RFID Human Body Temp Sensor

1. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 57, NO. 2, FEBRUARY 2010 95 Full Passive UHF Tag With a Temperature Sensor Suitable for Human Body Temperature Monitoring A. Vaz, A. Ubarretxena, I. Zalbide, D. Pardo, H. Solar, A. García-Alonso, Member, IEEE, and R. Berenguer, Member, IEEE distances than the previous ones, but require specific communi- cation protocols and offer poor temperature sensor performance (∼±1◦C accuracy), as compared with those required in health- care applications. Thus, there is a strong motivation to provide long-range high-performance UHF RFID temperature sensors suitable for next-generation wireless healthcare applications [1], [7]. In this brief, a high-performance long-range high- accuracy UHF RFID temperature sensor is presented. This brief is organized as follows: The tag architecture is reported in Section II. Section III describes the temperature sensor design. Section IV presents the RFID sensor layout. Finally, Section V discusses the performed measurements and the achieved read- ing distance. Abstract—A long-range UHF RF identification (RFID) sensor has been designed using a 0.35-µm CMOS standard process. The power-optimized tag, combined with the ultralow-power temper- ature sensor, allows an ID and a temperature reading range of 2 m from a 2-W effective radiated power output power reader. The temperature sensor is based on a ring oscillator, where the temperature dependence of the oscillation frequency is used for thermal sensing. The temperature sensor exhibits a resolution of 0.035◦C and an inaccuracy value lower than 0.1◦C in the range from 35◦C to 45◦C after two-point calibration. The average power consumption of the temperature sensor is only 110 nW at ten conversions per second while keeping a high resolution and ac- curacy. These properties allow the use of the RFID as a batteryless sensor in a wireless human body temperature monitoring system. Index Terms—CMOS analog front end, digital core, high ac- curacy, low power, RF identification (RFID), temperature sensor, ultrahigh frequency (UHF). II. RFID SENSOR ARCHITECTURE Fig. 1 shows a simplified block diagram of the proposed tag sensor. It is formed by three main blocks, namely, the analog front end, the digital core and the sensor. I. INTRODUCTION N plications, including supply chain management, public trans- portation, access control, and many more [1]. Recently, the combination of RFID with sensory systems has extended the applications of RFID to environmental monitoring [2]–[5] or healthcare applications [6]. Those existing sensors, such as [2] and [3], are usually designed for general temperature measure- ment and can measure temperature from −40◦C to 85◦C at 1◦C accuracy, operating at the 13.56 MHz and 134.2 kHz fre- quency bands, respectively. More specific application sensors, such as [6], which are designed to measure the temperature of an animal, are able to measure temperature from 30◦C to 50◦C with higher accuracy (∼0.1◦C), operating at the 134.2 kHz frequency band. However, these sensors have the inconvenience of a limited reading range (a few centimeters) and high cost. To overcome this limitation, RFID sensor development has been focused on RFID tags using the UHF bands (868 MHz, 900 MHz, and higher). Recently reported designs, such as in [4] and [5], operating in those bands, offer higher reading OWADAYS, there is an undeniable and unstoppable trend to use RF identification (RFID) in a number of ap- A. Analog Front End The analog front end has been designed taking into account the system constraints presented in [8]. Therefore, all the blocks have been optimized for low power consumption to maximize the communication range. A 35% efficiency differen- tial Greinacher-based voltage multiplier is implemented using Schottky diodes. This block rectifies the incoming signal and stores the required energy to operate in the supply capacitor (1.4 nF). The differential Greinacher topology, as compared with the traditional Dickson topology, supplies a higher DC output voltage for the same given input voltage. Two series voltage regulators, i.e., 1.4 and 2.1 V, have also been imple- mented. The first one (1.4 V) supplies a regulated voltage to the digital core, whereas the second one (2.1 V) supplies a regulated voltage to the trimmed clock generator (480 kHz), the charge pump, the electrically erasable programmable ROM (EEPROM), and the temperature