A CMOS Analog Front-End Circuit for MEMS Based Temperature Sensor

Technology Volume 1, Issue 2, October-December, 2013, pp. 01-06, IASTER 2013 www.iaster.com, Online: 2347-6109, Print: 2348-0017 A CMOS Analog Front-End Circuit for MEMS Based Temperature Sensor Bollam Naresh Sudarshan, A. Ravi Sankar School of Electronics Engineering (SENSE), VIT University Chennai, India ABSTRACT This paper presents a CMOS analog interface circuit for Micro-electromechanical System s (MEMS) based temperature sensors. The proposed interface circuit encompasses four different blocks; (i) a current generator whose output is Proportional To Absolute Temperature (PTAT), (ii) an Integrator, (iii) a current starved voltage controlled oscillator (VCO) and (iv) a frequency to current converter. These four circuit blocks were simulated with the 180nm technology library using SPICE Specter of the Cadence EDA tool. The PTAT current generator generates an output current proportional to the temperature to be sensed. The PTAT current generator is realized using a Wilson current mirror and a resistor implemented by switching capacitor circuit. The output of the PTAT circuit is compared with the feedback current from frequency-to-current converter. The detector, Integrator, VCO and frequency to current converter combine for a frequency locked loop to provide the output. Keywords: Interface circuit, temperature sensor, frequency locked-loop, frequency-to-current converter, current starved VCO. I. INTRODUCTION Micro-Electro-Mechanical Systems (MEMS) that integrate electrical, mechanical and electromechanical components in the same substrate have the ability to physically respond to its environment and convert directly or indirectly the input to its electrical equivalent. Advancement of MEMS technology lies not only in the increased complexity and manufacturing sophistication but also in exploring new devices with novel functionalities. MEMS devices have a wide range of applications of which MEMS based temperature sensors are utilized in various fields. Precise temperature sensing is very essential in hazardous and high-temperature environments, medical applications involving catheters, etc. An accurate electronic interface circuit is very important for high precision temperature measurements. Wide temperature range, high conversion rate, low power & area, high accuracy and simple calibration method are a few of the desired properties of the electronic interface circuit for thermal sensors. In the recent past, researchers have reported [1,2] low power electronic interface circuits for MEMS based temperature sensors. These circuits consume power in the order of μw. Even though power consumption of circuits is reduced with reduced geometrical dimensions of the transistors any power supply, these are various issues that need to be addressed in the analog IC design [3-7]. These issues include the following; reduced transistor gain & lower dynamic range any poor noise margins [8-9]. In the present paper, we report simulation and analysis of a low power high conversion rate CMOS interface circuit for temperature sensing over the range 70-125 ⁰C. This work is based on the research results reported by [10] where circuit consumes power in the order of less than 100μW. Very low power consumption can be achieved by operating the complete circuit in the sub-

threshold region. However, as a proof-of-concept we have simulated the circuit in the saturation region and further work needs to be carried out for achieving a very low power in the order of 10-15 µwatts. The CMOS interface circuit encompasses five modules. These modules were simulated in Virtuoso module of the Cadence EDA tool. The complete circuit configuration in described in section II whereas simulation results are described in section III. II. CIRCUIT CONFIGURATION The block diagram of the proposed electronic interface circuit is shown in Fig.1. The circuit encompasses the following modules; a PTAT current generator, a current subtractor followed by an Integrator, a voltage controlled oscillator (VCO) and a frequency to current converter. The CMOS circuit is designed to generate an output clock proportional to the temperature to be sensed. Initially, The PTAT current generator provides an output current proportional to the temperature. The four circuit blocks, i.e. the current subtractor, the Integrator, the VCO and the frequency to current converter are connected to form a frequency locked loop. The current subtractor detects the difference between I PTAT current from PTAT Current generator and output current I OUT, which is generated by the Frequency to current convertor. Then the Integrator integrates the difference between the two currents, the resultant output of the Integrator V OUT voltage is proportional to the difference between I PTAT and I OUT. The current starved VCO proportional to generate the desired clock frequency F PTAT as dependent on the integrator output voltage V OUT. The frequency to current convertor accepts the desired clock frequency F PTAT to produce the I OUT current that is proportional to the F PTAT.The current subtractor continuously compares I OUT with I PTAT and readjusts V OUT with respect to both currents. This feedback loop operation is repeated till I OUT current is equal to I PTAT current. So the resultant desired clock frequency F PTAT is proportional to the absolute temperature. Fig. 1. Various Modules of Temperature to Frequency Converter. III. SIMULATION ENVIRONMENT AND RESULTS The proposed circuit was designed using a Virtuoso module of the Cadence EDA tool. The simulation analyses were carried out with a SPICE Spectra module of the Cadence EDA tool using the 180nm technology library. A supply voltage of 1.8V power supply and a 1MHz nonoverlap clock frequency was provided to the PTAT current generator to verify the dependence of output current I PTAT and VCO frequency F PTAT on method variations. 2

