Passive and Semi-Passive Wireless Temperature and Humidity Sensors Based on EPC Generation-2 UHF Protocol
This paper proposes passive and semi-passive wireless temperature and humidity sensors based on electronic product code (EPC) global Class-1 Generation-2 UHF communication protocol. The wireless sensors consist of a sensor key chip and off-chip temperature and humidity sensors. The sensor key chip integrates RF/analog front-end circuit, digital baseband processor, nonvolatile memory, on-chip temperature sensor, and sensor interface. The sensor interface connects the off-chip sensors and the sensor key chip. The sensor key chip with the on-chip temperature sensor can operate without battery power (passive mode), and also can co-operate with the off-chip temperature and humidity sensors powered by battery (semi-passive mode). The RF/analog front-end circuit provides the dc power to the sensor key chip and communicates with the interrogator passively. Advanced low-power techniques are adopted to reduce the power consumption of the sensor key chip. The sensor key chip is fabricated in 0.18-μm CMOS process. In passive mode, the maximum wireless sensitivity of on-chip sensor is -15.1/-11.2 dBm for reading and sensing operation, respectively, and the temperature sensing error is -1 °C/0.8 °C over operating range from -20 °C to 50 °C. It achieves a reading/sensing distance of over 9.5/6 m with 4-W effective isotropic radiated power (EIRP) by the commercial interrogator. In semi-passive mode, the temperature and humidity sensing distance of off-chip sensors is 2.7 m.
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Passive and Semi-Passive Wireless Temperature
and Humidity Sensors Based on EPC Generation-
2 UHF Protocol
ARTICLE in IEEE SENSORS JOURNAL · APRIL 2015
Impact Factor: 1.76 · DOI: 10.1109/JSEN.2014.2375180
3 AUTHORS, INCLUDING:
Chinese Academy of Sciences
1 PUBLICATION 0 CITATIONS
Chinese Academy of Sciences
63 PUBLICATIONS 390 CITATIONS
IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015 2403
Passive and Semi-Passive Wireless Temperature
and Humidity Sensors Based on EPC
Generation-2 UHF Protocol
Shuang-Ming Yu, Peng Feng, and Nan-Jian Wu, Member, IEEE
Abstract—This paper proposes passive and semi-passive
wireless temperature and humidity sensors based on electronic
product code (EPC) global Class-1 Generation-2 UHF communication
protocol. The wireless sensors consist of a sensor
key chip and off-chip temperature and humidity sensors. The
sensor key chip integrates RF/analog front-end circuit, digital
baseband processor, nonvolatile memory, on-chip temperature
sensor, and sensor interface. The sensor interface connects
the off-chip sensors and the sensor key chip. The sensor key
chip with the on-chip temperature sensor can operate without
battery power (passive mode), and also can co-operate with the
off-chip temperature and humidity sensors powered by battery
(semi-passive mode). The RF/analog front-end circuit provides
the dc power to the sensor key chip and communicates with
the interrogator passively. Advanced low-power techniques are
adopted to reduce the power consumption of the sensor key
chip. The sensor key chip is fabricated in 0.18-μm CMOS
process. In passive mode, the maximum wireless sensitivity of
on-chip sensor is −15.1/−11.2 dBm for reading and sensing
operation, respectively, and the temperature sensing error is
−1 °C/0.8 °C over operating range from −20 °C to 50 °C.
It achieves a reading/sensing distance of over 9.5/6 m with
4-W effective isotropic radiated power (EIRP) by the commercial
interrogator. In semi-passive mode, the temperature and humidity
sensing distance of off-chip sensors is 2.7 m.
Index Terms—Wireless sensors, temperature sensors, humidity
sensors, sensor interface, baseband processor, EPC global Class-1
Generation-2 protocol, passive tags, low power design.
