A High-Precision CMOS Temperature Sensor with Thermistor ...
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sensors Article A High-Precision CMOS Temperature Sensor with Thermistor Linear Calibration in the (-5 ◦ C, 120 ◦ C) Temperature Range Chua-Chin Wang * ID , Zong-You Hou and Jhih-Cheng You Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung 80424, Taiwan; [email protected] (Z.-Y.H.); [email protected] (J.-C.Y.) * Correspondence: [email protected]; Tel.: +886-7-525-2000 (ext. 4144) Received: 3 June 2018; Accepted: 28 June 2018; Published: 5 July 2018 Abstract: A high-precision Complementary Metal-Oxide-Semiconductor (CMOS) temperature sensor for (-5 ◦ C, 120 ◦ C) temperature range is designed and analyzed in this investigation. The proposed design is featured with a temperature range selection circuit so that the thermistor linear circuit automatically switches to a corresponding calibration loop in light of the temperature range besides the analysis of the calibration method. It resolves the problem that the temperature range of a single thermistor temperature sensor is too small. Notably, the output of the proposed design also attains a high linearity. The measurement results in a thermal chamber justifying that the output voltage is 1.96 V to 4.15 V, the maximum linearity error ≤1.4%, and the worst temperature error ≤1.1 ◦ C in the temperature range of -5 ◦ C to 120 ◦ C. Keywords: thermistor; temperature variation; sensitivity; linear calibration 1. Introduction In recent years, consumer electronic products are often equipped with many sensors to detect the surrounding environmental conditions, e.g., voltage, current, temperature, etc., to facilitate a rapid and appropriate response. Temperature is considered one of the most important environmental factors. For example, high temperature usually leads to increased power consumption and even system breakdown. Industrial sensors utilized in those temperature sensing scenarios are classified into four categories: infrared, thermocouple, resistance thermometer, and thermistor. The infrared spectrum of the measured product is distinct in the different temperatures. Thus, infrared thermometer measures the temperature in light of the infrared spectra [1,2]. Two dissimilar electrical conductors with different temperature coefficients compose a thermocouple, where an electrical junction is formed [3,4]. The thermocouple will generate a weak voltage when the one conductor of thermocouple is applied with a thermal gradient. When the temperature is measured from one conductor, the temperature of the other conductor is derived from the weak voltage according to the Seebeck effect [5]. The resistance thermometer has a linearity feature between the temperature and resistance [6,7]. Thus, the resistance of the resistance thermometer rises as the temperature rises such that the temperature is obtained from the resistance measurement. The sensing principle of the thermistor is very similar as that of the resistance thermometer. The major difference is that its material is mainly made of ceramic polymer. Therefore, the size is smaller and the reaction is faster. Table 1 summarizes features of the above four temperature detection methods. Thermistor is often used for industrially cost-effective applications because it has ruggedness, high sensitivity, accuracy, and low cost. However, the thermistor sensor has a serious flaw, namely, a highly nonlinear resistance-temperature characteristic [8]. The nonlinear characteristic Sensors 2018, 18, 2165; doi:10.3390/s18072165 www.mdpi.com/journal/sensors
Transcript of A High-Precision CMOS Temperature Sensor with Thermistor ...
sensors
Article
A High-Precision CMOS Temperature Sensor withThermistor Linear Calibration in the (−5 ◦C, 120 ◦C)Temperature Range
Chua-Chin Wang * ID , Zong-You Hou and Jhih-Cheng You
Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung 80424, Taiwan;[email protected] (Z.-Y.H.); [email protected] (J.-C.Y.)* Correspondence: [email protected]; Tel.: +886-7-525-2000 (ext. 4144)
Received: 3 June 2018; Accepted: 28 June 2018; Published: 5 July 2018�����������������
Abstract: A high-precision Complementary Metal-Oxide-Semiconductor (CMOS) temperature sensorfor (−5 ◦C, 120 ◦C) temperature range is designed and analyzed in this investigation. The proposeddesign is featured with a temperature range selection circuit so that the thermistor linear circuitautomatically switches to a corresponding calibration loop in light of the temperature range besidesthe analysis of the calibration method. It resolves the problem that the temperature range of a singlethermistor temperature sensor is too small. Notably, the output of the proposed design also attainsa high linearity. The measurement results in a thermal chamber justifying that the output voltage is1.96 V to 4.15 V, the maximum linearity error ≤1.4%, and the worst temperature error ≤1.1 ◦C in thetemperature range of −5 ◦C to 120 ◦C.
Keywords: thermistor; temperature variation; sensitivity; linear calibration
1. Introduction
In recent years, consumer electronic products are often equipped with many sensors to detect thesurrounding environmental conditions, e.g., voltage, current, temperature, etc., to facilitate a rapidand appropriate response. Temperature is considered one of the most important environmentalfactors. For example, high temperature usually leads to increased power consumption and evensystem breakdown. Industrial sensors utilized in those temperature sensing scenarios are classifiedinto four categories: infrared, thermocouple, resistance thermometer, and thermistor. The infraredspectrum of the measured product is distinct in the different temperatures. Thus, infrared thermometermeasures the temperature in light of the infrared spectra [1,2]. Two dissimilar electrical conductors withdifferent temperature coefficients compose a thermocouple, where an electrical junction is formed [3,4].The thermocouple will generate a weak voltage when the one conductor of thermocouple is appliedwith a thermal gradient. When the temperature is measured from one conductor, the temperature ofthe other conductor is derived from the weak voltage according to the Seebeck effect [5]. The resistancethermometer has a linearity feature between the temperature and resistance [6,7]. Thus, the resistanceof the resistance thermometer rises as the temperature rises such that the temperature is obtainedfrom the resistance measurement. The sensing principle of the thermistor is very similar as that of theresistance thermometer. The major difference is that its material is mainly made of ceramic polymer.Therefore, the size is smaller and the reaction is faster. Table 1 summarizes features of the above fourtemperature detection methods.