3.1 I PTAT current generator. The feature of the PTAT current generator is based on the Wilson current mirror as well as on constant-gm biasing circuit operated in saturation mode. The resistive load PTAT current generator is shown in Fig. 2. However in the proposed smart sensor circuit operates in saturation region to get the sensed I PTAT current. In the proposed circuit, resistive load has been used. The circuit with resistive load has some drawbacks. Fig. 2. Resistive load PTAT current generator. When the resistive load is used there is no possibility of changing the resistor value after the sensor fabrication process. Also a passive resistor occupies more silicon in the chip area. So the to avoid these drawbacks, capacitor switching circuit is used instead of resistor load. The capacitive load PTAT Current generator is as shown in Fig. 3 Fig. 3.Using a switched Capacitor PTAT current generator with the resistive load implemented. The PTAT current generator circuit works under ultra-low power as the I PTAT current is in nanoampers. All the transistors are operating in the saturation region. The saturation region drain current Id or I PTAT current is given by 1 W 2 I PTAT ncox VGS VTH (1) 2 L W Where µ n is the mobility of carriers, Cox is the gate oxide capacitance, is the transistor ratio. L The calculated I PTAT current as a function of temperature variations is as shown in Fig.4. Fig. 4. Simulated I PTAT Current As A Function of Temperature. 3

The PTAT current generator generates the I PTAT current with respect to temperature variation as well as on external reference clock frequency F REF. 3.2 Current subtractor and Integrator. The current subtractor detects the difference between I PTAT current coming from PTAT current generator and the output current I OUT, which is generated by frequency to current convertor. Then the Integrator integrates the difference between these two currents and gives the resultant output voltage V OUT, which is proportional to the difference between I PTAT and I OUT. 3.3 Current starved VCO. The Current starved VCO consists of a ring oscillator, is biased in the saturation region of the MOSFET as shown in Fig.5. The current starved VCO generates the desired oscillation frequency based on the Integrator output voltage Vout and it controls the oscillation frequency based on the time delay product td, which is given by following equation. VDD P1 Freq Iref Iref N1 Iref Vconst Fig. 5. Current starved VCO. F= ( 1 2N.t d ) (2) Where, N is the number of delay cells in the ring and td is the delay time in the cell. A technique is used to alter the time delay td, due to which, variations occur in the current on each phase of charging or discharging of capacitive load. This way current starved VCO controls the oscillation frequency. Thus this circuit can be termed as a current starved electrical converter and its output waveform is shown in Fig. 6. Fig. 6. Simulated output wave of the current starved VCO. 4

