IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 3, MARCH 2012 767

Current Excitation Method for ΔR Measurement inPiezo-Resistive Sensors With a 0.3-ppm Resolution

Neena A. Gilda, Member, IEEE, Sudip Nag, Student Member, IEEE, Sheetal Patil,Maryam Shojaei Baghini, Senior Member, IEEE, Dinesh Kumar Sharma, Senior Member, IEEE, and

V. Ramgopal Rao, Senior Member, IEEE

Abstract—This paper presents a new highly sensitive(10 µV/ppm) bidirectional current excitation method forpiezo-resistive sensor measurements, through excitation oftwo half bridges for ΔR measurement. The proposed circuitis insensitive to thermoelectric and stray noise effects sinceit measures the peak-to-peak value of the generated voltage.Measurement results using resistors show that variations as lowas 0.3 ppm are measurable from 15 ◦C to 80 ◦C with resistors.As an experimental application, 40 parts per billion variation ingas concentration using piezo-resistive SU-8 microcantilevers ismeasured by the proposed circuit at room temperature.

Index Terms—Bidirectional current source, cantilevers, currentexcitation, piezo resistivity, sensitivity, Wheatstone’s bridge.

I. INTRODUCTION

P IEZO-RESISTIVE effect has played a key role in wide ap-plication measurement systems for sensing strain, temper-

ature, pressure, force, displacement, humidity, etc. [1]. With theadvancements in silicon and polymer technology and discoveryof large piezo-resistive effects in silicon along with its impres-sive mechanical properties, complex microelectromechanicalsystems (MEMS) structures such as laboratory-on-chip havebeen developed [2], [3]. Piezo-resistive MEMS sensors have at-tracted attention in recent years due to their efficiency and formfactor. These advancements in piezo-resistive sensors demandbetter signal conditioning circuits to exploit these devices totheir full potential.

The wide variety of piezo-resistive sensors challenges thedesign of measurement platforms at very high precision level.Many schemes have been incorporated for improvement ofsensitivity. The most popular as well as the simplest arrange-ment of piezo-resistive sensors is in the form of Wheatstone’sbridge [Fig. 1(a)]. It provides a direct differential output voltageas a function of the change in piezo resistance (ΔR) of thesensor connected to one arm of the bridge. The sensitivity ofWheatstone’s bridge depends on the excitation voltage. For a

Manuscript received March 30, 2011; revised September 16, 2011; acceptedSeptember 19, 2011. Date of publication November 23, 2011; date of currentversion February 8, 2012. This work was supported in part by the Departmentof Information Technology, Government of India, through the Centre of Excel-lence in Nanoelectronics. The Associate Editor coordinating the review processfor this paper was Dr. Jesús Ureña.

The authors are with the Centre of Excellence in Nanoelectronics, De-partment of Electrical Engineering, Indian Institute of Technology Bombay,Mumbai 400076, India.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2011.2172118

Fig. 1. (a) Traditional Wheatstone’s bridge. (b) DC-current-actuated Wheat-stone’s bridge. (c) Current-driven two half bridges connected in cascade.

decent bridge sensitivity greater than the reported 0.6 mV/ppm[4], high excitation voltage is required, which may prevent low-voltage and low-power operation.

Other reported techniques based on Wheatstone’s bridgeinclude voltage-to-frequency conversion [1], lock-in amplifica-tion [4], and the constant current excitation of the bridge usingonly one excitation current source, as shown in Fig. 1(b), [5],in the resistance. The sensitivity of these methods is limited bythe factors such as base resistance of the Wheatstone’s bridge,thermoelectric voltages, and stray noise components. To miti-gate these problems, we propose a circuit which actuates twohalf bridges connected in the fashion shown in Fig. 1(c) withbidirectional constant current sources. The aim of the proposedcircuit is to design and realize a low-voltage highly sensitivemeasurement circuit for piezo-resistive sensors. The organiza-tion of this paper is as follows. In Section II, the proposedmethod and system design are described. Section III presentsthe experimental results and device testing of the proposedtechnique. Design considerations are explained in Section IV.Finally, Section V summarizes the important findings from thisstudy.