NOWADAYS, various kinds of wireless sensors have been
widely used in many applications because they can be
Manuscript received October 20, 2014; revised November 17, 2014;
accepted November 19, 2014. Date of publication November 26, 2014; date
of current version February 10, 2015. This work was supported in part by the
National Key Technology Research and Development Program through the
Ministry of Science and Technology of China under Grant 2012BAH20B02,
in part by the National High Technology Research and Development Program
of China under Grant 2012AA012301, in part by the National Science and
Technology Major Projects through the Ministry of Science and Technology
of China under Grant 2012ZX03004007-002, in part by the National Natural
Science Foundation of China under Grant 61306027, and in part by the
Academy-Locality Cooperation Program, Chinese Academy of Sciences,
Beijing, China. The associate editor coordinating the review of this paper
and approving it for publication was Prof. Sang-Seok Lee.
The authors are with the State Key Laboratory for Superlattices and
Microstructures, Institute of Semiconductors, Chinese Academy of Sciences,
Beijing 100089, China (e-mail: firstname.lastname@example.org; fengpeng06@
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/JSEN.2014.2375180
easily arranged in the fields –. For example, wireless sensors
are increasingly applied to extreme condition detecting,
high-risk environment surveillance, emergency rescue, frozen
food transportation, and tracking humidity and pressure of
scientific equipment .
The wireless sensors usually include wireless communication
technologies, such as ZigBee, Bluetooth and wireless
local area networks, to receive commands and transmit
data –. A ZigBee-based intelligent self-adjusting
sensor for home energy management service is implemented
by using ZigBee wireless technology for networking and
communication . Another ZigBee-based non-invasive wearable
monitoring device for physiological parameters, such as
temperature and heart rate of a human subject, has been
developed and reported . A Bluetooth module is selected
to act as a virtual serial data port for digital temperature sensors
in rotor temperature measurement . However, those
wireless technologies are usually used to form sensor networks
for environment monitoring and diagnostics of electric
drives . They have drawbacks of high power consumption,
large device size, high complexity of operation, and low
The passive RFID tag technology exhibits benefits of very
simple structure, low cost and low power. The tags are able
to passively communicate with the interrogator in a zeropowered
backscatter mechanism. Therefore, wireless sensors
based on the passive tag technology have raised increasing
interest among academic and industrial research –.
An energy-efficient wireless sensing within a passive multistandard
RFID transponder is enabled by using a successive
approximation analog-digital converter . Another wireless
temperature sensor tag chip is presented with the extra
circuit modules, such as external temperature sensor and
ADC . A passive RFID tag embedded temperature sensor
with time-domain readout scheme for a −30 °C to 60 °C
sensing range was reported . TELID412 RFID sensor by
MicroSENSYS  and Fenix RFID Tag by Farsens  are
designed as passive RFID sensor devices without sensor interface
for off-chip sensor devices. However, in many applications,
some non-CMOS-compatible sensors, such as humidity
and pressure sensors are also required to be integrated in the
Recently, wireless sensors which include a RFID tag
chip with a sensor interface and off-chip sensors has been
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2404 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015
proposed –. An ultra-low-power four-channel sensor
interface for wireless semi-active RFID transponders based
on a switched-capacitor sigma-delta modulator was described
in . However the analog signal interface is not compatible
with most general off-chip sensors with standard digital
signal interface. RFID sensor prototypes fabricated on a
printed circuit board using discrete components were designed
for full-passive, semi-passive and passive/semi-passive compatible
operation mode –. A solution including a
compact low power microcontroller and a RFID core extended
by a memory unit as data interface was presented in .
PE3001 UHF Tag IC by Productivity Engineering (PE) GmbH
was designed with a Serial Peripheral Interface (SPI) which
costs higher pin count than I2C and limits the number of
accessible off-chip sensors . In –, the sensors need
external MCU component and extra antenna with off-chip
RF-dc circuit to harvest the RF energy and supply the MCU
and off-chip sensors in passive operation mode. This will
greatly increase the cost and size of the sensor tag.
