LOW POWER CAPACITIVE HUMIDITY SENSOR READOUT IC WITH ON-CHIP TEMPERATURE
SENSOR AND FULL DIGITAL OUTPUT FOR USN APPLICATIONS
Young Chang Jo, Kun Nyun Kim
Tae Yang Nam
Medical IT convergence Research center
KETI(Korea Electronics technology Institute)
SeongNam-Si, South Korea
ycjo@keti.re.kr
Dept. of Electrical Eng. And Computer Science
Korea University
Seoul, South Korea
Abstract— In this paper, a low power CMOS integrated
capacitance-to-frequency converter with on-chip temperature
sensor and fully digital output designed for humidity sensor
interface is newly proposed, which is manufactured by using a
standard 0.35um 2P/3M CMOS process. This ROIC(Readout
IC) consists of on-chip PTAT temperature sensing circuit, low
power temperature-to-frequency converter, low power C-F
converter, digital compensation circuit, OTP memory, control
logic and serial I/O circuits. Capacitance interfacing capability
of the circuit is ultra-wide, up to 350pF, which is higher than
previously reported works1. At a clock speed of 1MHz, the
average power dissipation is as low as about 150uW with one
humidity measurement per one second, which is lower than ever
reported works2,3. And the minimum detectable capacitance is
as low as about 4.5fF.
I. INTRODUCTION
Recently, low cost, low power transducers have been an
attractive research for ubiquitous sensor network(USN)
applications. The USN sensor interface integrated circuits
have gone towards the direction of battery-operated and
portable systems, where the sensing elements and the
processing circuits can be integrated on the same chip or
package to realize multi-function intelligent USN sensors.
In order to fit the demands of commercial market on USN
sensor transducers, the proposed converter, which is
implemented as a capacitance-to-frequency converter, has the
low cost feature. It is due to that a digitized signal is produced
without realizing the analog to digital converter. Hence, the
hardware cost could be reduced. Besides, the output signal of
the proposed transducer is a pulse stream, it could be easily
sent and received over a wide range of transmission media,
such as radio, optical, IR, ultrasonic, and etc. Until now,
several achievements [2][3][4] had been developed. However,
the power consumption of these chips are 895uW and 270uW
respectively. To reduce power consumption of the circuit, a
novel design of a CMOS integrated capacitance-to-frequency
converter with power saving circuit designed for USN
applications is thus inspired and designed.
978-1-4244-5335-1/09/$26.00 ©2009 IEEE
II. CIRCUIT DESIGN
The simplified block diagram of the designed capacitive
type humidity sensor interface circuit with on-chip PTAT
temperature sensor is shown in Figure 1. The main block of
the chip consists of C-F converter, reference timer, PTAT
temperature sensor and oscillator, counting logic, OTP
memory and digital interface logic circuits.
The chip has digital calibration logic for sensor offset and
gain trimming. Under various ambient temperature, the chip
provide actual humidity data with digital calibration logic.
Due to the process and ambient temperature variation, there
could be some fluctuation of the output pulse frequency of the
C-F converter and reference timer circuit. However, all of the
pulse signal of the chip has simultaneous timing derived from
the same original reference pulse signals.
Figure 2 explains how the humidity counting logic works
with respect to the ambient relative humidity, Figure 3 also
shows how the temperature counting logic works with respect
R-Trimming
Humidity
Sensor
1
C-F
Convert
er
Mux
.
0
Freq.
Divid
R-Trimming
PTAT
Temp.
Sensor
AN
D
Ref.
Timer
Oscillator
1
OTP/Reg.
Freq.
Divid
Mux
.
Counte
r
Offset
Coeff.
-
x
I2
C
VDD,
VSS,
SCLK,
SDAT
A
Digital
Code
Output
(0000~
3FFF)
0
C-Trimming
Figure 1. Simplified block diagram of the proposed ROIC for humidity and
temperature sensors
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IEEE SENSORS 2009 Conference
to the ambient temperature. To reduce power dissipation,
the power saving circuit was applied to the analog and digital
blocks which is major power consuming circuits. The
capacitance to frequency converter block and reference timer
block consists of optimized size of CMOS Schmitt trigger
circuits without power-consuming operational amplifiers is
shown in Figure 4. And all the circuit blocks have individual
on/off power saving circuits which can be controlled
whenever the actual operation is needed. Figure 5 shows the
temperature controlled oscillator circuit.
Figure 5. Temperature controlled oscillator circuit
III. CHIP TEST RESULTS
Figure 2. Humidity counting logic works with respect to the ambient relative
humidity
The total chip size is 1.8 mm by 1.9 mm, output resolution
is 14bit for temperature sensor and 12bit for humidity sensor.
The photograph of the chip layout, fabricated chip and hybrid
sensor module are shown in Figure 6. Figure 7 shows the
measured results of output pulses for humidity mode.
Different pulse counts could be observed low humidity and
high humidity mode. Actually 34 pulses observed at low
humidity and 42 pulses observed at high humidity.
Figure 6. Photograph of the chip layout(left), fabricated ROIC (right)
Figure 3. Temperature counting logic works with respect to the ambient
temperature
(a)
Figure 4. Reference timer circuit with CMOS Schmitt trigger
(b)
Figure 7. Measured results of output pulses for humidity mode : (a) 34 pulses
observed at low humidity and (b) 42 pulses observed at high humidity
1355
Humidity code with diff. gain
Humidity output code(a.u.)