II. PROPOSED METHOD AND SYSTEM DESIGN

A. Basic Concept

The general configuration of the voltage-driven Wheatstone’sbridge is shown in Fig. 1(a). This differential voltage is ex-pressed as

Vo = [ΔR/2(2R + ΔR)] × VA ≈ (ΔR/4R) × V A. (1)

According to (1), the sensitivity of the output voltage ofvoltage-excited Wheatstone’s bridge [Vo/(ΔR/R)] is directly

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768 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 3, MARCH 2012

proportional to the excitation voltage (VA). Therefore, appli-cations which need high sensitivity (e.g., for less than 1-ppmchange in the resistance) may necessitate a large excitationvoltage. On the other hand, for a given value of the excita-tion voltage and bridge output voltage, the required value ofR decreases as ΔR reduces, which leads to a high powerconsumption. This aspect has been discussed in detail in thissection. High-voltage operation also imposes constraints onthe instrumentation amplifier (INA). Measurement of very lowresistance change is also subjected to the error sources suchas nonohmic contacts, device heating, thermoelectric offset,and unequal thermal drift of the components, which limit theresolution of the final system [6].

Researchers have incorporated different methods to get ridof the disadvantages for ultrasensitive ΔR measurements. Anac voltage excitation method has been used to remove thethermoelectric offset problems. However, the bridge will stillrequire high power to operate with high sensitivity.

In [5], a dc current excitation method has been used toremove the need for high-voltage excitation. However, theauthors in [5] use a single dc constant current source to actuatethe bridge, as shown in Fig. 1(b). This requires that all fourbase resistances (R) be identical; otherwise, the current througheach arm will be different. This will lead to a dc offset, whichdemands an unusually high input voltage range for the INAfollowing the bridge. Also, the resolution observed was in therange of 5–10 ppm. The method in [7] needs one referenceresistance instead of bridge condition, but eventually, it con-siders ideal op-amps and resistances for the measurements. Thesignal conditioning circuits explained in [1], [8], and [9] needto consider the jitter problems at output frequency due to thedelay introduced from switches.

In this paper, we present an ac current excitation method withtwo identical bidirectional current sources. Fig. 1(c) shows thebasics of the method. As shown in the figure, the end points oftwo arms at one side are disconnected so that the bridge can bedriven with two identical current sources, connected at one endof each half bridge. Base resistor R and value of current sourcesset the proper bias point for the INA connected at the output ofthe half-bridge configuration. In practical conditions, when twoarm resistors are ideally equal, mismatch between amplitudesof current sources (shown by δI) and mismatch between twoarm resistors (shown by δR) induce an error in the peak-to-peakoutput voltage, as given by the following, neglecting second-order effects:

Vo = [(I + δI) × (R + δR) − I × R]

− [I × R − (I + δI) × (R + δR)]

∼= 2(I × δR + δI × R). (2)

To cancel the error, given by (2), a trimming procedure isrequired before the measurement starts. This is achieved byusing a potentiometer connected at the reference arm of thesecond half bridge so that two half bridges will be equallybalanced. For trimmed half bridges, ΔR change in the sensing

Fig. 2. Block diagram of the complete system implemented using the pro-posed bidirectional current excitation method.

arm of the bridge leads to measured peak-to-peak value ofoutput voltage, given by

Vo = [(I + δI) × (R + δR) − I × (R + ΔR)]

− [I × (R + ΔR) − (I + δI) × (R + δR)]

∼=2(I × ΔR − I × δR − δI × R). (3)

Assuming that δI × δR is negligible, Vo can be approxi-mated by the following relation:

Vo = 2(I × ΔR − I × δR − δI × R). (4)

According to (4), trimming cancels out the effects of δI andδR at the output voltage of half bridges. Therefore, Vo willbe directly proportional to ΔR and not ΔR/R. It should benoted that two additional resistors in series with the sensor andreference resistors are required to adjust the common-modevoltage needed at the output of current sources. In order tomake it a general-purpose schematic, we have kept these twoadditional resistors.

To clarify the advantage of low-voltage operation of our pro-posed circuit compared to the conventional voltage excitationmethod, large-signal voltage levels of two corresponding cir-cuits for the same ΔR/R and Vo are compared here. Referringto Fig. 1 and according to (1) and (4), we can conclude thatVA/4 (for the case of voltage excitation of Wheatstone’s bridge)corresponds to Vref + 2IR (for the case of current excitationusing a half-bridge configuration). Vref can be as low as 0 V,which means that VA = 8IR, i.e., a factor-of-eight reduction inthe case of our proposed current excitation method.

