thermal conductivity detector for gas chromatography: very wide gain range acquisition system and...

974 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 62, NO. 5, MAY 2013

Thermal Conductivity Detector for GasChromatography: Very Wide Gain Range Acquisition

System and Experimental MeasurementsFabio Rastrello, Pisana Placidi, Member, IEEE, Andrea Scorzoni, Member, IEEE, Enrico Cozzani,

Marco Messina, Ivan Elmi, Stefano Zampolli, and Gian Carlo Cardinali

Abstract—The aim of this paper is to present an acquisitionsystem featuring a very wide gain range and experimental mea-surements of a new micromachined thermal conductivity detector(μTCD), applied downstream of a gas-chromatography (GC) sys-tem. We describe a simple and innovative electronics for μTCDcontrol and data acquisition, outlining its resistance control, na-tive imbalance compensation, and automatic gain control (AGC)algorithm. The acquisition electronics features two parallel ampli-fication stages with programmable gain: a high-gain stage (gain:70–1280) and a low-gain or attenuating stage (gain: 0.6–30). Theresulting detection range turns out to be very wide, with full scalesranging between 3.9 mV and 6.5 V while voltage is acquired witha 10-b-resolution analog-to-digital converter. Measurements andsensitivity tests have been carried out by connecting our μTCDand acquisition system downstream of the microfluidic section andGC column of a commercial GC system. Sensitivity measurementson several toluene masses gave very good results, having observeda system sensitivity of 15.2 ± 0.6 μVs/ng. This high sensitivitywill enable the μTCD to be used in many portable applicationslike in-line quality control, and industrial security and safety. Wealso show the good operation of the AGC algorithm.

Index Terms—Automatic imbalance compensation, gas chro-matography (GC), gas sensor, micromachining, resistance control,thermal conductivity (TC) detector (TCD), wide programmablegain range.

I. INTRODUCTION

THERMAL conductivity (TC) detectors (TCDs) are well-known sensors for flow measurement and gas analysis [1],

[2]. They belong to the group of thermally based sensors [1]. Infact, the sensible elements of a TCD are positive temperaturecoefficient conductive filaments, connected in a Wheatstonebridge configuration. The temperature of these filaments de-pends on the surrounding fluid. Therefore, TCDs can measurea flow change or reveal the presence of a substance as animbalance of the bridge, owing to resistance changes due to

Manuscript received June 22, 2012; revised October 31, 2012; acceptedNovember 1, 2012. Date of publication January 23, 2013; date of currentversion April 3, 2013. The Associate Editor coordinating the review processfor this paper was Dr. Salvatore Baglio.

F. Rastrello, P. Placidi, and A. Scorzoni are with the Department of Elec-tronic and Information Engineering, University of Perugia, 06125 Perugia, Italy(e-mail: [email protected]).

E. Cozzani, M. Messina, I. Elmi, S. Zampolli, and G. C. Cardinali are withthe Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delleRicerche, 40129 Bologna, Italy.

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.2012.2236723

TC variations that the flow or the gases cause in the gas mixturearound the sensor.

Unlike ionization sensors that are mass sensitive (i.e., flameionization detectors), TCDs are nondestructive concentration-based sensors. Thus, their integration with micromachiningtechniques is very common [3], and miniaturization does notworsen their performances. In fact, given a fixed sample massin a detector chamber, decreasing the detector inner volumeincreases the sample concentration.

TCDs are often used downstream of a gas-chromatographicsystem [2], [3]. Gas chromatography (GC) is a technique of gasanalysis which works in two phases: first, gas separation is donewith chromatographic columns, and then, gas detection is donewith specific sensors. TC-based sensors are very suitable forthis kind of system, because they are sensitive to all gases, theycan be micromachined with other parts of the system, and theyare low-power sensors.

In literature, papers can be found about TCD fabricationprocess, optimization, and characterization [3]–[5], other worksare focused also on their modeling and measurements [6]–[10],and other papers describe reusable control and readout elec-tronics for TCDs [5], [6], [11]–[14] or for similar gas sensors[15], [16].

In this paper, we extend the work presented in [6] about ournew low-power micromachined TCD (μTCD) and the controland acquisition electronics. We designed the electronics forconstant resistance control, in order to regulate the averagetemperature of the sensible elements; the network to acquire thesignal with a very wide programmable gain range and with anautomatic gain control (AGC) algorithm; and the electronics forthe native imbalance compensation (NIC) of the μTCD. Thisarchitecture can be reusable in similar systems or for similarthermally based sensors.

Measurements and sensitivity tests have been carried out byconnecting our μTCD and acquisition system downstream ofthe microfluidic section and GC column of a commercial GCsystem. Other measurements with a full prototype system areunder development, including a characterization of its detectionlimit.

This paper is organized as follows. In Section II, we describethe μTCD device and its new features. In Section III, we outlinethe electronic system architecture, focusing on the control elec-tronics, the acquisition algorithm, and the NIC. In Section IV,we show the measurements and characterization of the μTCD.Finally, the conclusions of this work will be presented.