Thermistor is often used for industrially cost-effective applications because it has ruggedness,high sensitivity, accuracy, and low cost. However, the thermistor sensor has a serious flaw,namely, a highly nonlinear resistance-temperature characteristic [8]. The nonlinear characteristic
Sensors 2018, 18, 2165; doi:10.3390/s18072165 www.mdpi.com/journal/sensors
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will dramatically increase calculation circuit’s area and cost. Many researchers have devoted effortsto resolve this problem, e.g., [9,10]. Sarkar et al. uses an inverting operational amplifier with a linearcorrection resistor to correct the thermistor [10]. However, it has serious temperature error at the lowestand highest temperatures. Bandyopadhyay et al. proposed a linearization scheme for thermistor-basedsensing in biomedical applications [11]. This circuit, however, has large output error due to manyresistors with resistance variations. Kumar et al. showed a block diagram of a neural network basedmethod [12]. However, it is extremely inconvenient in a space-limited scenario due to the demand ofhigh computing power.
Table 1. The comparison table of the temperature sensor methods.
Infrared Thermocouple Resistance Thermometer Thermistor
Sensing temperature range (◦C) −50~400 −200~1200 −200~500 −90~150Volume ≈200 × 50 × 50 mm ≈Ø20 × 2000 mm ≈Ø10 × 2000 mm ≈Ø0.5 × 1 mmReaction Fast Fast Slow Fast
Disadvantage Limited measurementmethod
Need a referencetemperature Expensive Nonlinear
2. Architecture of Proposed Temperature Sensor
Figure 1 shows the proposed temperature sensor in this work, including a thermistor (RBT(T)),a thermistor linearity calibration circuit with switches, and a temperature range auto-selecting circuit.A thermistor linearity calibration circuit with switches is in charge of carrying out predictablelinearization because the characteristic function of the thermistor is nonlinear. However, a singlethermistor senses a relatively narrow temperature range. Besides, the linear correcting output, VBT(T),only attains high linearity when the sensed temperature is close to the selected center temperature.Therefore, the temperature range auto-selecting circuit selects feedback resistance and calibrationresistance automatically to enlarge the sensing temperature range and enhance the linearity bydetecting VBT(T). All of the sub-circuits in Figure 1 will be analyzed theoretically and designedin the following text.
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dramatically increase calculation circuit’s area and cost. Many researchers have devoted efforts to resolve this problem, e.g., [9,10]. Sarkar et al. uses an inverting operational amplifier with a linear correction resistor to correct the thermistor [10]. However, it has serious temperature error at the lowest and highest temperatures. Bandyopadhyay et al. proposed a linearization scheme for thermistor-based sensing in biomedical applications [11]. This circuit, however, has large output error due to many resistors with resistance variations. Kumar et al. showed a block diagram of a neural network based method [12]. However, it is extremely inconvenient in a space-limited scenario due to the demand of high computing power.
Table 1. The comparison table of the temperature sensor methods.
Infrared Thermocouple Resistance
Thermometer Thermistor
Sensing temperature range (°C)
−50~400 −200~1200 −200~500 −90~150
Volume ≈200 × 50 × 50 mm ≈Ø20 × 2000 mm ≈Ø10 × 2000 mm ≈Ø0.5 × 1 mm Reaction Fast Fast Slow Fast
Disadvantage Limited measurement method
Need a reference temperature Expensive Nonlinear
2. Architecture of Proposed Temperature Sensor
Figure 1 shows the proposed temperature sensor in this work, including a thermistor (RBT(T)), a thermistor linearity calibration circuit with switches, and a temperature range auto-selecting circuit. A thermistor linearity calibration circuit with switches is in charge of carrying out predictable linearization because the characteristic function of the thermistor is nonlinear. However, a single thermistor senses a relatively narrow temperature range. Besides, the linear correcting output, VBT(T), only attains high linearity when the sensed temperature is close to the selected center temperature. Therefore, the temperature range auto-selecting circuit selects feedback resistance and calibration resistance automatically to enlarge the sensing temperature range and enhance the linearity by detecting VBT(T). All of the sub-circuits in Figure 1 will be analyzed theoretically and designed in the following text.
S_1
Thermistor linearity
calibration circuit with switches
sw1
Temperature
VBT_in
RBT(T)
sw3sw2
reset
VHVL
sw_int
S_0
Temperature range
auto-selecting circuit
pre_Q0pre_Q1
VBT(T)
Figure 1. The block diagram of the proposed temperature sensor.
2.1. Thermistor Linearity Calibration Circuit with Switches
Referring to Figure 2, thermistor linearity calibration circuit with switches is composed of six switches, three calibration resistors, Rcal1, Rcal2, and Rcal3, three feedback resistors, Rf1, Rf2, and Rf3, and two operational amplifiers. Notably, VBT_in is an external DC bias. Thermistor linearity calibration
Figure 1. The block diagram of the proposed temperature sensor.