The proposed method also provides a flexibility of choosinga wider range of resistor values, compared to voltage excitationmethod. This is because the differential output voltage is pro-portional to ΔR in the proposed method, while it is proportionalto ΔR/R in the voltage excitation method. This means that, fora given ΔR, the amplitude of the output voltage can be adjustedindependent of R in the proposed method, as is evident in (4).

Fig. 2 shows the complete system implementation and mea-surement setup of the proposed current excitation method forsensor applications. The system is composed of three mainanalog modules. The first module is a square-wave currentgenerator, followed by a current scaler which divides/multiplies

GILDA et al.: CURRENT EXCITATION METHOD FOR ΔR MEASUREMENT IN PIEZO-RESISTIVE SENSORS 769

Fig. 3. Schematic of the proposed current actuator circuit.

its input current by a factor as needed for the bridge base load.The third module is an INA. An INA with a maximum gain of90 dB is selected here. The output of the INA is fed to a lock-inamplifier. The square wave driving the current sources is usedas the reference signal of the lock-in amplifier, so that randomnoise and drift components away from the signal frequency arerejected.

B. Bidirectional Current Generator

A novel bidirectional current generator, shown in Fig. 3, isused for the excitation of the half-bridge model. The bidirec-tional current generator is composed of two diode bridges andtwo constant dc current sources. Dual current source generatorREF200 from Texas Instruments (TI) [10], composed of two100-μA constant current sources with ±0.5% accuracy, is usedhere. A 5-Vpp square wave with a frequency of 10 kHz anda common-mode dc level is used to drive the diode bridges,as shown in Fig. 3. DC offset is required to operate the dualcurrent source generator IC above the voltage compliance oftwo diodes, i.e., (0.7 V + 0.7 V = 1.4 V). Two diodes of theopposite arms of the bridge are ON at a time to decide the flowof direction of the current through the load.

C. Current Scaler

In order to control the gain and maintain the voltage dropacross the half bridge within the input voltage range of the INA,it is necessary to scale the current according to the different

base resistance values of the half bridge. This can be achievedwith the use of a current scaling topology, as shown in Fig. 3.This configuration uses a low-noise op-amp AD8656 IC fromAnalog Devices (2.7 nV/

√Hz at f = 10 kHz) with ultralow

input bias current (1 pA) and rail-to-rail input/output [11]. Thisop-amp forms a current divider configuration with feedbackresistors R1 and R2, to scale the input current as per therequirements of base resistance of the half bridges, here denotedby R. Capacitor C is used to attenuate the output current atfrequencies above the operation frequency of the current scaler.Currents in the range of 1 nA to 100 μA can be scaled by usingdifferent values of R1. For this application, a square-wave inputcurrent source is scaled down to 1, 0.5, and 0.1 μA.

D. INA

The output of single-element varying half-bridge configu-ration is fed to the INA, AD8222 from Analog Devices withvariable gain (1 to 10 000), high common-mode rejection ratio(126 dB at gain = 100), and low input noise (8 nV/

√Hz at

1 kHz) [12].

E. Lock-in Amplifier

An SR530 lock-in amplifier from Stanford Research Systemis used to measure the output voltage of INA at a particularoperating frequency (10 kHz here) [13]. Therefore, the effect ofnoise components arising in the circuit, such as stray static andmagnetic pickup or line noises, is highly attenuated. The lock-inamplifier is in synchronization with the circuit excitation squarewave.

F. Power Supply

The entire system board is powered by a 9-V battery, fol-lowed by a 5-V voltage regulator 7805 from National Semicon-ductor which generates a 5-V dc voltage from an input voltageof 9 V [14]. Vref is generated by a voltage divider, followed bya unity gain buffer configuration using an op-amp.

III. EXPERIMENTAL RESULTS AND DEVICE TESTING

To demonstrate a state-of-the-art and highly precise applica-tion of the proposed current excitation method, characterizationand calibration of the circuit are carried out with the help ofpurely resistive half bridges and then tested on an actual micro-cantilever platform. The bridge is initially characterized withthe different base resistances, such as 100, 330, and 470 kΩ, asshown in Fig. 1(c). The testing arm of the bridge is composed ofa combination of the potentiometer combination (1 + 500 kΩ)as a variable resistance to get a balanced bridge condition andadjust the dc offset levels. A 100-turn potentiometer is used forvery precise trimming for matching the sensor resistance withthe testing arm resistance. The excitation currents are scaledto 1, 0.5, and 0.1 μA, respectively, to keep the output voltageof the bridge within the common-mode input range of theINA. The frequency of the excitation voltage (square wave) is10 kHz, and its amplitude is 5 Vpp with the offset set at

770 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 3, MARCH 2012

Fig. 4. Photograph of test board for testing microcantilever response.