Comparing with the wireless sensors, where a microcontroller
and an off-the-shelf RFID chip are externally connected,
an ASIC single-chip solution and easier data exchange strategy
between the RFID chip and the microcontroller is more
compact and has a lower power consumption. Therefore, it is
very important to develop a sensor single key chip with wireless
low power communication circuits, on-chip low power
sensor circuits and sensor interface for operating the off-chip
This paper proposes passive and semi-passive wireless temperature
and humidity sensors based on EPC global Class-1
Generation-2 (EPC Generation-2) UHF communication
protocol. The wireless sensors consist of a sensor key chip
and off-chip temperature and humidity sensors. The sensor
key chip integrates an on-chip temperature sensor, a sensor
interface and a digital baseband processor. The sensor interface
uses I2C serial bus to connect the off-chip sensors and the
sensor key chip. Some advanced low power techniques are
adopted to effectively reduce the power consumption of the
key chip and to increase the sensing distance. Comparing with
the above-mentioned related works, the proposed wireless sensors
have the following features. First, the sensor key chip with
the on-chip temperature sensor can operate without battery
power (passive mode) and also co-operate with the off-chip
sensors which are powered by a battery (semi-passive mode),
which can be operated by a commercial interrogator. Second,
the sensor key chip implements single-chip solution instead
of using discrete MCU to reduce the power consumption and
cost. Third, the integrated I2C sensor interface can connect
the off-chip sensors and the sensor key chip directly. Fourth,
the sensors are equipped with only one UHF antenna which is
designed and patterned directly on FR4 substrate to perform
both energy-harvesting and communication. This will reduce
the size and cost of the wireless sensors.
The paper is organized as follows. Firstly, the wireless
sensor architecture is presented in Section II. Next, the critical
circuit blocks are designed in Section III. The measurement
results are shown in Section IV. Finally, conclusions are drawn
in Section V.
Fig. 1. Architecture of the wireless sensors.
II. WIRELESS SENSOR ARCHITECTURE
Fig. 1 shows the architecture of the wireless sensors. The
wireless sensors consist of a sensor key chip and off-chip
temperature and humidity sensors. The sensor key chip integrates
RF/analog frontend circuit, digital baseband processor,
non-volatile memory (NVM), on-chip temperature sensor,
and off-chip sensor interface. The communication between
the wireless sensors and the interrogator is based on the
EPC Generation-2 UHF protocol. The wireless sensors can
operate in two modes: passive mode and semi-passive mode.
In passive mode, the wireless sensors can measure temperature
by the on-chip sensor passively. In semi-passive mode, the
wireless sensors can semi-passively measure temperature and
humidity by the off-chip sensors, which are powered by a
battery. The performance of the off-chip temperature sensor is
better than on-chip temperature sensor.
The proposed wireless sensors have several advanced merits.
The interrogator can identify the wireless sensors when there
are a mass of wireless sensors in the application fields.
Secondly the wireless sensors can also execute temperature
sensing operation in passive mode under some battery-less
conditions. Furthermore, when the humidity or high-accuracy
temperature measurements are required, the sensor key chip
can activate the off-chip sensors and perform the measurements
in the semi-passive mode. Therefore, the wireless sensors
can adapt to the various application scenarios flexibly and
improve the battery lifetime.
The communication process between the interrogator and
wireless sensors is described as follows. The interrogator sends
energy and information to the sensor key chip by a modulated
UHF RF carrier. The sensor key chip harvests electrical energy
from the RF carrier, and activates itself. Then the sensor key
chip receives the commands and data from the interrogator
and sends information back by a backscattering scheme. The
communication link between the interrogator and sensor key
chip is half-duplex.
The RF/analog frontend circuit converts the energy of
RF signal into DC power to energize the sensor key chip, and
demodulates (modulates) the received (transmitted) signal. The
digital baseband processor decodes the demodulated signal
and generates the encoded data for modulation. Besides,
the baseband processor executes the received commands and
controls the on-chip and off-chip sensors. The sensor interface,
compatible with the I2C serial bus standard, supports the
communication between the sensor key chip and off-chip
YU et al.: PASSIVE AND SEMI-PASSIVE WIRELESS TEMPERATURE AND HUMIDITY SENSORS 2405
Fig. 2. Block diagram of the digital baseband processor.
sensors. The CMOS-compatible NVM is designed to store the
information such as EPC code, security information or sensing
data after power off.