sensor. By using the 2.1-V voltage regulator, the power-supply rejection ratio (PSRR) re- quirements for the clock generator and the temperature sensor are relaxed to meet the required performance. A voltage limiter (Fig. 2) has been implemented to avoid possible damages in the circuits due to voltage surges whenever the reader and the tag are very close. The proposed voltage limiter takes advantage of the existing band-gap reference to provide a low-variation limiting voltage of 2.95 V. As shown in Fig. 2, the supply voltage (VSUPPLY∼ VPOR) is divided down (VX) using R2 and R3 and compared with the band-gap reference (VBGN) using differential amplifier M1− M4. When VX is higher 1549-7747/$26.00 © 2010 IEEE Manuscript received July 17, 2009; revised October 6, 2009. Current version published February 26, 2010. This work was supported in part by the Depart- ment of Education, Universities and Research, Basque Country Government. This paper was recommended by Associate Editor J. S. Chang. The authors are with the Integrated Circuits for Communications Systems (COMMIC) Group, Department of Electronics and Communications, Centro de Estudios e Investigaciones Técnicas de Guipúzcoa (CEIT) and Technological Campus, University of Navarra (Tecnun), 20018 San Sebastian, Spain (e-mail: rberenguer@ceit.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCSII.2010.2040314

2. 96 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 57, NO. 2, FEBRUARY 2010 Fig. 1. Simplified block diagram of the proposed RFID sensor. recover the baseband data. The envelope detector (D1−D4and CS−CP) uses the same structure as that of the Greinacher voltage multiplier with a smaller number of stages. During the power-up of the tag, the envelope signal V1+increases as does the reference signal VREF+, since diode D5is working in the forward-bias region. Capacitors CF are charged up. After the power-up time, the reference signal value is VREF+= V1+− VFORWARD−D5. Since V1+> VREF+, the logic output level of the ASK demodulator is 1. When the envelope signal V1+starts to decrease, the Schottky diode D5cuts off and works in the reverse region, preventing the capacitors CF to discharge and the signal VREF+to decrease. When V1+< VREF+, the output logic level of the ASK demodulator changes to 0. This way, baseband data are ASK demodulated. The proposed demodula- tor is very robust against voltage ripple from the envelope de- tector and noise captured by the antenna. Finally, a reset block has been implemented to reset the digital core after 175 μs from power on reset (POR) activation. The measured total currentconsumptionofthecompleteanalogfrontendis7.4μA. Fig. 2. Voltage limiter architecture. B. Digital Core The digital core controls the communication flow with the reader following the electronic product code (EPC) Gen 2 communication protocol [9]. The core used for this work is presented in [10], and its architecture is shown in Fig. 1. The incoming signal from the analog front end is detected and demodulated in the digital demodulator. The decoder obtains the operation code and the arguments of the instruction. The control module controls the system with a finite-state machine. It does the necessary operations by accessing the memory and the register bank. Finally, the transmitter modulates the answer. The accesses to the tag memory (EEPROM) are handled by the memory access module. To reduce the power consumption, the unnecessary modules are deactivated during operation [11]. For that, a power management module, which controls the status of each block, is implemented. The simulated average power consumption over time of the digital core for a req rn command is approximately 1.25 μW. This command performs the typical operations, making use of Fig. 3. Architecture of the differential ASK demodulator. than VBGN, transistor M6switches on and, in turn, activates transistor M8, which carries most of the current, preventing the supply voltage from increasing. The simulated limiting voltage variation (including the voltage limiter, 2.1 V voltage regulator, andband-gapreferenceblocks)againstprocessandtemperature (35◦C to 45◦C) variations is ±0.05 V. Fig. 3 shows the simplified circuit schematic of the ASK demodulator. It uses the envelope detection of the RF incoming signal and its comparison with a reference signal VREF+ to