2.1. Thermistor Linearity Calibration Circuit with Switches
Referring to Figure 2, thermistor linearity calibration circuit with switches is composed of sixswitches, three calibration resistors, Rcal1, Rcal2, and Rcal3, three feedback resistors, Rf1, Rf2, and Rf3,and two operational amplifiers. Notably, VBT_in is an external DC bias. Thermistor linearity calibration
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circuit with switches is used to enhance the linearity of the sensing voltage of the thermistor and rejectthe noise coupled from others. Take sw1 turn-on and sw2, sw3 turn-off as an example. Figure 2 thenbecomes Figure 3. The resistance of the thermistor is shown as follows:
RBT(T) = R0eβ( 1T −
1T0
) (1)
where R0 is the resistance of the thermistor at the 25 ◦C, β is the temperature constant of the thermistor,T is the sensed temperature, and T0 is 25 ◦C. VBT(T) can be written as Equation (2):
VBT(T) = VBT_in ×[
1 +Rf1
RBT(T) + Rcal1
]. (2)
All the terms in Taylor series expansion of Equation (2) are truncated except the first three terms.VBT(T) is re-organized as follows:
VBT(T) = VBT(Tc) + (T − Tc)VBT′(Tc) +(T − T c)
2
2!VBT′′ (Tc) (3)
where Tc is the central temperature of the selected temperature range. The coefficient of the quadraticterm needs to be zero to linearize VBT(T). Thus, the coefficient of the quadratic term is expressed asEquation (4):
d2
dt2 VBT(T)|T = Tc =d2
dt2
{VBT_in ×
[1 +
Rf1RBT(Tc) + Rcal1
]}= 0. (4)
Equation (4) only needs to deal with the rightmost term where VBT_in is a constant. The quadraticdifferential of Equation (4) will be written as Equation (5):
d2
dt2 (1 +Rf1
RBT(Tc)+Rcal1)|T = Tc =
Rf1
[Rcal1 + RBT(Tc)R
′′BT(Tc) − 2R′BT(Tc)
2]
[Rcal1 + RBT(Tc)]3 |T = Tc = 0. (5)
When Equation (5) holds, the numerator must be zero as follows:
Rcal1 + RBT(Tc)R′′BT(Tc) − 2R′BT(Tc)
2 = 0. (6)
Rcal1 is then derived as follows:
Rcal1 =2R′BT(Tc)
2
R′′BT(Tc)− RBT(Tc). (7)
Rcal1 is re-organized as Equation (8) according to Equation (1):
Rcal1 =β − 2·Tc
β + 2·Tc× RBT(Tc). (8)
Namely, the coefficient of the quadratic term will become 0 by selecting Rcal1 with an appropriateresistance. Besides, this work adds more calibration resistors and feedback resistors to enlarge thesensing temperature and maintain the high linearity of the VBT(T). Therefore, thermistor linearitycalibration circuit with switches changes sensing temperature range by selecting the appropriatecalibration resistor and feedback resistor. Referring to Equation (2), RBT(T) is derived to attain thefollowing result:
RBT(T) =Rf1(
VBT(T)VBT_in
)− 1− Rcal1. (9)
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According to Equations (2), (8), and (9), temperature (T) can be indirectly estimated by RBT(T)based on this conclusion:
T =1 ln(
Rf1(VBT(T)/VBT_in)−1
− Rcal1
R0)
β
+ 1t0
. (10)
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T=1
ln(
Rf1( VBT(T) VBT_in⁄ )-1 - Rcal1
R0)β + 1
t0
. (10)
R cal2
R f2
VBT_in
R cal3
R f3
R BT(T)
R cal1
R f1
VBT(T)
sw1
sw2
sw3
sw1
sw2
sw3
Thermistor linearity calibration circuit with switches
Figure 2. Schematic of thermistor linearity calibration circuit with switches.
VBT_in
R BT(T) R cal1
R f1
VBT(T)
Simplified Thermistor linearity calibration circuit
Figure 3. Schematic of simplified thermistor linearity calibration circuit. (sw1 on, and sw2 and sw3 off).
Figure 4 shows the illustration of the output of thermistor linearity calibration circuit with one of the three switches turned on. Namely, three different output voltage curves (VBT1, VBT2, and VBT3) are corresponding to the output voltage curves of VBT(T) in −5–35 ℃, 30–80 ℃, and 75–120 ℃, separately, in our design. VH (voltage high) and VL (voltage low), which are external DC bias voltages, are the reference voltages of temperature range auto-selecting circuit. VH is set to the maximum voltage of VBT2 and VBT3. By contrast, VL is set to the minimum voltage of VBT1 and VBT2. If VBT(T) > VH or VBT(T) < VL, temperature range auto-selecting circuit turn off the current switch and turn on another switch depending on VBT(T) higher than VH or lower than VL. On the other hand, when the VBT(T) is between VH and VL, it means that the thermistor linearity calibration circuit has been selected to the correct sensing temperature range. In addition, the sensing temperature ranges t1~t2 and t3~t4 are overlapping sensing temperature ranges of VBT3, VBT2, and VBT1, respectively. If there is no overlapping between two adjacent ranges, a bouncing hazard will
Figure 2. Schematic of thermistor linearity calibration circuit with switches.
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T=1
ln(
Rf1( VBT(T) VBT_in⁄ )-1 - Rcal1
R0)β + 1
t0
. (10)
R cal2
R f2
VBT_in
R cal3
R f3
R BT(T)
R cal1
R f1
VBT(T)
sw1
sw2
sw3
sw1
sw2
sw3
Thermistor linearity calibration circuit with switches
Figure 2. Schematic of thermistor linearity calibration circuit with switches.