III. DESIGN AND IMPLEMENTATION OF CIRCUIT BLOCKS
A. Low-Power Digital Baseband Processor
Fig. 2 shows the structure of the proposed digital baseband
processor which is fully compatible with EPC Generation-2
UHF protocol. The system consists of power management
(PM) module, PIE decoder, encoder, command decoder,
state controller, NVM controller, sensor controller, off-chip
sensor interface, cyclic redundancy check (CRC) check
module and slot counter.
In each communication round, the baseband processor
receives the encoded commands and data from the RF/analog
frontend circuit at a rate of 26.7kbps to 128kbps. Then, the
PIE decoder decodes the data symbol and starts the data
processing. The CRC check module calculates the CRC-16
over the received commands or transmitted data to detect error
or protect the certain information. If the CRC check result is
error, all data are discarded and frame is terminated.
The command decoder analyzes and identifies all commands
received from the interrogator. According to the decoded
commands, the state controller performs the corresponding
operations with the input data, such as enabling or disabling
the NVM controller, sensor controller and off-chip sensor
These commands are responsible for reading data from or
writing data to the NVM, activating the on-chip temperature
sensor, and controlling the off-chip temperature and humidity
sensors through the sensor interface module. Finally the
encoder collects the output frames from the State Controller,
CRC code from CRC check module, and random number
generated in slot counter. The slot counter is designed
based on the Slotted ALOHA algorithm as anti-collision
mechanism . Then, the encoder encodes the output data
according to FM0/Miller encoding method, and sends them
to the modulator for backscattering. The PM module adaptively
and dynamically controls the operations of the circuit
modules of baseband processor to effectively reduce its power
We design a set of internal customized commands to
operate the on-chip temperature sensor and off-chip sensors.
Fig. 3. Functional illustration of Power Management Module.
The baseband processor firstly sends the certain command
to start up the on-chip temperature sensor. After finishing
the measurement operation, the baseband processor receives
the sensing result data and saves them into the NVM. The
customized commands also can start up the off-chip sensors,
receive the sensing result data and save them into the NVM
through the I2C interface. The interrogator can read out
the sensing result data in the NVM and finish the sensing
The designed baseband processor has some advanced
merits. Firstly, it can implement the internal customized
commands and manage the on(off)-chip sensors. Secondly,
it integrates the general I2C serial bus, which can connect any
sensors with I2C serial bus. Thirdly, it is compatible with EPC
Generation-2 protocol. Finally, the wireless sensors operate
in the half-duplex fashion so that a novel PM method can
be designed to activate/shut down the certain circuit modules
adaptively and to reduce the instantaneous power consumption
of the baseband processor.
Because the sensor key chip integrates the on-chip sensor
and I2C serial bus, the power consumption of the chip will
be increased. To improve the operating distance, the power
consumption of the sensor key chip must be minimized. Fig. 3
illustrates the function of the power management module,
which reduce the baseband power consumption.
The operation of the baseband processor is divided into
three kinds of states: RECEIVE state, PROCESS state and
TRANSMIT state. When the baseband processor operates in
RECEIVE state, only PIE decoder and CRC check modules
are activated. After receiving the whole message, processor
turns in to PROCESS state. PM module disables the PIE
decoder and CRC check module, and enables command
decoder and state controller. Then according to the received
commands, state controller performs the corresponding operations,
and prepares the output data. Finally, in TRANSMIT
state, encoder and slot counter module are activated to encode
the message and send the data to the modulator. Thus, the
PM module enables the operating modules and disables the
idle modules dynamically to reduce the power consumption
of the baseband processor.
2406 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015
Fig. 4. Clock gating and operand isolation techniques.
Fig. 5. Block diagram of the on-chip temperature sensor.
