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IEEE JOURNAL OF SOLIII-STATE CIRCUITS, VOL. 31, NO . 7, JULY 1996

Supply voltageAverage supply current

(3V, 2 samples/sec)RangeError

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Micropower CMOS TemperatureSensor with Digital OutputAnton Bakker and Johan H.

Abstract-A CMOS smart temperature sensor with digital out-put is presented. It consumes only 7 gW. Toachieve this extremelylow-power consumption, the system is equipped with a facilitythat switches off the supply power after each sample. The circuituses substrate bipolars as a temperature sensor. Conversion to thedigital domain is done by a sigma-delta converter which makesthe circuit highly insensitive to digital interference. The completesystem is realized in a standard CMOS process and measuresonly 1.5 mm2. In the temperature range from -40 to +12QC,the inaccuracy is 3 ~ 1 C fter calibration at two temperatures.The circuit operates at supply voltages down to 2.2 V.

H ui j s ing , Senior Member, IEEE

TABLE ITARGET PECIFICATIONSI Min. I Nom. I Max. I Unit

I. INTRODUCTIONOW-COST' high-performance temperature sensors areL ncreasingly required for the following applications: 1)on VLSI chips to control the dissipation; 2 ) in microsystems to

compensate for temperature cross sensitivity of other sensors;3) in strongly automated production plants; and 4) in auto-mated consumer products like cars and domestic appliances.

Next to the demands on costs and performance, i t is alsoneeded to reduce the power consumption of the temperaturesystem to a minimum . The reasons fo r this are to be able to usethe sensor in battery operated products, to reduce the errorscaused by self-heating, and to be able to add the sensor toa VLSI chip without causing a significant increase in powerconsumption of the complete system.The micropower CMOS smart temperature sensor describedhere perfectly fits in this low-cost, high-performance, and low-power market. It has its application in the automotive industry,where it is meant to measure the temperature in car tires.This is first for safety reasons and second to compensate forthe temperature sensitivity of a pressure sensor which is alsolocated in the tire.

Krummenacher and Oguey [ l ] presented a smart CMOStemperature sensor with a power consumption of 120 pW.For our application, however, the power consumption shouldnot exceed 10 p W . This is required to guarantee a lifetimeof the battery of at least seven years, which is equal to th emaximum lifetime of the tire.

Another important disadvantage of the circuit of Krummen-acher and Oguey is that they use a lateral bipolar transistor asManuscript received December 4, 1995; revised February 5 , 1996. Thiswork was supported by the European Committee (ESPRIT Project 9011,acronym Slopsys).The authors are with the Delft Institute of Microelectronics and SubmicronTechnology (DIMES), Delft University of Technology, 2628 CD Delft, TheNetherlands.Publisher Item Identilier S 0018-9200(96)04470-8.

a temperature sensor. This device can only be used in specialprocesses.

We decided to use the substrate transistor, which is muchmore compatible with standard CMOS technology. This im-plies, however, the design of a complete new circuit topology,including offset-reduction techniques. Without degrading theaccuracy too much, we obtained a power consumption of 60pW at continuous operation. Including a power-down facility,this could be reduced to 7 p W at a nominal sample rate of2 samplesh .The com plete temperature measurement system consists ofa temperature sensor, a bandgap voltage reference, and ananalog to digital converter on one single chip. The interfaceto an external microcontroller is 8 b parallel. The targetspecifications are shown in Table I.

11. INTEGRATEDEMPERATUREENSORSIn CMOS technology, there are three possible devices which1) CMOS transistors operating in weak inversion;2) lateral bipolar transistors;3) vertical bipolar transistors.Vittoz [2] showed that i t is possible to make voltage ref-

erences using CMOS transistors operating in weak inversion.However, for temperature sensing, these devices suffer fromlimited reproducibility on absolute value and temperaturecoefficient, which are both depe nding on the threshold voltage.

A lateral bipolar transistor can be used to make an accuratetemperature sensor [l] , [ 3 ] .The quality of the lateral bipolartransistor depends heavily, however, on th e IC process. Inmany processes, especially when no attention has been paidto optimize this device, the current gain of this transistor isvery low (

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934 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 31, NO . 7, JULY 1996

0 220 320 420 600 --+T(K)220 320 420 600 --+T(K)Fig. 1. Derivation of \;tat and I/;'f.'ssFig. 2. Basic PTAT circuit

the parasitic vertical bipolar) is difficult to control and canbe up to 50%. Considering also that data on these parametersis hard to obtain, good circuit design with these devices ishardly feasible.

The vertical bipolar substrate transistor seems to be the bestcandidate for integrated temperature sensors [4].The quality ofthis transistor is comparable to transistors in standard bipolarprocesses. The characterization of these devices may not bevery elaborate in most processes, but is usually sufficientfor our purpose. The main problem is the lack of a freecollector terminal, which is inherently connected to substrate.This problem can, however, be solved on circuit level.