3. VAZ et al.: UHF TAG WITH A TEMPERATURE SENSOR SUITABLE FOR HUMAN BODY TEMPERATURE MONITORING 97 of delay stages NS; and the output voltage swing VH− VL [12], i.e., I f = (1) NS· CL· (VH− VL). The cascoded bootstrapped current source generates the temperature-dependent supply current required by the pulse generator. As shown in the following equation, the supply current I can be expressed as a function of the threshold voltage Vth; the transconductance k? of transistor N6; and the resistance R3: Nand dimensions W/LN6 Fig. 4. Block diagram of the temperature sensor. 1 I =Vth + ?W ? ? k? k? N6R2 R3 3 N L ? ? +1 2Vth ?W 1 ? + (2) ? ?2. ? ? ?W R3 N6R3 k? N6R3 N L N L If R3? and (W/L)N6?, then the supply current I given by the current source can be approximated to I ≈Vth (3) R3. The relationship governing the temperature variation of the threshold voltage Vthand the resistance R3is shown by Vth(T) =Vth(T0)(1 + TCVth(ΔT)) R3(T) =R3(T0)?1 + TC1R3(ΔT) + TC2R3(ΔT)2? where the temperature coefficients TCVthand TC1 are negative for the process where the sensor is implemented, and ΔT = T − T0. By using (4) and (5) in (3), the current I can be expressed as a function of temperature and can further be approximated using Taylor series, i.e., (4) (5) Fig. 5. Circuit diagram of the temperature-dependant oscillator block. all the working states. Thus, its power distribution can be used as the generic power consumption of the digital core. I =Vth(T0) R3(T0) ?1 + (TCVth− TC1R3)(ΔT) + (TC12 − TC13 III. TEMPERATURE SENSOR The block diagram of the proposed temperature sensor is presented in Fig. 4. It is formed by the digital control logic (CTRL_LOGIC module); the oscillator (OSC block), whose output frequency depends on the sensed temperature, and the 15-bit binary counter, which counts the number of pulses delivered by the oscillator in a fixed time interval. The measurement process begins with the rising edge of the “Start” signal that comes from the digital core. Then, the CTRL_LOGICmodulegeneratesthreebinarysignalstocontrol theoperationtimesoftheotherblocks.First,the“Osc_EN” sig- nal connects the oscillator to the power supply. After an oscil- lator frequency stabilization time of 8.33 μs, the “Temp_meas” signal determines the fixed time interval of ∼166.7 μs, in which pulses will be counted. The CTRL_LOGIC module uses, as a timereference,the4-bittrimmedandtemperature-compensated 480 kHz clock given by the clock generator module [12], [13]. Finally, the falling edge of the “Enable” signal indicates the end of the measurement and resets the counter. Fig. 5 shows the circuit diagram of the ring oscillator with a temperature-dependent output frequency. The output frequency is fixed by the supply current I; the dimensions of the pulse generator transistors (N7−N11 and P9−P13), which set the load capacitor CLseen at the output of each stage; the number R3− TCVthTC1R3)(ΔT)2 R3(ΔT)3+ ···?. (6) If |ΔT| < 20◦C, as it is the case of human body temperature monitoring, the contribution of the higher order terms is small and, hence, can be neglected. The temperature dependence of the supply current I for |ΔT| < 20◦C is shown as ∂I ∂T= Vth(T0)/R3(T0)[TCVth− TC1R3]. (7) Therefore, the oscillation frequency is expected to linearly change with temperature variations in the range between 35◦C and 45◦C, and its temperature dependence is mainly provided by the variation of both R3resistance and Vthof transistor N6 with temperature. The simulated frequency sensitivity of the oscillator is 209 kHz/◦C. Transistors P3to P8form a wide- swing P-type cascode current mirror, providing the sensor, together with the series voltage regulator (2.1 V), a high PSRR, below 0.1◦C for ±15% variation on VSUPPLY. The pulse generator subcircuit is basically a ring oscillator (composed of an odd number of current-controlled inverter cells and a retroactive loop). Inverters P13−N11and P14−N12(out of a