VBT_in
R BT(T) R cal1
R f1
VBT(T)
Simplified Thermistor linearity calibration circuit
Figure 3. Schematic of simplified thermistor linearity calibration circuit. (sw1 on, and sw2 and sw3 off).
Figure 4 shows the illustration of the output of thermistor linearity calibration circuit with one of the three switches turned on. Namely, three different output voltage curves (VBT1, VBT2, and VBT3) are corresponding to the output voltage curves of VBT(T) in −5–35 ℃, 30–80 ℃, and 75–120 ℃, separately, in our design. VH (voltage high) and VL (voltage low), which are external DC bias voltages, are the reference voltages of temperature range auto-selecting circuit. VH is set to the maximum voltage of VBT2 and VBT3. By contrast, VL is set to the minimum voltage of VBT1 and VBT2. If VBT(T) > VH or VBT(T) < VL, temperature range auto-selecting circuit turn off the current switch and turn on another switch depending on VBT(T) higher than VH or lower than VL. On the other hand, when the VBT(T) is between VH and VL, it means that the thermistor linearity calibration circuit has been selected to the correct sensing temperature range. In addition, the sensing temperature ranges t1~t2 and t3~t4 are overlapping sensing temperature ranges of VBT3, VBT2, and VBT1, respectively. If there is no overlapping between two adjacent ranges, a bouncing hazard will
Figure 3. Schematic of simplified thermistor linearity calibration circuit. (sw1 on, and sw2 and sw3 off).
Figure 4 shows the illustration of the output of thermistor linearity calibration circuit with one ofthe three switches turned on. Namely, three different output voltage curves (VBT1, VBT2, and VBT3)are corresponding to the output voltage curves of VBT(T) in −5–35 , 30–80 , and 75–120 , separately,in our design. VH (voltage high) and VL (voltage low), which are external DC bias voltages, are thereference voltages of temperature range auto-selecting circuit. VH is set to the maximum voltage ofVBT2 and VBT3. By contrast, VL is set to the minimum voltage of VBT1 and VBT2. If VBT(T) > VHor VBT(T) < VL, temperature range auto-selecting circuit turn off the current switch and turn onanother switch depending on VBT(T) higher than VH or lower than VL. On the other hand, whenthe VBT(T) is between VH and VL, it means that the thermistor linearity calibration circuit has beenselected to the correct sensing temperature range. In addition, the sensing temperature ranges t1~t2and t3~t4 are overlapping sensing temperature ranges of VBT3, VBT2, and VBT1, respectively. If thereis no overlapping between two adjacent ranges, a bouncing hazard will occur between adjacent rangeselections when the temperature is close to the switch point of two adjacent ranges.
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occur between adjacent range selections when the temperature is close to the switch point of two adjacent ranges.
VBT1
Temperature (℃)
VB
T(T
)
VL
VH
VBT2VBT3
t1 t2 t3 t4
sw1 offsw2 offsw3 on
sw1 offsw2 onsw3 off
sw1 onsw2 offsw3 off
(V)
Figure 4. The illustration of VBT(T).
2.2. Temperature Range Auto-Selecting Circuit (TRASC)
The temperature range auto-selecting circuit is shown in Figure 5, where two operating modes are used. The reset mode sets the initial state of temperature range auto-selecting circuit. The other is work mode, where VBT(T) is compared with VH and VL to auto-select corresponding resistors and enter the pre-defined sensing temperature range. The functionality of temperature range auto-selecting circuit will be described in detail in the following text.
pre_Q1
S _0S _1
shift register
reset
Q0
Q1
sw3
2 to 4 decoder
Cx
sw2
sw1
S x Rising E dge Detector
enVH
VL
VBT(T)
pre_Q0
Rising E dge Detector
Rising E dge Detector
comp1
comp2
Sr
sw_int
Figure 5. Schematic of temperature range auto-selecting circuit.
• Reset Mode
Figure 4. The illustration of VBT(T).
2.2. Temperature Range Auto-Selecting Circuit (TRASC)
The temperature range auto-selecting circuit is shown in Figure 5, where two operating modes areused. The reset mode sets the initial state of temperature range auto-selecting circuit. The other is workmode, where VBT(T) is compared with VH and VL to auto-select corresponding resistors and enterthe pre-defined sensing temperature range. The functionality of temperature range auto-selectingcircuit will be described in detail in the following text.
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occur between adjacent range selections when the temperature is close to the switch point of two adjacent ranges.
VBT1
Temperature (℃)
VB
T(T
)
VL
VH
VBT2VBT3
t1 t2 t3 t4
sw1 offsw2 offsw3 on
sw1 offsw2 onsw3 off
sw1 onsw2 offsw3 off
(V)
Figure 4. The illustration of VBT(T).
2.2. Temperature Range Auto-Selecting Circuit (TRASC)
The temperature range auto-selecting circuit is shown in Figure 5, where two operating modes are used. The reset mode sets the initial state of temperature range auto-selecting circuit. The other is work mode, where VBT(T) is compared with VH and VL to auto-select corresponding resistors and enter the pre-defined sensing temperature range. The functionality of temperature range auto-selecting circuit will be described in detail in the following text.
pre_Q1
S _0S _1
shift register
reset
Q0
Q1
sw3
2 to 4 decoder
Cx
sw2
sw1
S x Rising E dge Detector
enVH
VL
VBT(T)
pre_Q0
Rising E dge Detector
Rising E dge Detector
comp1
comp2
Sr
sw_int
Figure 5. Schematic of temperature range auto-selecting circuit.