Iout(nA) International Journal of Research in Electronics & Communication 3.4 Frequency to current convertor. In Fig.7 shows the frequency to current convertor. VDD Vref Iout c3 VDD s/w3 vco s/w4 c2 Fig. 7. Frequency to Current Convertor. This circuit generates the output I OUT current and it is proportional to the desired oscillation frequency F PTAT of the current starved VCO 16 14 12 10 8 6 4 2 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Frequency(Ghz) This circuit consists of a switched capacitor resistor, VCO and operational amplifier which is fixed to reference voltage V REF.The switched capacitor resistor is operated by the desired oscillation clock frequency from the current starved VCO, and it works as a resistor with a resistance of (C 2.F PTAT ) -1.So that I OUT output current is followed by Fig. 8. Simulated I OUT Current as a Function of Frequency I OUT = F PTAT.C 2.V REF. (3) The simulated I OUT current as shown in Fig. 8. This I OUT is forwarded into the current subtractor through a current mirror. In feedback loop operation, the circuit continuously observes the I OUT current and I PTAT current to check whether these currents are equal or not. Mostly I OUT current is equal to I PTAT current because of the feedback loop operation. 3.5 Complete configurations Fig. 9. Shows the complete circuit configuration of the temperature to frequency convertor. The current starved VCO consist of seven current starved inverters connected in a ring. The clock frequency pulses are used for the purpose of non overlapping of switched capacitors in the PTAT current generator as well as in the frequency to current convertor in order to protect simultaneous shorting of switches. The capacitor C 3 is used to remove the high frequency noise, which is affecting the switching operation. The simulated output frequency F PTAT as a function of temperature is shown in Fig. 10. Fig. 9. Temperature to Frequency Convertor Circuit. 5

Fig. 10. Simulated output frequency (F PTAT ) as a function of temperature IV. CONCLUSION In the present work, an electronic interface circuit for a MEMS based temperature sensor has been proposed. The circuit consists of various building blocks that include PTAT current generator, current starved VCO, frequency to current convertor, current subtractor and Integrator. All the modules were simulated by using SPICE Specter of the Cadence EDA tool and the simulation results are presented. The functional verification of the circuit has been carried out by operating the transistors in the active region. An extremely low power consumption can be achieved by the transistors in the sub-threshold region. REFERENCES [1] V. Szekely, Cs. Marta, Zs. Kohari, M. Rencz, CMOS sensors for on-line thermal monitoring of VLSI circuits, IEEE Trans. Very Large Scale Integration (VLSI) Syst. 5 (3) (1997) 270 276. [2] P. Krummenacher, H. Oguey, Smart temperature sensor in CMOS technology, Sens. Actuators A21 A23 (1990) 636 638. [3] A.P. Chandrakasan, D.C. Daly, J. Kwong, Y.K. Ramadass, Next generation micropower systems, in: Proceedings of the IEEE Symposium on VLSI Circuits, Honolulu, USA, June 17 20, 2008, pp. 2 5. [7] M. Tuthill, A switched-current, switched-capacitor temperature sensor in 0.6-um CMOS, IEEE J. Solid-state Circuits 33 (7) (1998) 1117 1122. [4] G. Wang, G.C.M. Meijer, The temperature characteristics of bipolar transistors fabricated in CMOS technology, Sens. Actuators A 87 (2000) 81 89. [5] B. Razavi, Design of Analog CMOS Integrated Circuits, McGraw Hill, 2000. [6] K. Ueno, T. Hirose, T. Asai, Y. Amemiya, CMOS smart sensor for monitoring the quality of perishables, IEEE J. Solid-state Circuits 42 (4) (2007) 798 803. [8] A. Bakker, J.H. Huijsing, Micropower CMOS temperature sensor with digital output, IEEE J. Solid-state Circuits 31 (7) (1996) 933 937. [9] G.C.M. Meijer, G. Wang, F. Fruett, Temperature sensors and voltage references implemented in CMOS technology, IEEE Sens. J. 1 (3) (2001) 225 234. [10] Ken Ueno, Tetsuya Asai, Yoshihito Amemiya, Low-power temperature-to-frequency converter consisting of subthreshold CMOS circuits for integrated smart temperature sensor. Sensor and Actuators A 165(2011)132-137. 6