4. 98 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 57, NO. 2, FEBRUARY 2010 Fig. 6. Sensor performance for different simulated corners. retroactive loop) regenerate the train of pulses delivered by the ring oscillator to reach full logic levels. As far as sensing is concerned, the supply current of the ring oscillator varies with temperature; in consequence, the output frequency varies with temperature in a proportional way, as shown in (1). Thus, the number of pulses N that the binary counter will count in a controlled time interval, fixed by the signal “Temp_meas,” is temperature dependent. To correct process variation errors, calibration points N1and N2at two different temperatures T1and T2are stored in the EEPROM of the RFID sensor. Then, when a temperature measurement at T3is required by the reader, instead of sending three values, namely, the measured point N3and the two calibration points N1 and N2, the sensor responds the difference between the measured value at T3 and the calibration value at T1 (Y3= N3− N1) and the difference between the calibration values at T2and T1(Y2= N2− N1). This way, the number of bits to be sent back to the reader, and consequently the required power to operate, is reduced with a minimal extra cost in the digital part. By knowing Y2and the temperatures where the sensor was calibrated a priori (T1and T2), the reader is able to obtain the sensor characteristic pattern (Y = a · T + b). Finally, through Y3, the reader is able to determine the temperature T3of the sensor. Fig. 6 shows the sensor performance for the different simulated corners (Typical, Fast, Slow, WZ, and WO). As can be observed, all corners exhibit an inaccuracy value of ∼±0.1◦C after sensor calibration. The occupied area of the proposed temperature sensor is only 0.084 mm2. The area is mainly dominated by the control logic and biasing resistors R1, R2, and R3(Fig. 6). R1and R2are implemented using an n-well resistor, whereas large resistor R3 (∼1 MΩ) is implemented using High Resistive poly (HR-poly) resistance (∼10 kΩ/?). Fig. 7. (b) Detail of the digital core. (c) Temperature sensor of the RFID sensor. (a) Microphotograph of the analog front end of the RFID sensor. Fig. 8. Fabricated prototype of the complete RFID sensor. the charge pump are located at the bottom left part. Fig. 7(c) shows a detail of the temperature sensor. The complete RFID sensor where the previous parts (analog front end, temperature sensor, and digital core) were integrated all together was also fabricated. The chip size is approximately 2200 × 1800 μm2. It has been implemented using a low- cost two-poly four-metal layer 0.35-μm CMOS process. The complete RFID sensor was packaged in a ceramic LCC 16-pin package and assembled to a matched impedance dipole antenna (Fig. 8). IV. LAYOUT Three different chips were fabricated to be able to character- ize each part separately: the analog front end, the temperature sensor, and the digital core (core, charge pump, and EEPROM). Fig. 7(a) shows the analog front-end chip, where the RF input pads are located at the left side of the chip. The clock-trimming pads are located at the top, and the clock output is located at the bottom. Other pads allow checking the right operation of other key blocks such as voltage regulators, band-gap reference, etc. Fig. 7(b) shows a detail of the digital core. The EEPROM and V. MEASUREMENT RESULTS The measurement and characterization of the complete RFID sensor have been performed in two steps: First the complete RFID sensor was built using the individual parts that were sep- arately fabricated: the analog front end, the temperature sensor,

5. VAZ et al.: UHF TAG WITH A TEMPERATURE SENSOR SUITABLE FOR HUMAN BODY TEMPERATURE MONITORING 99 TABLE I RFID TEMPERATURE SENSOR COMPARISON combined with the low-power temperature sensor, allows an ID and a temperature reading range of 2 m from a 2-W ERP output power reader. The temperature sensor exhibits a resolution of 0.035◦C and an inaccuracy value lower than 0.1◦C after two- point calibration in the range between 35◦C and 45◦C. The temperature sensor average power consumption is only 110 nW at ten conversions per second. These properties allow the use of theRFIDsensorasabatterylesssensorinashort-rangewireless human body temperature monitoring system. Fig. 9. calibration in the range from 35◦C to 45◦C. Sensor output and sensor temperature inaccuracy after two-point and the digital core. This way, it was possible to individually check the correct operation of each block. The proper response of the digital core to different EPC commands was checked, making it suitable to be used with most EPC commercial readers. After individually checking the proper operation of each block, in a second step, the complete RFID sensor, as afore- mentioned, was packaged in a ceramic LCC 16-pin package and assembled to a matched impedance dipole antenna (Fig. 8). The measured input impedance of the chip at the frequency of 868 MHz was 6–47j Ω. Successful ID and temperature communication between the reader operating at 868 MHz and 2-W effective