• Reset Mode
Figure 5. Schematic of temperature range auto-selecting circuit.
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• Reset Mode
When reset is low, Q1 and Q0 will be asserted low. By contrast, when reset is pulled up high andsw_int is dropped to low, Sx will be asserted low, sw1 and sw3 will be off, and sw2 will be on. Thus,VBT(T) follows the transfer function of VBT2. Finally, if sw_int is pulled up high, Temperature rangeauto-selecting circuit enters the work mode.
• Work Mode
Temperature range auto-selecting circuit keeps tracking VBT(T) vs. VH and VL to determinewhich switch to turn on. If TRASC is kept in the work mode, Thermistor linearity calibration circuitwith switches will be changed to appropriate temperature range according to the following rules.
� VBT(T) > VH: When VBT(T) is higher than VH, the sensing temperature region will change fromVBT3 to VBT2 or VBT2 to VBT1 until VL < VBT(T) < VH. Notably, when VBT(T) is in the VBT1,the sensing temperature region stays the same.
� VL < VBT(T) < VH: When VBT(T) is between VH and VL, the state is kept in sametemperature range.
� VBT(T) < VL: When VBT(T) is lower than VL, the sensing temperature region will change fromVBT1 to VBT2 or VBT2 to VBT3 until VL < VBT(T) < VH. Notably, when VBT(T) is in the VBT3,the sensing temperature region stays the same.
In short, Figure 6 shows the flow chart for TRASC how to select which switch be turned on.
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When reset is low, Q1 and Q0 will be asserted low. By contrast, when reset is pulled up high and sw_int is dropped to low, Sx will be asserted low, sw1 and sw3 will be off, and sw2 will be on. Thus, VBT(T) follows the transfer function of VBT2. Finally, if sw_int is pulled up high, Temperature range auto-selecting circuit enters the work mode.
• Work Mode
Temperature range auto-selecting circuit keeps tracking VBT(T) vs. VH and VL to determine which switch to turn on. If TRASC is kept in the work mode, Thermistor linearity calibration circuit with switches will be changed to appropriate temperature range according to the following rules.
VBT(T) > VH: When VBT(T) is higher than VH, the sensing temperature region will change from VBT3 to VBT2 or VBT2 to VBT1 until VL < VBT(T) < VH. Notably, when VBT(T) is in the VBT1, the sensing temperature region stays the same.
VL < VBT(T) < VH: When VBT(T) is between VH and VL, the state is kept in same temperature range.
VBT(T) < VL: When VBT(T) is lower than VL, the sensing temperature region will change from VBT1 to VBT2 or VBT2 to VBT3 until VL < VBT(T) < VH. Notably, when VBT(T) is in the VBT3, the sensing temperature region stays the same.
In short, Figure 6 shows the flow chart for TRASC how to select which switch be turned on.
S ensing start
Thermistor linear calibration
Comparing VBT(T) with VH, VL
Which switch on ? Which switch on ?Which switch on ?
VBT(T) > VH
VL< VBT(T) < VH
VBT(T) < VL
sw3 is turned on and others are turned off(Temperature of DUT
< 30 °C )
sw2 is turned on and others are turned off
(30 °C < Temperature of DUT < 80 °C )
sw1 is turned on and others are turned off(Temperature of DUT
> 80 °C )
sw3 on
sw2 on
sw1 or sw2 on
sw1 on
sw2 or sw3 on
sw3 on sw1 on
It is not from VL< VBT(T) < VH
Note : DUT = Device Under Test
Figure 6. The flowchart of temperature range auto-selecting circuit (TRASC).
A total of four states are required for temperature range auto-selecting circuit, which are S00, S01, S10, and S11. They are, respectively, described as shown in Figure 7. Referring to Figure 5 again, the inputs of this state machine is S_1 and S_0. By contrast, the outputs of this state machine is Q1 and Q0.
Figure 6. The flowchart of temperature range auto-selecting circuit (TRASC).
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A total of four states are required for temperature range auto-selecting circuit, which are S00, S01,S10, and S11. They are, respectively, described as shown in Figure 7. Referring to Figure 5 again, theinputs of this state machine is S_1 and S_0. By contrast, the outputs of this state machine is Q1 and Q0.
� S00 mode: sw1 is turned on and others are turned off. VBT(T) is in VBT1.� S01 and S10 mode: sw2 is turned on and others are turned off. VBT(T) is in VBT2.� S11 mode: The sw3 is turned on and others are turned off. VBT(T) is in VBT3.
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S00 mode: sw1 is turned on and others are turned off. VBT(T) is in VBT1. S01 and S10 mode: sw2 is turned on and others are turned off. VBT(T) is in VBT2. S11 mode: The sw3 is turned on and others are turned off. VBT(T) is in VBT3.
S00 S01 S10 S1100/00 11/01
00/10
11/11
01/01
01/10
00/11, 01/11
01/00,11/00
00/01
11/10 S_1, S_0/Q1, Q0
input/output
Figure 7. State machine of temperature range auto-selecting circuit.
3. Implementation and Measurement
The proposed temperature sensor is carried out using TSMC 0.5 μm CMOS high voltage mixed-signal based LDMOS USGAL 2P3M polycide (T50UHV). Figure 8 shows the die photo of the proposed temperature sensor, where the entire chip area is 1933.52 μm × 1703.22 μm, and the core area is 710 μm × 460 μm. The measurement setup of the proposed design on silicon is shown in Figure 9. The thermistor is placed in the programmable compact temperature and humidity chamber (MHK-120 [13], TERCHY, Nantou, Taiwan). The power supply (E3631A DC Power Supply, Rohde and Schwarz, Munich, Germany) is in charge of the required supply voltages. The oscilloscope (Agilent 54855A) is used to demonstrate VBT(T).