Fig. 3. PTAT circuit with chopper

vssFig. 4. Simplified schematic of complete analog interface.

At an emitter current density ratio of eight, the temperaturecoefficient of this PTAT voltage is 0.2 mV/"C. Amplifying thisvoltage (uAVb,) and adding it to a base-emitter voltage (&,I)results in a so-called bandgap reference voltage independentof temperature (V,,f).B . A -D Conversion

EA converters have proven to be very suitable in low-frequency, high-performance applications. In our case, havinga very low-frequency signal (

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BAKKER AND HUIJSING: MICROPOWER CMOS TEMPERATURE SENSOR WITH DIGITAL OUTPUT

T I

Fig. 5. Schematic of first-order current-mode ZA converter.

ck-in4G-t

bitstream

sensorreference

ower-on

'temp

I efEA

converter counter-ias timerreset enableI +reset /enabe-'1 timer Icontrol

sample-ready

935

I IFig. 6. Block schematic of complete temperature system.

required accuracy (1C M 200 p V ) which makes an offsetcompensation technique necessary.There are two topologies for offset compensation: Autoze-roing and chopping [8]. The autozeroing technique is morecomplicated to combine with a current-mode EA converter,because of the not continuously available output signal. Achoice has therefore been made to implement the choppingtechnique.B. Chopper Circuit

The PTAT circuit with chopper is shown in Fig. 3. Theopamp, as schematically shown in Fig. 2, consists of twoamplifying stages. The first stage of the opamp is formed by. . M8, while the second one is formed by Mg. The Millercapacitor C,, is needed for stabilization of the circuit.The chopper circuit is usually placed across the entireopamp . In our case, however, the chopper is placed only acrossthe first stage of the opamp including the mirror. The offsetof the mirror is therefore also removed. The advantage of

Fig. 7. Micrograph of' the realized chip.

not placing the chopper across the complete opamp is thatthe second (Miller compensated) stage acts as a low-pass

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936 IEEE JO U R N A L OF SOLID-STATE CIRCUITS, VOL. 31, NO. 7 , JULY 1996

chi0 14

(c) 3errort :0

-1-2-3-4-5-6-7

-20 0

Fig. 8. Error before calibration

(c) 1.0 ............................................................................................................1.51error chip 2chip 1chip 3

-0.5-1.0 I I I I I I II I I I I

-40 -20 0 20

Fig. 9. Error after calibration at - 2 O O C and 100C.

filter, which makes an additional low-pass filter unnecessary.Another advantage is that the lack of an additional low-passfilter also increases the bandwidth of the complete system.This circuit is patent pending [SI.

C. Analog Interface ElectronicsA simplified schematic of the analog interface is shown

in Fig. 4. The left part is the PTAT circuit. The base-emittervoltage of Q 2 is transformed into a current by Rbe. Thereference current Irefs the addition of the current throughM2, which is PTAT and the current through M4, whichis proportional to Vbe, thus making a current independentof temperature. The temperature dependent current I telnpsformed by th e PTAT current through A.49 minus a small currentproportional to Vb to shift the point at which I t emps zerofrom -273C (0 K) to -61C. This is done to enlarge theuseful range of the E A converter. The choice for -61C asa lowest temperature point is mainly dictated by the need tohave a ratio, which makes an accurate scaling of the currentmirrors possible. The values of RPtatan d Rbe are 110 kR an d560 kR, espectively, which are relatively large, but neededto reduce power consumption. The opamp which is used togenerate the current proportional to V,, does not need to bechopped because the offset drift is fairly small compared tothe signal (1C =2 mV).

40 60 80 100 120-T (c)

D. A-D ConverterThe principle of the first-order E A converter is shownin Fig. 5. The bitstream at the output is proportional toI t em p/ I r e f .his signal is led to a counter, which acts as afirst-order digital decimation filter. The integrator is reset at

the start of each sample. The value of the capacitor is 60 pF.E. Complete Temperature System

A block schematic of the entire temperature system is shownin Fig. 6. The control block is needed to control the power-down m ode and the counter. The clock signal has to be dividedby two to guarantee a precise 50% duty-cycle, which is neededfor the chopper. Both the digital and the analog part can beswitched off by the power-on signal.F. Layout Design and Fabrication

The circuit has been realized in a double-poly 2 pm CMOSprocess. Special attention has been paid to the matching oftransistors and resistors. The complete circuit measures I .5mm2 (excluding bond pads), of which 20% is occupied bypoly resistors and 10% by poly-poly capacitors. A micrographis shown in Fig. 7. This chip, which measures 12 mm, alsoincorporates the pressure sensor interface, which is needed forthe tire control application. The analog part of the temperaturesystem is marked by a white rectangle. A second version is

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0.1 C50 samples/sec

PSR I ! I 0.1 I CN I

currently being processed in a state-of-the-art 0.7 pm process,and measures only 0.7 mm. This reduction could be reachedmainly by resizing the digital part and by replacing the polyresistors of 25 Q /U by high-resistive (undoped) poly of 2kS1/0.