Fig. 5. DC characteristic of the half-bridge configuration under test with100-kΩ base resistance and 1-µA excitation current at 15 ◦C and 80 ◦Ctemperatures.

common-mode dc levels, which is further converted into acurrent square wave at circuit level. The set of readings is takenat different temperatures ranging from 23 ◦C, 45 ◦C, and 80 ◦C.The actual test board is shown in Fig. 4.

Figs. 5–7 show the circuit response for sensitivity tests withbase resistances of 100, 300, and 470 kΩ, respectively. Thesefigures illustrate the calculated and observed values of theoutput voltage as a function of change in the resistance atdifferent temperatures. Calculations are carried out by con-sidering the temperature variation profiles according to therespective datasheets of resistors [15]. The differences betweenthe calculated and observed values of the output voltages areattributed to variations in resistor values with the temperatureand INA offset, which illustrates that the circuit is immuneto temperature-dependent nonidealities. The lock-in amplifierdisplays the noise values. A line is fitted on the data points, anda maximum percentage of deviation from this line, i.e., linearmodel of the proposed method, is calculated as a percentage ofnonlinearity, as mentioned in Table I.

Figs. 8 and 9 show the histograms of INA output voltagefor two cases: voltage-driven Wheatstone’s bridge and cur-rent excitation method. The Gaussian distribution is used withmaximum mismatch of ±0.1% in the base resistances andmaximum ±0.5% in excitation currents for changing parameterproperties. The simulations are run with the help of NI Multisim11.0 software [16] using model libraries available for INA IC,op-amps, and other components from vendors. These modelscover all practical specifications, including offset and tolerance

Fig. 6. DC characteristic of the half-bridge configuration under test with330-kΩ base resistance and 0.5-µA excitation current at 15 ◦C and 45 ◦Ctemperatures.

Fig. 7. DC characteristic of the half-bridge configuration under test with470-kΩ base resistance and 0.1-µA excitation current at 15 ◦C and 45 ◦Ctemperatures.

values. Multisim 11.0 uses transient analysis for 100 runs at10-kHz operating frequency. For the accuracy of measure-ments, real accuracy setting was done with Multisim 11.0, at27 ◦C temperature to account all the variances, as recommendedin the user manual [16]. A current-excited circuit uses morecomponents than the voltage-excited circuit to excite the half-bridge arms, and hence, it shows a higher fixed error in the meanvalue of the output voltage, compared to the voltage-excitedcircuit. Since this error is fixed, an initial trimming, as wasmentioned in Section II-A, nullifies it. As shown in the figures,the proposed current-excited circuit reduces the variance of themeasured quantity at the output of INA and shows much tighterdistribution of mean value of peak-to-peak output voltage forthe current excitation method [17].

The circuit response for sensitivity is further tested with apiezo-resistive microcantilever sensor fabricated in our labo-ratory connected at the sensing arm of the bridge at roomtemperature (Fig. 10). A microcantilever with an integratedpiezo resistor performs electrical transduction of strain in termsof change in the resistance. When a microcantilever is func-tionalized with a molecular coating that is selective to targetmolecules and exposed to the analyte, a surface stress develops.This is because of the resultant interactions between the surfacecoating and the analyte, leading to a differential surface stressbetween the top and bottom surfaces of the microcantilever. Themicrocantilevers are made of SU-8 (Microchem, MI), which isa transparent negative-tone epoxy-based photoresist. An SU-8/carbon-black composite (Conductex 7067 Ultra ColumbianChemicals) sandwiched between two SU-8 layers is used inthe piezo-resistive mode of transduction [18]. These devices

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TABLE IMEASURED PERFORMANCE CHARACTERISTICS OF TEST CIRCUIT WITH RESISTORS

Fig. 8. Monte Carlo simulation results of (a) voltage-driven Wheatstone’s bridge and (b) the proposed current excitation method without cantilever deflection.

Fig. 9. Monte Carlo simulation results of (a) voltage-driven Wheatstone’s bridge and (b) the proposed current excitation method with cantilever deflection.