0018-9456/$31.00 © 2013 IEEE

RASTRELLO et al.: TCD FOR GC: GAIN RANGE ACQUISITION SYSTEM AND EXPERIMENTAL MEASUREMENTS 975

Fig. 1. (a) Picture of the micromachined device with its Pyrex cover. (b) (Top)Layout and (bottom) picture of a single conductive filament of the μTCD. Theholes realized among the filament meanders promote heat exchange with theflowing gas when the device is operating.

Fig. 2. Cross section of the μTCD encapsulated with the Pyrexmicrochambers.

II. μTCD DEVICE

The developed μTCD (Fig. 1) is formed by two couples ofnominally equal platinum resistors, arranged in a Wheatstonebridge configuration, defined over suspended dielectric mem-branes obtained through silicon micromachining techniques.The chip contains the four resistors of the bridge: its overalldimensions are 5 mm × 5 mm, i.e., they are not pushed tothe limits of technology to favor the interconnection of fluidicchannels and chip handling, since the surface actually employedby the resistors is much smaller.

The silicon chip containing the suspended structures isencapsulated, through a wafer bonding technique, with a me-chanically micromachined Pyrex wafer implementing ultralow-volume microchambers (both the reference channel and theanalytical channel feature a square cross section of 250×250 μm2, Fig. 2).

III. ELECTRONIC SYSTEM ARCHITECTURE

In Fig. 3, the block diagram of the electronic system isdepicted. Referring to that figure, starting from the left, weobserve the following.

1) Regarding the power supply section, the 50 Hz mains istransformed to low AC voltage, then converted to DC bya full-wave rectifier with smoothing capacitor producingDC with a superimposed 100-Hz ripple voltage; thisprovides the input to a number of linear regulators whoseoutputs feed all the electronic components.

2) A circuit for powering the μTCD and to define its setpoint (i.e., the electrical resistance, Section III-A).

3) A network to compensate the native imbalance of thebridge (NIC, Section III-C).

4) A wide gain range amplification circuit, which allows theuser to choose the amplifying gain (Section III-B).

5) The information signal is given by the imbalance betweenterminals 2 and 3, i.e., ΔV = V2 − V3, and the two chan-nels of amplified output are connected with two analog-to-digital converter (ADC) pins of the microcontroller(μC).

6) A protection network performs an analog control on thetotal voltage drop on the μTCD, in order to avoid too highand dangerous currents on it.

7) The feedback network keeps the common mode (CM)voltage of terminals 2 and 3 of the μTCD (CM = (V2 +V3)/2) at the constant value of the analog referencevoltage (Vref ).

8) The digital control is done with a PIC16F876A μC (RISCCPU, 20-MHz clock frequency, 10-b ADC, and threeinternal 8/16 bit timers/counters) by Microchip Tech-nology Inc. [17]. All the commands to the system andthe data acquired by the μC are exchanged between thePIC16F876A and a terminal via the RS232 protocol.

A. Set Point and Powering

In literature, we can find various interfacing circuits forthermally based sensors. In [10], a readout electronic circuit forresistive chemical sensors is proposed based on the resistance-to-time conversion technique, useful for resistance and parasiticcapacitance measurements, but it showed some problems inthe estimation of resistance values smaller than 10 MΩ. Theauthors in [11] propose an integrated phase-domain ADC,but at that time, its operation performances still had to beexperimentally verified. In [12] and [13], we find a reusablecontrol electronics based on a pulsewidth-modulated (PWM)control signal, digitally created by an ARM 7, for an ultralow-power resistance temperature detector. It is a good approachfor that class of sensor, because it minimizes the power con-sumption, but it generates temperature ripples, too. The authorsin [5] proposed an electrothermal feedback control based ona comparator working in a sort of PWM mode. In [14], anoscillator-based system for resistance measurements is pro-posed. This is an approach that is more robust, but it needs anapplication-specific integrated circuit. A comparison betweenthis work and previous described architectures is summarizedin Table I.

In this paper, we propose a constant resistance control basedon a classic opamp configuration, with digitally programmablevalues. Compared with the other solutions described before,this is a very simple, effective, and low-cost architecture. InFig. 4, the control circuit is depicted, and the imbalance of thebridge becomes ΔV = VSIGN+ − VSIGN−.

We use a digital trimmer from Analog Device Inc. (AD5293,20 kΩ of total resistance, and 1024 positions [18]) to choosethe values of RBW and RWA = 20 kΩ−RBW to set theoverall resistance of the Wheatstone bridge Rbridge = (R1 +R2)//(R3 +R4) as

VEXC+ − Vref

I∼= VEXC+ − VEXC−

2I

=Rbridge

2= R1//R3

∼= Rfb × RBW

RWA(1)


Fig. 3. Block diagram of the electronic system. In the picture of the layout of the μTCD, the bonding pads with the same number are commoned.