V. MEASUREMENTESULTSThe chip has been encapsulated in a standard IC package

and has been exposed to temperatures in the range from -40to +120C. The circuit is working at supply voltages varyingform 2.2 to 5.0 V. The current consumption is 25 pA atcontinuous operation (50 samplesk) and 3 p A at a samplerate of 2 samplesh, thus having a nominal power consumptionof 3 /L A * 2.2 V = 7 pW. Due to the cascoding of allcurrent mirrors and bias circuits, the power supply rejection islimited to O.IC /V, which is very goo d for circuits operatingat voltages as low as 2.2 V. The noise is only 0 . 1 C over theentire temperature range. The specifications of the measuredcircuit are shown in Table 11.

The error over temperature before calibration is shown inFig. 8. The error is below h 7 C for the entire temperaturerange for three samples, which is in accordance with the cal-culated error of k 8C.Fig. 9shows the error after calibrationat -20C and 100C. This shows that the residual error aftercalibration is h1C.

VI. CONCLUSIONA smart temperature sensor has been presented which offersextremely low power consumption and small chip area. The

circuit has been made in standard CM OS technology. Calibra-tion is done in an external microcontroller. The specificationof very low power consumption and wide temperature rangemakes this smart temperature system very suitable to beused as a subsystem in various existing and future electronicsystems.A next generation of this system will use more elaborateoffset reduction techniques which will further reduce the

errors. The total residual error will then be due to the offsetin the base-emitter voltage of the bipolar temperature sensor,which will be h3 C . The required accuracy of *l C can thenbe met with a single calibration of Vbe at room temperature.

ACKNOWLEDGMENTThe authors wish to thank F. R. Riedijk for fruitful discus-

sions.REFERENCES

P. Krummenacher and H. Oguey, Smart temperature sensor in CMOStechnology, Sensors and Actuato rs, vols. A21L.423, pp. 636-638, 1990.E. A. Vittoz, A low-voltage CMOS bandgap reference, IEEE J. Solid-State Circuits, vol. SC-14, pp. 573-577, 1979.-, MOS transistors operated in the lateral bipolar mode and theirapplication in CMOS technology, IEEE J. Solid-State Circuits, vol.M . Ferro, F. Salerno, and R. Castello, A floating CMOS bandgapvoltage reference for differential applications, IEEE J. Solid-StateCircuits, vol. 24, pp. 690-697, 1989.G. C. M. Meijer, Thermal sensors based on transistors, Sensors andActuators, vol. 10, pp. 103-125, 1986.G . C. M. Meijer, A. J. M. Boodamp, and R. J. Duguesnoy, Anaccurate biomedical temperature transducer with on-chip microcomputerinterfacing, IEEE J . Solid-State Circuits,vol. 23, pp. 1405-1410, 1988.F. R. Riedijk and J. H. Huijsing, An integrated absolute temperaturesensor with sigma-delta A-to-D conversion, Sensors and Actuato rs, vol.A34, pp. 249-256, 1992.C. C. Enz, E. A. Vittoz, and F. Krummenacher, A CMOS chopperamplifier, IEEE J. Solid-State Circuits, vol. SC-22, pp. 335-342, 1987.A . Bakker and J. H. Huijsing, Chopper amplifier with internal filter,Dutch patent application no. 10.01231, Sept. 18, 1995.

SC-18, pp. 273-279, 1983.

Anton Bakker was born in Amsterdam, TheNetherlands, on July I I , 1968 He received theM Sc. degree in electrical engineering from the DelftUniversity of Technology, Delft, The Netherlands,in 1991.Since then, he has been involved in thedevelopment of mixed-mode analog-digital circuitson digitdl gate-arrays In 1993, he joined theElectronic Instrumentation Labordtory at the DelftUniversity of Technology where he is now workingtoward the Ph.D degree on the Subject of CMOStemperature microsystems

JohanH. Huijsing (SM81 was born in Bandung,Indonesia, on May 21, 1938. He received the M.Scdegree in eleLtrical engineering from the Delft Uni-versity of Technology, Delft, The Netherlands, in1969, and the Ph D degree from the same universityin 1981 for work on operational amplihers Hi\thesis was Integrated Circuits for Accurate LinearAnalogue Electric Signal Procesaing, supervised byProf Dr Ir J. DavidseSince 1969 he has been a memberof the Researchand Teaching Staff of the Electronic InstrumentdtionLaboratory, Department of Electrical Engineering, Delft UniverWy of Technology, where he 15 now Profe\sor of Electronic Instrumentation He teachescourses on electrical measurement technique\, electronic instrumentation,operational amplifiers and analog-to-digitdl converters His field of researchis analog circuit design (operational amplihers, analog multipliers, etc 1 dndintegrated smart wnsors (signal conditioning on the sensor chip, frequencyand digital converters which incorporate sensors, bus interfaces, etc ) He isthe duthor or coauthor of some 110 scientific papers dnd 15 patents