772 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 3, MARCH 2012

Fig. 10. Photograph of test setup for microcantilever response testing.

Fig. 11. Actual piezo-resistive microcantilever response; the exposure startedat zero time second and removed at the 18th second; and test 1 and test 2 aretwo different tests carried out on the same cantilever at different day timings tocheck out temperature independence as well as repeatability.

offer a higher sensitivity because of the lower Young’s modulusof the polymer composite material, compared to silicon-basedcantilevers.

Microcantilevers are used to detect explosive vapors afterfunctionalizing the surface using selective materials such as4-mercaptobenzoic acid (4-MBA) which acts as a binding layerfor explosive molecules. Nitroaromatic explosive moleculessuch as trinitrotoluene (TNT) form a hydrogen bonding with4-MBA [2], [3], [19]. 4-MBA can easily form a stablemonolayer on the gold surface through thiol chemistry, whichis the process used for coating the cantilever. Therefore, oneside of the microcantilevers was coated with gold for a selectivefunctionalization of 4-MBA.

A micro diaphragm brushless pump [20] operated at 3-V dcsupply with a flow rate of 250 mL/min was used for suctionof air and TNT molecules. The present experiments were per-formed with actual TNT samples at a concentration of subpartsper billion. The cantilever resistance is measured to be 390 kΩ.An excitation current of 0.1 μA is used for the Wheatstonebridge. Fig. 11 shows the cantilever response when exposed toTNT molecules for different exposure times. It reaches a peakvalue of around 250 mV with a few seconds of response timeand recovers as soon as the exposure is removed. Fig. 12 is theactual image of a polymer composite microcantilever die, andFig. 11 shows the response of microcantilevers when exposed

Fig. 12. SEM image of polymer composite microcantilever die [18].

to a TNT sample for 20 s. The experiments were repeated overthe day to test the repeatability and temperature independenceof the circuit. Table I summarizes the performance charac-teristics of the measurement circuit with a resistor connectedat the sensor arm of the two half-bridge configurations, andTable II summarizes the performance characteristics of thesame circuit when an actual SU-8 microcantilever is used in thesensor arm.

IV. DESIGN CONSIDERATIONS

The circuit board measures 3.3 cm × 4.4 cm. The perfor-mance of the board is summarized in Table II. The valuesare based on the actual performance of the test circuit andcomponent datasheets. The measurement circuit and modifiedhalf-bridge configuration use the same board. The differencesbetween the calculated and observed values of the output volt-ages in the graph can be attributed entirely to the capacitor andresistor tolerances. As the whole circuit resides on one PCBalong with the half-bridge configuration, no effect of parasiticcapacitances due to cables arises. The offset voltages and biascurrents of the active components produce a constant dc offsetof 6 μV at the output of INA. This dc offset does not affectthe measurements as the output of lock-in amplifier displaysthe amplitude of the square-wave signal. The effect of driftin excitation currents is negligible. This is because both thecurrent sources will have close drifts since they are on the samechip. Similarly, both the op-amps used for current scaling areon a single chip, and hence, their offset voltages will have closedrifts.

V. CONCLUSION

This paper has presented a new measurement system basedon a novel bidirectional current excitation circuit in “half-bridge” configuration for a wide range of piezo-resistivesensory measurements. The proposed technique is deployed

GILDA et al.: CURRENT EXCITATION METHOD FOR ΔR MEASUREMENT IN PIEZO-RESISTIVE SENSORS 773

TABLE IIMEASURED PERFORMANCE CHARACTERISTICS OF TEST CIRCUIT

WITH ACTUAL SU-8 MICROCANTILEVERS

around a bridge with resistive as well as microcantilever sensorsmade up of piezo-resistive SU-8 materials. It is shown thatresistor resolution as low as 0.3 ppm can be measured andthe sensor resistance variations for gas concentration changesdown to 40 subparts per billion can be detected. The presentedcircuit and test setup are designed as a part of an ongoingresearch effort to develop a portable wireless electronic-nose-based explosive detection system.

ACKNOWLEDGMENT

The authors would like to thank the Terminal BallisticsResearch Laboratory, India, for providing the TNT vapor gen-erator for the experiments. The authors would also like to thankAnvesha A, Vinayak Hande, Viral Thaker, Seena V, KadayintiNaveen, Sanjay Khairnar, and Sandeep Surya at IIT-Bombayfor the support.

