.00/ | 10.00(in air)/ 2.50(in metal) |
[ | 860~928 | semi-passive | EPC C1G2/ | -20.00~70.00/ | 10.00(in air) |
[ | - -(UHF) | semi-passive | EPC C1G2 | -25.00~-20.00/ | - - |
[ | 860~960 | passive/ | EPC C1G2/ | -40.00~85.00/ - - | - - |
[ | 860~928 | semi-passive | EPC C1G2/ | -30.00~70.00/ | 10.00 |
[ | 860~960 | passive | ISO18000-6C | -10.00~20.00/ | 0.50 |
As shown in Table
In this paper, the integration technology of RFID and sensor is summarized and analyzed. Three typical architectures and key technologies of ADC, TDC, and FDC for TS are discussed. On this basis, the overall architecture of UHF RFID-TSC is given. Then the existing research results and commercial products were compared and analyzed from the aspects of technical architectures, power consumption, temperature measurement range and accuracy, communication distance, protocol standards, and other dimensions. It is believed that the extended sensor interface can realize the automatic perception of temperature, humidity, pressure, and other environmental information by RFID, which greatly expands the traditional RFID application scenarios. Based on the above analysis, the development trend and prospect of UHF RFID-TSC are listed as follows.
SPI and I2C bus as the sensor interface of RFID chip can greatly expand the function of RFID and have achieved success, and some products have been successfully applied. For example, the AMS/SL900A provides an easy-to-use interface for access to external sensors [
The current mainstream air interface protocol of is EPC Global C1 G2/ISO 18000-6C, and most products are fully compatible. In addition, HF and UHF work frequencies have different advantages. For better commercial applications, multiprotocol and multifrequency fusion innovation are a good way to develop. For example, a sensor chip [
In the IoT systems, RFID is no longer a simple individual identification function. It has evolved into a feature-rich intelligent wireless sensor node. Most importantly, RF technology solves the problem of relying on power in traditional sensor technology. For example, the sensing unit and the off-chip sensing interface make RFID a wireless sensor node [
Although some achievements have been made in UHF RFID-TSC research, the overall size of the product is too large, it is still mainly attached to the surface of the object, and the concealment of the tag cannot be achieved. Therefore, on the basis of keeping distance and realizing temperature sensing function, how to realize miniaturization and miniaturization of products, how to implant them into the object to realize invisibility, and how to sense the temperature and other information inside the object are still a hot and difficult point worth researching.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the Project of Scientific Operating Expenses from Ministry of Education of China (Grant no. 2017PT19) and Open Research Fund of Beijing Key Laboratory of Big Data Technology for Food Safety, Beijing Technology and Business University.