radiated power (ERP) and the RFID sensor have been achieved, in the laboratory environment, over a distance of approximately 2 m, which is much higher than the reading range (a few centimeters) obtained from RFID temperature sensors operating in the 134.2 kHz or 13.56 MHz frequency bands [2], [3], [6]. Fig. 9 shows, in decimal format, the measured output of the sensor for different chip temperatures and samples. The sensor measurement was performed in a climate chamber using a tag built using the different modules (analog front end, sensor, field-programmable gate array (FPGA), and digital core replica power consumption module). The sensor exhibits a linear behavior and a temperature resolution smaller than 0.035◦C. Temperature inaccuracy of the sensor after two- point calibration is also shown in Fig. 9. The inaccuracy of the sensor is found to be ∼±0.1◦C for the temperature range from 35◦C to 45◦C, making it suitable for human body temperature monitoring. The average power consumption of the temperature sensor module is only 110 nW at ten conversions per second while keeping high accuracy and resolution. Table I compares the performance of the designed temperature sensor with other temperature sensors for RFID available in the literature. It can be observed that the proposed temperature sensor adoptsanapproachthatisfavorableforlowinaccuracyandhigh resolutionwhilekeepingpowerconsumptionverylow.Thearea of the sensor is comparable or even smaller than other works after considering the translation of different technologies. REFERENCES [1] “IDTechEx report,” RFID Market Projections 2008 to 2018, Jan. 2007. [2] “New low-cost temperature sensor,” RFID Journal, Jul. 2002. [Online]. Available: http://www.rfidjournal. com/article/view/28/1/1 [3] K. Opasjumruskit, T. Thanthipwan, O. Sathusen, P. Sirinamarattana, P. Gadmanee, E. Pootarapan, N. Wongkomet, A. Thanachayanont, and M. Thamsirianunt, “Self-powered wireless temperature sensors ex- ploit RFID technology,” Pervasive Comput., vol. 5, no. 1, pp. 54–61, Jan.–Mar. 2006. [4] H. Shen, L. Li, and Y. Zhou, “Fully integrated passive UHF RFID tag with temperature sensor for environment monitoring,” in Proc. 7th Int. Conf. ASIC, Oct. 2007, pp. 360–363. [5] N.Cho,S.-J.Song,S.Kim,S.Kim,andH.-J.Yoo,“A5.1-µW,UHFRFID tag chip integrated with sensors for wireless environmental monitoring,” in Proc. 31st Eur. ESSCIRC, Sep. 2005, pp. 279–282. [6] V. K. Chan and E. Mejia, “Interrogation device and method for scanning,” U.S. Patent 7 432 825, Oct. 7, 2008. [7] “Dagstuhl seminar,” Assisted Living Systems: Models, Architectures and Engineering Approaches, Wadern, Germany, Sep. 2007. [8] D. Pardo, A. Vaz, S. Gil, J. Gomez, A. Ubarretxena, D. Puente, R. Morales-Ramos, A. Garcia-Alonso, and R. Berenguer, “Design criteria for full passive long range UHF RFID sensor for human body temperature monitoring,” in Proc. IEEE RFID Conf., Mar. 2007, pp. 141–148. [9] EPCTM Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID—Protocol for Communications at 860 MHz–960 MHz, Oct. 2008. V1.2.0. [10] I. Zalbide, J. Vicario, and I. Velez, “Power and energy optimization of the digital core of a Gen2 long range full passive RFID sensor tag,” in Proc. IEEE RFID Conf., 2008, pp. 125–133. [11] M. Keating, D. Flynn, R. Aitken, A. Gibbons, and K. Shi, Low Power Methodology Manual for System-on-Chip Design. Berlin, Germany: Springer-Verlag, 2007. [12] K. Sundaresan, P. E. Allen, and F. Ayazi, “Process and temperature compensation in a 7-MHz CMOS clock oscillator,” IEEE J. Solid-State Circuits, vol. 41, no. 2, pp. 433–442, Feb. 2006. [13] N. Tran, B. Lee, and J. W. Lee, “Development of long-range UHF-band RFID tag chip using Schottky diodes in standard CMOS technology,” in Proc. IEEE RFIC Symp., Jun. 2007, pp. 281–284. [14] Z.ShenghuaandW.Nanjian,“Anovelultralowpowertemperaturesensor for UHF RFID tag chip,” in Proc. IEEE Asian Solid-State Circuits Conf., Nov. 2007, pp. 464–467. [15] P. Chen, C.-C. Chen, C.-C. Tsai, and W.-F. Lu, “A time-to-digital- converter-based CMOS smart temperature sensor,” IEEE J. Solid-State Circuits, vol. 40, no. 8, pp. 1642–1648, Aug. 2005. [16] Y. Lin, D. Sylvester, and D. Blaauw, “An ultra low power 1 V, 220 nW temperature sensor for passive wireless applications,” in Proc. IEEE Custom Integr. Circuits Conf., Sep. 2008, pp. 507–510. VI. CONCLUSION A long-range RFID sensor has been designed using a 0.35-μm CMOS standard process. The power-optimized tag,