Besides of PM method, other low-power circuit design
techniques are also adopted to reduce the power consumption
of baseband processor. As shown in Fig. 4, clock gating
and operand isolation are used to reduce power consumption
of the modules. These power optimization techniques are
adopted at RTL and logic synthesis level. Clock gating reduces
the switching activity of the sequential cells by turning off
the clock signal. Furthermore, operand isolation is applied
to arithmetic modules to reduce the internal and dynamic
power consumption of the combinational cells. The power
consumption of these cells is reduced by eliminating unwanted
transitions of their inputs when their outputs do not affect any
Supply voltage reduction is also an effective method to
reduce the dynamic power consumption. In order to minimize
the power consumption, the processor circuit is designed
to operate at 0.8V. On the other hand, the operating clock
frequency is minimized to further reduce the dynamic power.
The clock frequency is optimized by considering the trade-off
between the sensor performance and the power consumption.
B. Low-Power On-Chip Temperature Sensor and
I2C Serial Bus Interface for Off-Chip Sensors
The low power on-chip temperature sensor is powered by
the energy provided by RF rectifier. The block diagram of
the on-chip temperature sensor is shown in Fig. 5. It consists
of a bipolar sensing core, a low power 2nd-order sigma delta
( ) ADC, a dynamic element match (DEM) control module
and a bias circuit. The ADC includes a modulator,
a decimation filter and a clock generator. The bipolar sensing
core which is biased by a p: 1 ratio currents, produces two
voltage signals VBE1 and VBE2, which are complementary to
absolute temperature (CTAT), and can be expressed as follows:
VBEn = (
) · ln(
), n = 1,2 (1)
where k is the Boltzmann constant, q is the electron charge,
T is the temperature in Kelvin, IC is the collector current
and IS is the PNP’s saturation current. The voltage difference
VBE = VBE1 − VBE2 is proportional to absolute temperature
(PTAT), which can be expressed as:
VBE = kT
· ln(p) (2)
The relationship among the absolute temperature T ,
VBE2 and VBE can be expressed as :
T = A
α + X
where A is a constant, X = VBE2/ VBE and α is the gain
factor which is chosen to make a band-gap reference voltage
VREF = VBE2 + α VBE. For the temperature range from
−20 °C to 50 °C, in consideration of process corners and
design margins, X will not exceed the range from 5 VBE
to 24 VBE. Then the charge balance scheme can be used to
digitize X in the ADC, and α can be simply trimmed in
the digital backend. In the actual implementation, a 2nd-order
ADC is designed to realize noise shaping and improve
the resolution and conversion speed, and reduce the power
consumption. The modulator implemented with differential
switched-capacitor circuits, adopts DEM method to reduce
the mismatch effect of current sources in the bipolar core
and sampling capacitors in the modulator. As a result of the
2nd-order noise shaping, 13 bits temperature data can be
obtained with only 128 bit streams. The measured temperature
data are stored in the NVM, which can be read out through
The off-chip sensor interface is integrated in this wireless
sensor key chip to manage off-chip temperature and humidity
sensors. It is compatible with I2C serial bus interface standard.
The off-chip sensors and sensor key chip are connected with
two bus lines: a serial data line and a serial clock line.
The EPC operation commands control the sensing commands
and data transmissions through the sensor interface. The
off-chip sensors are recognized as slave (master) devices when
receiving (transmitting) the operation commands (the sensing
data) from (to) the sensor key chip. The interrogator first starts
up the off-chip sensors by a measurement command. After
finishing the sensing operation, the off-chip sensors send back
the sensing result digital data to the sensor key chip. Then,
the result data is stored in NVM. Finally the interrogator can
read out the result data in NVM by an EPC read operation
command and shows the measured temperature and humidity.
C. CMOS UHF Rectifier
The rectifier converts input electromagnetic waves into
DC power for the wireless sensor key chip. The UHF rectifier
in this paper is compatible with a standard CMOS process.
YU et al.: PASSIVE AND SEMI-PASSIVE WIRELESS TEMPERATURE AND HUMIDITY SENSORS 2407
Fig. 6. Circuit architecture of the CMOS rectifier.