65000
(a)
60000
Linear Fit(Linearity : 99.63%, SD : 535.08)
High gain_1st
High gain_2nd
High gain_3rd
High gain_4th
High gain_5th
High gain_6th
Low gain_1st
Low gain_2nd
Low gain_3rd
Low gain_4th
Low gain_5th
Low gain_6th
55000
50000
45000
40000
35000
30000
25000
Linear Fit(Linearity : 99.69%, SD : 1000.26)
20
40
60
80
Relative Humidity (%RH)
Figure 10. Output digital code of ROIC at various relative humidity with
different gain setting
(b)
Figure 8. Measured results of output pulses for temperature mode : (a) 34
pulses observed at room temperature and (b) 39 pulses observed at high
temperature
Humidity digital code (a.u.)
55000
50000
45000
40000
1st measure
2nd measure
3rd measure
4th measure
5th measure
6th measure
35000
20
40
60
1st @40 degree
2nd @40 degree
3rd @40 degree
4th @40 degree
5th @40 degree
6th @40 degree
1st @50 degree
2nd @50 degree
3rd @50 degree
4th @50 degree
5th @50 degree
6th @50 degree
1st @60 degree
2nd @60 degree
3rd @60 degree
4th @60 degree
5th @60 degree
6th @60 degree
Linear Fit(Linearity : 99.69%, SD : 1000.3)
Linear Fit(Linearity : 99.775%, SD : 812.8)
55000
50000
45000
40000
35000
30000
Linear Fit(Linearity : 99.5%, SD : 1183.3)
25000
70
8
20
9
40
10
60
11
80
12
100
Relative Humidity (%RH)
Figure 11. Output digital code at various relative humidity with different
ambient temperature
The relationship between humidity output code and
ambient temperature is shown in Figure 11. The plots
describe the humidity output code decrease as temperature
increase with the slope of about 200 code per unit degree
Celsius. Figure 12 shows the frequency change of the
temperature oscillator output pulse at various ambient
temperature. As shown in Figure 12 output frequency
increase as ambient temperature increase. Figure 13 shows
the summarized relationship between temperature change and
frequency of the temperature oscillator output pulse. Figure
14 shows an experimental output digital code of the proposed
D
Linear Fit
(Linearity : 99.775%, SD : 814.8)
30000
25000
Humidity output code(a.u.)
Figure 8 shows the measured results of output pulses for
temperature mode. Obviously, some different pulse counts
could be observed under the low temperature and high
temperature mode respectively. Actually 34 pulses observed
at room temperature and 39 pulses observed at high
temperature.
Figure 9 shows an experimental final output digital code
of the proposed circuit at various relative humidity, from
20%RH to 80%RH. The linearity of the data is about
99.775% and standard deviation is about 814.8 code. In this
case the humidity resolution is estimated as about
0.00245 %RH. The humidity gain(slope) is calculated as 408
code per unit % of relative humidity change. The proposed
ROIC was designed with gain adjustable circuit structure.
Figure 10 shows output code at various relative humidity with
different gain setting.
60000
D
circuit at various temperature, from -40 C to 120 C.
80
Relative Humidity (% RH)
Figure 9. Output digital code of ROIC at various relative humidity
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CFC : -40 degree
Temperature output code(a.u.)
50000
CFC : 40 degree
45000
40000
35000
1st measure
2nd measure
3rd measure
4th measure
5th measure
6th measure
30000
25000
Linear Fit(Linearity : 99.394%,
SD : 1346)
20000
-60 -40 -20
0
20
40
60
80 100 120 140
Temperature (degrees celsius)
Figure 14. Output digital code of ROIC at various temperature
IV. CONCLUSIONS
CFC : 120 degree
CFC : 80 degree
Figure 12. The frequency change of the temperature oscillator output pulse at
various ambient temperature
The linearity of the data is about 99.394% and standard
deviation is about 1346 code. In this case the temperature
D
Temp. output frequency (KHz)
resolution is estimated as about 0.0058 C. The temperature
gain(slope) is calculated as 172 code per unit degree of
ambient temperature change.
30
25
Temp. output frequency
Linear Fit
(Linearity : 99.7%, SD:0.65)
In this paper, a low power CMOS integrated C-F converter
with on-chip temperature sensor and fully digital output
designed for humidity sensor interface is newly proposed,
which is manufactured by using a standard 0.35um 2P/3M
CMOS process. Capacitance interfacing capability of the
circuit is ultra-wide, up to 350pF. The average power
dissipation is as low as about 150uW with one humidity
measurement per one second. And the minimum detectable
capacitance is as low as about 4.5fF. The total chip size is 1.8
mm by 1.9 mm, output resolution is 14bit for temperature
sensor and 12bit for humidity sensor. The temperature
resolution of the chip is about ±0.0058℃ at 25℃ and the
humidity resolution is ±0.00245 %RH at 25℃. Also, the
proposed ROIC have programmable gain and offset trimming
circuits for humidity/temperature signal calibration.
20
ACKNOWLEDGMENT
15
This work was supported by the IT R&D program of
MKE/IITA, Republic of Korea [2006-S-054-03].
10
-60 -40 -20
0
20
40
60
80 100 120 140
REFERENCES
Temperature (degrees celsius)
Figure 13. The summarized relationship between temperature change and
frequency of the temperature oscillator output pulse
[1] C.T. Chiang, C.S. Wang and Y.C. Huang, pp. 954-957, Proc. IEEE
Sensors, 2007.
[2] P. Bruschi, N. Nizza, and M. Piottto, “A Current-Mode, DualSlope,Integrated Capacitance-to-Pulse Duration Converter,” IEEE J.SolidState Circuits, vol. 42, no. 9, pp. 1884-1891, Sep. 2007.
[3] A. Thanachayanont and S. Sangtong, “Low Voltage Current-Sensing
CMOS Interface Circuit for Piezo-Resistive Pressure Sensor,” ETRI Journal,
vol. 29, no. 1, pp. 70-78, Feb. 2007.
[4] G. Ferri and P. D. Laurentiis, pp 437-441, Sensors and Actuators, vol.
76,1999.
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