REFERENCES

[1] V. Ferrari, A. Ghisla, Z. Kovács Vajna, D. Marioli, and A. Taroni, “ASICfront-end interface with frequency and duty cycle output for resistive-bridge sensors,” Sens. Actuators A, Phys., vol. 138, no. 1, pp. 112–119,Jul. 2007.

[2] V. Seena, A. Rajorya, P. Pant, S. Mukherji, and V. R. Rao, “Polymermicrocantilever biochemical sensors with integrated polymer compositesfor electrical detection,” Solid State Sci., vol. 11, no. 9, pp. 1606–1611,Sep. 2009.

[3] N. S. Kale, S. Nag, R. Pinto, and V. R. Rao, “Fabrication and charac-terization of a polymeric microcantilever with an encapsulated hotwireCVD polysilicon piezoresistor,” J. Microelectromech. Syst., vol. 18, no. 1,pp. 79–87, Feb. 2009.

[4] S. Nag, N. S. Kale, V. Rao, and D. K. Sharma, “An ultra-sensitive DeltaR/R measurement system for biochemical sensors using piezo resistivemicro-cantilevers,” in Proc. IEEE Eng. Med. Biol. Soc. Conf., 2009,pp. 3794–3797.

[5] S. Sangtong and A. Thanachayanont, “Low-voltage CMOS instrumen-tation amplifier for piezoresistive transducer,” ETRI J., vol. 29, no. 1,pp. 70–78, Feb. 2007.

[6] A. Loui, F. T. Goericke, T. V. Ratto, J. Lee, B. R. Hart, and W. P. King,“The effect of piezoresistive microcantilever geometry on cantilever sen-sitivity during surface stress chemical sensing,” Sens. Actuators A, Phys.,vol. 147, no. 2, pp. 516–521, Oct. 2008.

[7] C. Falconi, C. Di Natale, A. D’Amico, and M. Faccio, “Electronic inter-face for the accurate read-out of resistive sensors in low voltage-low powerintegrated systems,” Sens. Actuators A, Phys., vol. 117, no. 1, pp. 121–126, Jan. 2005.

[8] N. M. Mohan and V. J. Kumar, “A novel signal conditioning circuitfor push-pull-type resistive transducers,” Meas. Sci. Technol., vol. 16,pp. 1848–1852, 2005.

[9] K. Mochizuki and K. Watanabe, “A linear resistance-to-frequency con-verter,” in Proc. IEEE IMTC, 1995, pp. 339–343.

[10] REF200 Datasheet, Burr-Brown Corporation, Tucson, AZ, 1988.[Online]. Available: http://focus.ti.com/lit/ds/symlink/ref200.pdf

[11] AD8656 Datasheet, Analog Devices, Norwood, MA, 2005.[Online]. Available: http://www.analog.com/static/imported-files/data_sheets/AD8655_8656.pdf

[12] AD8222 Datasheet, Analog Devices, Norwood, MA, 2006–2010.[Online]. Available: http://www.analog.com/static/imported-files/data_sheets/AD8222.pdf

[13] SR530 Lock in Amplifier User Manual,Stanford Res. Syst., Inc., Sunny-vale, CA, 1997. [Online]. Available: http://www.thinksrs.com/downloads/PDFs/Manuals/SR530m.pdf

[14] 7805 Datasheet, Fairchild Semicond. Corporation, San Jose, CA, 2001.[Online]. http://www.electrokits.com/downloads/pdf/7805-datasheet-fairchild.pdf

[15] Resistor Datasheet, Jul. 2008. [Online]. Available: http://www.panasonic.com/industrial/components/pdf/AOA0000CE1.pdf

[16] NI Multisim Software 11.0, Evaluation Version, National Instruments,Austin, TX, [Online]. Available: http://www.ni.com/multisim/

[17] Evaluation of Measurement Data—Guide to the Expression of Uncer-tainty in Measurement, JCGM 100, 2008.

[18] L. A. Pinnaduwage, J. E. Hawk, V. Boiadjiev, D. Yi, and T. Thundat,“Use of microcantilevers for the monitoring of molecular binding to self-assembled monolayers,” Langmuir, vol. 19, no. 19, pp. 7841–7844, 2003.