Fig. 7. Voltage control methods of the rectifier.
As shown in Fig. 6, the CMOS UHF rectifier is based on a
modified charge pump structure. It consists of six cascaded
rectifier cells and an oscillator. A switched capacitor circuit
technique is used to provide active bias to solve the
threshold voltage problem in MOS diodes. The MOS diodes
(NM1 and PM1) are biased by two circuits (Bias1 and Bias2)
respectively to make them work in sub-threshold region. As a
result, the voltage loss during charge transferring caused by
the threshold voltage of the switch transistors is reduced. This
technique improves the voltage gain of the rectifier cell and
the power efficiency of the rectifier.
Because the incoming electromagnetic wave intensity into
the wireless sensors varies over a 30 dB range, to provide a
stable DC voltage and power, some voltage control methods
must be designed. Fig. 7 shows the schematic diagram of the
voltage control circuits, which include regulator, limiter and
protector. For low level of electromagnetic wave intensity, the
voltage is stabilized by the regulator only. For middle level
of electromagnetic wave intensity, the voltage is stabilized by
the regulator and limiter. The limiter can clamp the voltage
below a certain value. For high level of electromagnetic wave
intensity, the voltage is stabilized by the regulator, limiter and
protector using feedback mechanism. The protector consists
of voltage comparator and a switch connecting to the input
ports of rectifier. The comparator is used to compare output
voltage of rectifier and reference voltage, and the comparison
result controls the switch to change the input impedance of
rectifier. As a result, the input signal intensity can be reduced
and the sensor key chip can be protected.
D. Low Power Single-Poly NVM
NVM embedded in the sensor key chip is used to store
information such as EPC code, security codes and sensing data
Fig. 8. (a) Block diagram of the non-volatile memory. (b) Schematic of the
memory bit cell.
Fig. 9. Microphotograph of the sensor key chip.
after power off , . The architecture of the proposed
NVM is shown in Fig. 8(a). It consists of a non-volatile cell
array, a volatile register array, a bit line controller, a column
decoder, a sense amplifier column, a charge pump and a
control logic module. The non-volatile cell array and register
array have the equal numbers of corresponding bits. The data
in the non-volatile array can be first loaded into the register
array for the subsequently read operation with a faster speed
and less power. The charge pump generates a high voltage
and a medium voltage for programming. The column decoder
selects the active column. The sense amplifier column detects
the output data of active-column cells during read operation.
Fig. 8(b) shows the schematic of the memory bit cell.
To minimize the process cost and power consumption, the
NVM is realized with single-poly differential floating gate cell
compatible with standard CMOS process and bi-directional
FN-tunneling to achieve reliable low power programming. The
memory bit cell consists of FN tunneling junctions M1-M4
and coupling capacitors C1 and C2. The electrons can tunnel
through M1 and M2 (M3 and M4) to inject into (depart from)
the floating gates.
IV. MEASUREMENT RESULTS
The sensor key chip has been fabricated in 0.18μm
standard CMOS process with the chip area of 1.44 mm2.
Fig. 9 shows the microphotograph of the sensor key chip.
2408 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015
Fig. 10. The test platform for the wireless sensors.
Fig. 11. Measurement communication signal waves between wireless sensors
Fig. 12. Measured backscattered data of FM0-encoded symbols.
Fig. 13. Power consumption for optimization techniques.
Prototype passive and semi-passive wireless temperature and
humidity sensors based on the sensor key chip have been
fabricated on FR4 substrate. As shown in Fig. 10, the wireless
sensors include the sensor key chip, off-chip temperature and
humidity sensors, dipole copper antenna and lithium battery.
Fig. 10 shows the test platform of the wireless sensors.
The commercial interrogator is used to send energy and a
set of commands and data through the antenna. We use a
RFID protocol analyzer to monitor the communication information
between wireless sensors and interrogator. Fig. 11
shows measurement communication signals between wireless
sensors and interrogator. The interrogator uses Select Command
to selects a particular sensor population, and uses Query
command to start the communication process. The selected
sensor will backscatter a 16 bits random number (RN16)
to the interrogator. Then the interrogator will send ACK
command which echoes the RN16 to acknowledge the sensor.