Figure 10 shows the scenario of three switches when the ambient temperature is changed: (a) from 35 °C up to 40 °C and (b) from 80 °C up to 85 °C. Referring to Figure 10, when the ambient temperature changes from low to high, the switch is turned on in sequence from sw3 to sw2 and then to sw1. By contrast, the scenario of the three switches, when the ambient temperature varies: (a) from 70 °C down to 65 °C and (b) from 30 °C down to 25 °C, is shown in Figure 11. Referring to Figure 11 again, when the ambient temperature changes from high to low, the switch is turned on in sequence from sw1 to sw2 and then to sw3. To make sure that the proposed chip is functional in the range of −5 °C to 120 °C, a total of 26 temperature cases, i.e., −5, 0, 5, …, 120 °C, are tested and measured. Figure 12 summarizes the measurement result of the VBT(T). The maximum error with respect to a linear asymptotic line is as low as 1.4%. Then, the error of the sensed temperature compared with the real temperature is attained in Figure 13. The maximum error with respect to real temperature is as low as 1.1 °C (≈3.1%).
Figure 7. State machine of temperature range auto-selecting circuit.
3. Implementation and Measurement
The proposed temperature sensor is carried out using TSMC 0.5 µm CMOS high voltagemixed-signal based LDMOS USGAL 2P3M polycide (T50UHV). Figure 8 shows the die photo ofthe proposed temperature sensor, where the entire chip area is 1933.52 µm × 1703.22 µm, and thecore area is 710 µm × 460 µm. The measurement setup of the proposed design on silicon is shown inFigure 9. The thermistor is placed in the programmable compact temperature and humidity chamber(MHK-120 [13], TERCHY, Nantou, Taiwan). The power supply (E3631A DC Power Supply, Rohde andSchwarz, Munich, Germany) is in charge of the required supply voltages. The oscilloscope (Agilent54855A) is used to demonstrate VBT(T).
Figure 10 shows the scenario of three switches when the ambient temperature is changed: (a) from35 ◦C up to 40 ◦C and (b) from 80 ◦C up to 85 ◦C. Referring to Figure 10, when the ambient temperaturechanges from low to high, the switch is turned on in sequence from sw3 to sw2 and then to sw1.By contrast, the scenario of the three switches, when the ambient temperature varies: (a) from 70 ◦Cdown to 65 ◦C and (b) from 30 ◦C down to 25 ◦C, is shown in Figure 11. Referring to Figure 11 again,when the ambient temperature changes from high to low, the switch is turned on in sequence fromsw1 to sw2 and then to sw3. To make sure that the proposed chip is functional in the range of −5 ◦C to120 ◦C, a total of 26 temperature cases, i.e., −5, 0, 5, . . . , 120 ◦C, are tested and measured. Figure 12summarizes the measurement result of the VBT(T). The maximum error with respect to a linearasymptotic line is as low as 1.4%. Then, the error of the sensed temperature compared with the realtemperature is attained in Figure 13. The maximum error with respect to real temperature is as low as1.1 ◦C (≈3.1%).
Sensors 2018, 18, 2165 8 of 12Sensors 2018, 18, x 8 of 12
1933.52 μm
1703.22 μm
Temperature range auto-selecting circuit
Thermistor linearitycalibration circuit with switches
460 μm
710 μm
Figure 8. Die photo of the proposed temperature sensor.
Agilent 54855A
VDDVBT(T)
Programmable compact temperature & humidity chamber
Rohde & S chwarz E 3631A
Printed Circuit Board
R BT(T)
sw2
sw1
sw3
VSS
Figure 9. Measurement setup of the proposed Temperature sensor.
Four chips of the proposed temperature sensor are used to carry out the measurements, where each one was under five trials of 26 temperature tests. Figure 14 shows the summary of these
Figure 8. Die photo of the proposed temperature sensor.
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1933.52 μm
1703.22 μmTemperature range auto-
selecting circuit
Thermistor linearitycalibration circuit with switches
460 μm
710 μm
Figure 8. Die photo of the proposed temperature sensor.
Agilent 54855A
VDDVBT(T)
Programmable compact temperature & humidity chamber
Rohde & S chwarz E 3631A
Printed Circuit Board
R BT(T)
sw2
sw1
sw3
VSS
Figure 9. Measurement setup of the proposed Temperature sensor.
Four chips of the proposed temperature sensor are used to carry out the measurements, where each one was under five trials of 26 temperature tests. Figure 14 shows the summary of these
Figure 9. Measurement setup of the proposed Temperature sensor.
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Four chips of the proposed temperature sensor are used to carry out the measurements, where eachone was under five trials of 26 temperature tests. Figure 14 shows the summary of these measurementresults. The standard deviation of measurement results is smaller than ±3σ as shown in Figure 15.
The performance comparison of the proposed design and several recent works is tabulated inTable 2. Notably, the proposed design attains the widest sensing temperature range, which is the rangeof −5 ◦C to 120 ◦C. Notably, the maximum error 1.1 ◦C, which is also the best to date.
Table 2. Performance comparison of temperature sensors.