[19] V. Seena, A. Fernandes, P. Pant, S. Mukherji, and V. R. Rao, “Polymernanocomposite nanomechanical cantilever sensors: Material characteriza-tion, device development and application in explosive vapour detection,”Nanotechnology, vol. 22, no. 29, p. 295 501 (11 pp), Jul. 2011.

[20] NMP 05 B Brushless Pump. [Online]. Available: http://www.knf.com.cn/uploads/files/OEM/vacuum_pump/micro_pump/NMP05(09,015)S,M,L,B.pdf

Neena A. Gilda (M’11) received the B.Eng. degreefrom Shivaji University, India, in 2008.

She is currently a Research Assistant with theDepartment of Electrical Engineering, Indian Insti-tute of Technology (IIT) Bombay, Mumbai, India.She is working on an analog system-on-chip designfor sensor applications. She has four internationalconference papers on different topics like analog de-sign, embedded system design, and image processingand one patent on her ongoing work. Her area ofresearch focuses on instrumentation development for

an explosive detector (e-nose) and the development of a point-of-care cardiacdiagnostic system initiated at IIT Bombay by the Government of India.

774 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 3, MARCH 2012

Sudip Nag (S’11) received the B.Eng. degree withthe first rank in university and distinction fromNagpur University, Nagpur, India. He is currentlyworking toward the Ph.D. degree in the area ofbioelectronics, with micro- and nanoelectronics asspecialization, in the Department of Electrical En-gineering, Indian Institute of Technology Bombay,Mumbai, India.

He was a Research Assistant (Fellow) with the In-dian Institute of Technology Bombay prior to joiningdoctoral degree program. He was associated with the

development of ultraminiaturized wearable computer for cardiac patients withmobile-algorithm operated telemedicine (Silicon Locket). He has also designedand developed numerous industry projects, such as wirelessly powered stim-ulator and recorder for neuronal implants, instrumentation for cardiac markerdetection, radiation-hardened supervisory controller, and aircraft cockpit con-troller. He has published ten journal/conference papers and is the holder of twopatents. His research interests include bioinspired systems, bioinstrumentation,low-power analog, and nanofabrication.

Mr. Nag is a member of IEEE Engineering in Medicine and BiologySociety and IEEE Communications Society. He has served as IEEE StudentBranch Vice Chairman at his undergraduate institute and held other importantofficer positions. He is a recipient of the Budding Innovators Award 2008, theIEEE International Summer School and Symposium on Medical Devices andBiosensors 2006 Student Award, the Academic Excellence Award 2004, andthe First Prize awards in competitions at various levels.

Sheetal Patil received the Ph.D. degree in electron-ics science from the University of Pune, Pune, India,in 2004.

She was a Research Scientist with the Depart-ment of Electrical Engineering, University of SouthFlorida, Tampa, where she worked on “BioMEMSAu nanowires based biosensors for lung cancer cellsdetection.” After working as a Research FacultyMember with the Bio-MEMS Group, Departmentof Mechanical Engineering, University of Maryland,College Park, she joined the Department of Elec-

trical Engineering, Indian Institute of Technology (IIT) Bombay, Mumbai,India, as a Senior Research Associate. She is currently a “Lead Manager(R&D)” with “NanoSniff Technologies Pvt. Ltd.,” a company incubated atIIT Bombay, Powai, Mumbai. She has a vast experience in microfabrication,material characterization, and chemical biosensors for specific applicationstoward biochemical sensing. She has published more than 22 peer-reviewedjournal publications in these fields.

Dr. Patil is a recipient of the Fast Track Young Scientist Proposal Award fromthe Department of Science and Technology, Government of India, in 2009.

Maryam Shojaei Baghini (M’00–SM’09) receivedthe M.S. and Ph.D. degrees in electrical engineeringfrom Sharif University of Technology, Tehran, Iran,in 1991 and 1999, respectively.

She worked for two years in industry on the designof analog ICs. In 2001, she joined the Indian Instituteof Technology (IIT) Bombay, Mumbai, India, asa Postdoctoral Fellow, where she is currently anAssociate Professor. She is the author/coauthor of79 international journal and conference papers, theinventor/coinventor of 11 patent applications, and

the coauthor of 2 books. Her current research interests include device–circuitinteraction in emerging technologies, high-performance low-poweranalog/mixed-signal/RF very large scale integration (VLSI) design andtest for various applications, analog/mixed-signal/RF electronic designautomation, power management for systems on chip, high-speed interconnects,and circuit design with organic thin-film components.