Finally, the sensor will backscatter the data. Fig. 12 shows
the measured backscattered data of FM0-encoded symbols.
Fig. 14. Measurement result of operating off-chip sensors with I2C serial
Fig. 15. Measured temperature error of the on-chip temperature sensor.
The wireless sensors have been further measured according
to EPC Generation-2 UHF protocol. The results show that
the wireless sensors are compatible with the communication
protocol and it can correctly execute reading, writing and
Fig. 13 shows a comparison of the measured power
consumption for two different baseband processor designs.
The first version of the design without power optimization
techniques consumes 18.5μW at minimum operating voltage
of 0.9V. By using power optimization techniques described
above, the baseband processor consumes only 7.9μW at
minimum operating voltage of 0.8V. The comparison results
show that a 57% power reduction is achieved by using power
Fig. 14 shows the measured communication process of the
I2C serial bus interface between the sensor key chip and
the off-chip temperature and humidity sensors. SCL signal
provides the off-chip sensors a continuous stable clock signal,
and SDA signal is used to transmit data and commands
between the sensor key chip and off-chip sensors. The communication
process is divided into three steps. In the first
step, the off-chip sensors receive the address and commands
from the sensor key chip. Then the off-chip sensors begin
to perform the measurement operation. After finishing the
measurement, they outputs the sensing data back to the sensor
key chip through SDA signal line. Finally, the sensing data
is stored in the NVM which can be read out by commercial
interrogator. The measurement results show that the sensor
interface can make the sensor key chip operate the off-chip
The measured performance of the rectifier indicates that it
achieves a power efficiency of about 30% at the load resistor of
20k at input power of −5dBm. The measured results of the
YU et al.: PASSIVE AND SEMI-PASSIVE WIRELESS TEMPERATURE AND HUMIDITY SENSORS 2409
PERFORMANCE SUMMARY AND COMPARISON
NVM show that the write power consumption is only 4.3μW
@1.2V supply voltage, which is acceptable in the wireless
sensors. The NVM also shows good endurance performance
of more than 105 write/erase times, and it is fully compatible
with standard CMOS process.
To test the temperature sensing performance, the wireless
temperature sensor, the interrogator antenna and a reference
temperature sensor are placed inside the temperature chamber
with a temperature constancy of ±0.3 °C. Fig. 15 and Fig. 16
show the typical measurement results of the on-chip passive
temperature sensor and the off-chip semi-passive temperature
sensor. The inaccuracy of the on-chip temperature sensor is
−1.0 °C/0.8 °C over operating range from −20 °C to 50 °C
with a resolution of 0.02 °C. The commercial off-chip temperature
sensor also shows good performance, and can achieve a
resolution of 0.04 °C over a range of −40 °C to 125 °C.
2410 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015
Fig. 16. Measured temperature error of the off-chip temperature sensor.
Fig. 17. Measured humidity error of the off-chip humidity sensor.
To test the humidity sensing performance, the wireless
sensors, the interrogator antenna and a reference humidity
sensor are placed inside a chamber in which the humidity
is adjustable. The commercial off-chip humidity sensor is
energized by a 3V lithium battery. Fig. 17 shows the typical
measurement results of the off-chip humidity sensor. The
off-chip humidity sensor can monitor the humidity with a
resolution of 0.4% over a range of 20% to 98%. Although
the sensing error is limited by the reference sensor and other
non-ideal effects, the wireless sensors show good performance.
Table I summarizes and compares the performances of the
wireless sensors with those of other works. The wireless sensors
successfully measured data using commercial interrogator
with 4W EIRP. It achieved a reading/sensing distance of over
9.5/6.0 m with on-chip sensor, and 2.7 m with off-chip sensors.