[14] [10] [11] [15] This Work
Year 2009TIM
2013SJ
2016SJ
2017HNICEM 2018
Implementation PCB PCB PCB PCB T50UHV CMOSPower (mW) N/A N/A N/A N/A 12
Output of Voltage (V) N/A 2~4.5 N/A N/A 1.9~4.1Max. Linearity Error (%) 1.7 3 1.3 9.05 1.4
Sensing temperature range (◦C) 0~120 30~120 30~110 −10~100 −5~120Sensitivity (mV/◦C) N/A 27 N/A N/A 17.6
Temperature Error (◦C) 2 2.7 1.1 2 1.1Reliability N/A N/A N/A N/A ≤ ±3σ
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measurement results. The standard deviation of measurement results is smaller than ±3σ as shown in Figure 15.
The performance comparison of the proposed design and several recent works is tabulated in Table 2. Notably, the proposed design attains the widest sensing temperature range, which is the range of −5 °C to 120 °C. Notably, the maximum error 1.1 °C, which is also the best to date.
Table 2. Performance comparison of temperature sensors.
[14] [10] [11] [15] This Work
Year 2009 TIM
2013 SJ
2016 SJ
2017 HNICEM
2018
Implementation PCB PCB PCB PCB T50UHV CMOS Power (mW) N/A N/A N/A N/A 12
Output of Voltage (V) N/A 2~4.5 N/A N/A 1.9~4.1 Max. Linearity Error (%) 1.7 3 1.3 9.05 1.4
Sensing temperature range (°C) 0~120 30~120 30~110 −10~100 −5~120 Sensitivity (mV/°C) N/A 27 N/A N/A 17.6
Temperature Error (°C) 2 2.7 1.1 2 1.1 Reliability N/A N/A N/A N/A ≤ ±3σ
(a) (b)
sw1=0→1sw1=0
sw2=0→1
VBT(T)
sw3=1→0
VBT(T)
sw2=0→1
sw3=0
Figure 10. The scenario of three switches when the ambient temperature: (a) 35–40 °C and (b) 80–85 °C.
(a) (b)
sw1=0sw1=1→0
sw2=0→1
VBT(T)
sw3=0
VBT(T)
sw2=1→0
sw3=0→1
Figure 11. The scenario of three switches when the ambient temperature (a) 70–65 °C and (b) 30–25 °C.
Figure 10. The scenario of three switches when the ambient temperature: (a) 35–40 ◦C and (b) 80–85 ◦C.
Sensors 2018, 18, x 9 of 12
measurement results. The standard deviation of measurement results is smaller than ±3σ as shown in Figure 15.
The performance comparison of the proposed design and several recent works is tabulated in Table 2. Notably, the proposed design attains the widest sensing temperature range, which is the range of −5 °C to 120 °C. Notably, the maximum error 1.1 °C, which is also the best to date.
Table 2. Performance comparison of temperature sensors.
[14] [10] [11] [15] This Work
Year 2009 TIM
2013 SJ
2016 SJ
2017 HNICEM
2018
Implementation PCB PCB PCB PCB T50UHV CMOS Power (mW) N/A N/A N/A N/A 12
Output of Voltage (V) N/A 2~4.5 N/A N/A 1.9~4.1 Max. Linearity Error (%) 1.7 3 1.3 9.05 1.4
Sensing temperature range (°C) 0~120 30~120 30~110 −10~100 −5~120 Sensitivity (mV/°C) N/A 27 N/A N/A 17.6
Temperature Error (°C) 2 2.7 1.1 2 1.1 Reliability N/A N/A N/A N/A ≤ ±3σ
(a) (b)
sw1=0→1sw1=0
sw2=0→1
VBT(T)
sw3=1→0
VBT(T)
sw2=0→1
sw3=0
Figure 10. The scenario of three switches when the ambient temperature: (a) 35–40 °C and (b) 80–85 °C.
(a) (b)
sw1=0sw1=1→0
sw2=0→1
VBT(T)
sw3=0
VBT(T)
sw2=1→0
sw3=0→1
Figure 11. The scenario of three switches when the ambient temperature (a) 70–65 °C and (b) 30–25 °C. Figure 11. The scenario of three switches when the ambient temperature (a) 70–65 ◦C and (b) 30–25 ◦C.
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2.00
2.50
3.00
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4.00
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-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
VBT
(T)
Temperature (oC)
Voltage of VBT(T)Measurement of VBT1
Measurement of VBT2
Measurement of VBT3
Linear regression of VBT1
Linear regression of VBT2
Linear regression of VBT3Max. linearity error -1.4%
Linear regression of VBT3y=0.05*x+2.35R^2=0.98
Linear regression of VBT2y=0.043*x+0.65R^2=0.99
Linear regression of VBT1y=0.028*x-0.17R^2=0.99
(V)
Figure 12. The measurement result of the VBT(T) with respect to a linear asymptotic line.
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-0.5
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-0.1
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Erro
r
Temperature (oC)Max. temperature error of measurement -1.1 ℃
(oC)
Figure 13. The measurement results of the proposed temperature sensor.
Figure 12. The measurement result of the VBT(T) with respect to a linear asymptotic line.
Sensors 2018, 18, x 10 of 12
1.50
2.00
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3.00
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VBT
(T)
Temperature (oC)
Voltage of VBT(T)Measurement of VBT1
Measurement of VBT2
Measurement of VBT3
Linear regression of VBT1
Linear regression of VBT2
Linear regression of VBT3Max. linearity error -1.4%
Linear regression of VBT3y=0.05*x+2.35R^2=0.98
Linear regression of VBT2y=0.043*x+0.65R^2=0.99
Linear regression of VBT1y=0.028*x-0.17R^2=0.99
(V)
Figure 12. The measurement result of the VBT(T) with respect to a linear asymptotic line.