Dr. Baghini serves in the Program Committee of several conferences, in-cluding the IEEE Asian Solid-State Circuits Conference, IEEE InternationalConference on VLSI Design, and Asia Symposium on Quality ElectronicDesign. She was a corecipient of IIT Bombay Industry Impact Award in 2008,the Best Research Award in Circuit Design at Intel Corporation Asia AcademicForum 2008, and the Third Award on R&D at the International Festival ofKharazmi in 2002. Her team of students won the first place in the DesignContests held by Cadence Design Systems, India, and Analog Devices, India,in 2006 and 2011, respectively.

Dinesh Kumar Sharma (M’98–SM’01) receivedthe Ph.D. degree from the University of Bombay,Mumbai, India.

He was with the Solid-State Electronics Group,Tata Institute of Fundamental Research, during1971–1991, except for 1976–1978, when he was aVisiting Scientist at the Laboratoire d’Electroniqueet des Technologies de l’Information, Grenoble,France, and 1985–1987, when he was with the Mi-croelectronics Center, Research Triangle Park, NC.Since 1991, he has been with the Department of

Electrical Engineering, Indian Institute of Technology Bombay, Mumbai,where he is currently a Professor and the Head of the department. Over thelast 35 years, he has worked in the areas of MOS device modeling, very largescale integration (VLSI) technology development, VLSI digital system design,mixed-signal design, and RF design. He has also contributed to research inprocess and device simulation, electrothermal modeling, and characterizationof MOS devices. He has published more than 50 papers in reputed journalsand conferences on these subjects. He maintains close contact with the micro-electronics industry in India. He has designed several ICs for the industry andhas conducted training courses for them in the areas of VLSI technology anddesign. Over the last few years, he has also been working on manpower trainingin the areas of microelectronics and VLSI design in India. He has served onseveral committees within the government, which are trying to improve thegeneral level of training in this area. He has also collaborated with the industryand coauthored a widely quoted report with Dr. F. C. Kohli of Tata ConsultancyServices on this subject. His current interests include RF and mixed-signalVLSIs, asynchronous design, and the effect of technology and device scalingon design architectures and tools.

Dr. Sharma is a Fellow of the Institution of Electronics and Telecommunica-tion Engineers (IETE) and serves on the editorial board of Pramana, whichis the journal of physics from the Indian Academy of Sciences. He was arecipient of the Bapu Sitaram Award of the IETE for Excellence in Researchand Development in electronics in 2001.

V. Ramgopal Rao (M’98–SM’02) received theM.Tech. degree from the Indian Institute of Tech-nology (IIT) Bombay, Mumbai, India, in 1991and the Dr. Ing. degree from the Universitaet derBundeswehr Munich, Munich, Germany, in 1997.

From 1997 to 1998 and again in 2001, he was aVisiting Scholar with the Department of ElectricalEngineering, University of California, Los Angeles.He is currently with IIT Bombay, where he is anInstitute Chair Professor in the Department of Elec-trical Engineering and the Chief Investigator for the

Centre of Excellence in Nanoelectronics. He has over 300 publications in thearea of electron devices and nanoelectronics in refereed international journalsand conference proceedings and has 16 patents issued or pending.

Prof. Rao is a Fellow of the Indian National Academy of Engineering,the Indian Academy of Sciences, and the National Academy of Sciences inIndia. He is a Distinguished Lecturer of the IEEE Electron Devices Societyand has served on the Program/Organizing Committees of a large number ofinternational conferences in the area of electron devices. He was the Chairmanof the IEEE AP/ED Bombay Chapter during 2002–2003 and currently serveson the Executive Committee of the IEEE Bombay Section besides being theVice-Chair of the IEEE Asia-Pacific Regions/Chapters Subcommittee. He isan Editor for the IEEE TRANSACTIONS ON ELECTRON DEVICES in theCMOS Devices and Technology area and serves on the editorial boards ofvarious other international journals. He was the recipient of the coveted ShantiSwarup Bhatnagar Prize in Engineering Sciences awarded by the HonorablePrime Minister, Government of India, in 2005 for his work on electron devices.He is also a recipient of the 2004 Swarnajayanti Fellowship Award from theDepartment of Science and Technology, the 2007 IBM Faculty Award, the 2008Materials Research Society of India Annual Prize, and the 2009 TechnoMentorAward from the Indian Semiconductor Association.