The maximum wireless sensitivity is −15.1dBm for reading
operation, −11.2dBm for on-chip passive temperature sensing
operation and −4.2dBm for off-chip semi-passive temperature
and humidity sensing operation, respectively.
Compared with the sensors in –, our proposed
wireless sensors integrate the sensor interface for operating
off-chip sensors to expand their application area. Our wireless
sensors also have longer operating distance because of the
ultra-low power NVM, on-chip temperature sensor, baseband
circuit and high efficiency RF rectifier. Compared with the
sensors in , our wireless sensors can perform temperature
sensing operation in passive mode without battery power.
Compared with the sensors in , our wireless sensors can
perform sensing operation without MCU. This can reduce
the power consumption and cost. Furthermore, our wireless
sensors are equipped with only one UHF antenna designed
and patterned directly on FR4 substrate to perform both
energy-harvesting and communication. This can reduce the
size and cost of the sensors.
This paper proposed passive and semi-passive wireless
temperature and humidity sensors based on EPC Generation-2
UHF protocol. A sensor key chip and off-chip temperature
and humidity sensors were integrated in the wireless sensors.
The sensor interface in sensor key chip was designed for
connecting the off-chip sensors. The wireless sensors can
be operated in two modes: passive mode and semi-passive
mode. Power management method and clock gating method
were adopted to reduce the power consumption. The sensor
key chip was fabricated in 0.18μm CMOS process occupying
an area of 1.44mm2 . The experimental results indicate
that the wireless sensors can measure temperature by the
on-chip temperature sensor passively, and temperature and
humidity by the commercial off-chip sensors semi-passively.
In passive mode, the maximum wireless sensitivity of the sensor
is −15.1/−11.2dBm for reading and sensing operation of
on-chip sensor, respectively. It achieves a reading/sensing
distance of over 9.5/6.0m with 4W EIRP. In semi-passive
mode, the wireless sensitivity is −4.2dBm for sensing operation
of off-chip sensors, and the sensing distance is 2.7 m.
The wireless sensors can be operated by the commercial
The authors would like to thank S. Zhang for his help in
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Shuang-Ming Yu was born in Liaoning,
China, in 1987. He received the B.S. degree in
electronic science and technology from the Beijing
University of Posts and Telecommunications,
Beijing, China, in 2010. He is currently pursuing
the Ph.D. degree at the State Key Laboratory
for Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences,
Beijing. His current research interest includes
ultralow-power circuit design, digital circuits
design, and radio frequency identification tags.
Peng Feng was born in Chongqing, China, in 1983.
He received the B.S. degree in electronics from
Sichuan University, Chengdu, China, in 2006, and
the Ph.D. degree in microelectronics and solidstate
electronics from the Institute of Semiconductors,
Chinese Academy of Sciences, Beijing, China,
where he has been an Assistant Professor with the
Institute of Semiconductors since 2011. His current
research interests include radio frequency energy
harvesting, embedded CMOS nonvolatile memory,
CMOS-embedded sensors, and RFID tags.
Nan-Jian Wu (M’05) was born in Zhejiang, China,
in 1961. He received the B.S. degree in physics from
Heilongjiang University, Harbin, China, in 1982, the
M.S. degree in electronic engineering from Jilin
University, Changchun, China, in 1985, and the
D.Sc. degree in electronic engineering from the
University of Electronic-Communications, Tokyo,
Japan, in 1992. In 1992, he joined the Research
Center for Interface Quantum Electronics and
the Faculty of Engineering, Hokkaido University,
Sapporo, Japan, as a Research Associate. In 1998, he
was an Associate Professor with the Department of Electro-Communications,
University of Electronic Communications, Tokyo. Since 2000, he has been a
Professor with the Institute of Semiconductors, Chinese Academy of Sciences,
Beijing, China. In 2005, he visited the Research Center for Integrated Quantum
Electronics, Hokkaido University, as a Visiting Professor. In 2009, he
became an Honorable Guest Professor at the Research Institute of Electronics,
Shizuoka University, Shizuoka, Japan. His research is in the field of mixedsignal
LSI design and semiconductor quantum devices