-1.3
-1.1
-0.9
-0.7
-0.5
-0.3
-0.1
0.1
0.3
0.5
0.7
0.9
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1.3
-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
Erro
r
Temperature (oC)Max. temperature error of measurement -1.1 ℃
(oC)
Figure 13. The measurement results of the proposed temperature sensor.
Figure 13. The measurement results of the proposed temperature sensor.
Sensors 2018, 18, x 10 of 12
1.50
2.00
2.50
3.00
3.50
4.00
4.50
-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
VBT
(T)
Temperature (oC)
Voltage of VBT(T)Measurement of VBT1
Measurement of VBT2
Measurement of VBT3
Linear regression of VBT1
Linear regression of VBT2
Linear regression of VBT3Max. linearity error -1.4%
Linear regression of VBT3y=0.05*x+2.35R^2=0.98
Linear regression of VBT2y=0.043*x+0.65R^2=0.99
Linear regression of VBT1y=0.028*x-0.17R^2=0.99
(V)
Figure 12. The measurement result of the VBT(T) with respect to a linear asymptotic line.
-1.3
-1.1
-0.9
-0.7
-0.5
-0.3
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0.1
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-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
Erro
r
Temperature (oC)Max. temperature error of measurement -1.1 ℃
(oC)
Figure 13. The measurement results of the proposed temperature sensor.
Figure 14. The summary of measurement results of the proposed temperature sensor.
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Figure 14. The summary of measurement results of the proposed temperature sensor.
Figure 15. The standard deviation of the measurement results.
4. Conclusions
In this paper, a temperature sensor with thermistor linear calibration with automatic temperature range selection is proposed. Detailed schematic design and analysis of the proposed design are given as well. The proposed design can extend sensing temperature range through a temperature range auto-selecting circuit. In addition, the measurement result of the proposed output demonstrates a high linearity, where the linearity error is smaller than 1.4%. Moreover, not only are the sensitivity and the sensing temperature range enhanced, the temperature error, which is less than 1.1 °C, is also better than that of the traditional temperature sensors by far.
Author Contributions: Z.-Y.H. and J.-C.Y. contributed to design, implementation and measurement of the proposed temperature sensor. C.-C.W. and Z.-Y.H. wrote the manuscript of this work. C.-C.W. conducted an internal review as the project leader. All authors contributed in conceiving of analyzing data and performing experiments.
Funding: This research was partially funded by Ministry of Science and Technology (MOST), Taiwan, under grant MOST 106-2221-E-110-065-, MOST 107-2218-E-110-004-, and MOST 107-2912-I-110-507-.
Acknowledgments: This research was partially supported by Ministry of Science and Technology under grant MOST 106-2221-E-110-065-, MOST 107-2218-E-110-004-, and MOST 107-2912-I-110-507-. The authors would like to express their deepest gratefulness to CIC (Chip Implementation Center) of NARL (Nation applied Research Laboratories), Taiwan, for the thoughtful chip fabrication service.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Zhang, Y.; Yao, Z.; Yang, Y.; Zhang, L.; Pan, X. A verification method of thermo-infrared remote sensing temperature retrieval algorithm with a CFD model. Am. J. Geogr. Res. Rev. 2018, 1, 1–10.
2. Wang, F.; Qin, Z.; Song, C.; Tu, L.; Karnieli, A.; Zhao, S. An improved mono-window algorithm for land surface temperature retrieval from Landsat 8 thermal infrared sensor data. Remote Sens. 2015, 7, 4268–4289.
3. Mukherjee, A.; Sarkar, D.; Sen, A.; Dey, D.; Munshi, S. An analog signal conditioning circuit for thermocouple temperature sensor employing thermistor for cold junction compensation. In Proceedings of the International Conference on Control, Automation, Robotics and Embedded Systems (CARE), Jabalpur, India, 16–18 December 2013; pp. 1–5.
Figure 15. The standard deviation of the measurement results.
4. Conclusions
In this paper, a temperature sensor with thermistor linear calibration with automatic temperaturerange selection is proposed. Detailed schematic design and analysis of the proposed design are givenas well. The proposed design can extend sensing temperature range through a temperature rangeauto-selecting circuit. In addition, the measurement result of the proposed output demonstrates a highlinearity, where the linearity error is smaller than 1.4%. Moreover, not only are the sensitivity and thesensing temperature range enhanced, the temperature error, which is less than 1.1 ◦C, is also betterthan that of the traditional temperature sensors by far.
Author Contributions: Z.-Y.H. and J.-C.Y. contributed to design, implementation and measurement of theproposed temperature sensor. C.-C.W. and Z.-Y.H. wrote the manuscript of this work. C.-C.W. conductedan internal review as the project leader. All authors contributed in conceiving of analyzing data andperforming experiments.
Funding: This research was partially funded by Ministry of Science and Technology (MOST), Taiwan, under grantMOST 106-2221-E-110-065-, MOST 107-2218-E-110-004-, and MOST 107-2912-I-110-507-.
Acknowledgments: This research was partially supported by Ministry of Science and Technology under grantMOST 106-2221-E-110-065-, MOST 107-2218-E-110-004-, and MOST 107-2912-I-110-507-. The authors would liketo express their deepest gratefulness to CIC (Chip Implementation Center) of NARL (Nation applied ResearchLaboratories), Taiwan, for the thoughtful chip fabrication service.
Conflicts of Interest: The authors declare no conflict of interest.
References
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