TABLE ICOMPARISON BETWEEN THE PROPOSED SYSTEM AND THE STATE OF THE ART

Fig. 4. Simplified schematic of the electronics for the control of theμTCD. The μTCD’s terminal numbering corresponds to that in Fig. 3.RBW and RWA are so called because A and B are the two outermost terminalsof the 20-kΩ resistor of the digital potentiometer, while W (wiper) is the middlepoint between A and B.

where VEXC+ and VEXC− are the voltages at terminals 1 and 4of the μTCD, respectively, Vref = 2.5 V is the analog referencevoltage, I is the Wheatstone bridge current, Rfb is the feedbackresistance of our electronics, and RBW and RWA are the tworesistances set by the AD5293 for the set point.

In (1), we suppose identical behaviors of the couples R1−R4

and R2−R3, this being a reasonable assumption due to thesymmetry of our μTCD. Observe that Rbridge is an increasingfunction of its own temperature, being for each resistor ofthe bridge (Ri): Ri = R0(1 + TCR0 ×ΔT ), where R0 issupposed to be the same resistance at 0 ◦C for all the resistors ofthe μTCD, TCR0 is the Pt temperature coefficient of resistanceat 0 ◦C, and ΔT is the temperature increase over 0 ◦C. Weemphasize that (1) relates Rbridge with the constant resistancevalues Rfb, RBW , and RWA and that the constant resistancecontrol can only be obtained using a positive feedback in thearchitecture in Fig. 4. A software limit on the minimum valueof RWA (i.e., RWA > 488 Ω), an overall 20-kΩ resistancebetween the output of the operational amplifier and Vref , avery low value of Rfb (10 Ω), and an Rbridge of 300 Ω atambient temperature (20 ◦C) assure that the negative feedbackof the operational amplifier is always stronger than the positiveone and, at the same time, assure a limit on the maximumselectable set point (i.e., a limit on I) to prevent damagesto the μTCD. The advantage of this architecture is that, byexploiting (1), the Rbridge/2 is very accurately controlled, withan analog feedback that avoids resistance (i.e., temperature)ripples. Moreover, choosing a thermally stable material forRfb, we minimize the effects of its resistance variations withtemperature, while RBW and RWA, showing up as a ratio in(1), do not affect Rbridge with their possible temperature or


Fig. 5. Schematic of the INA2. Observe that R5 = R6, R7 = R9, andR8 = R10. The operational amplifiers are OPA2137, and Rgain is obtainedwith a digital trimmer AD5293.

tolerance variations (35 ppm/◦C and less than 1% tolerance).Due to the discrete nature of this network to fix the set point,the number of the operative resistances of the sensible elementof the μTCD is discrete and limited, but the 1024 positions ofthe trimmer give a satisfactory number of selectable values.Moreover, the AD5293 is digitally controlled with the SPIprotocol, and the device can be connected in a daisy chaincontrol configuration with other ones of the embedded system.

SPICE simulations, exploiting the model in [6] to reproducethe electrothermal performance of the μTCD, confirmed thecorrect behavior of the electronics.

B. Wide Gain Range Amplification

The amplification network is formed by two parallel andcomplementary digitally programmable instrumentation ampli-fiers (INAs). The first one (INA1) is an AD8555 (gain between70 and 1280, 255 positions, and programmable output offsetbetween 0 and 5 V, [19]). We choose this component to haveonly one gain stage to amplify signals from tens of microvoltsup to tens of millivolts, as observed from calculations andSPICE simulations calibrated with 1–1000-ppm variations ofN2 concentration (sample) in He (carrier) on a μTCD madeof elements similar to that modeled, simulated, and measuredin [9].

The second INA (INA2) conforms to the well-knownschematic in Fig. 5, and it is designed to amplify or, if needed,attenuate input signals from tens of millivolts up to 6.5 V. It hasbeen created with buffered inputs and a digitally programmablegain resistor (Rgain), in order to have a gain varying in a rangebetween 0.6 and 30. Referring to Fig. 5, the Vout expression is

Vout = (VSIGN+−VSIGN−)×(1 +

2R5

Rgain

)×(R8

R7

). (2)

Therefore, we used another AD5293, in a daisy chain controlconfiguration with the others, to set the Rgain value and changethe overall gain. Because of the nonideal INA2 (i.e., resistorsand potentiometer’s tolerances), a calibration process for thegain is needed. Please consider that bias currents and offsetvoltages have negligible effects on the characteristics of theINA2 since we amplify relatively high differential voltagesand we perform this circuit calibration. The calibration wasimplemented using a custom circuit and dedicated firmware.

Fig. 6. Flow chart of the AGC algorithm. See the text for details.

The μTCD was replaced with a calibrator which contains a5-V linear regulator and an operational amplifier in invertingconfiguration. Thus, the calibrator circuit is combined withthe AD5293 of the NIC circuit (Section III-C) to create aprogrammable voltage divider. By varying the wiper positionof the AD5293, a range of differential voltages (unbalancingsignals) at the inputs of the INA2 were generated. Then,using a dedicated calibration firmware, we apply a knownsweeping unbalancing signal (from approximately 50 mV upto 4.7 V), and increasing the gain (i.e., decreasing Rgain) atthe same time, we measure the voltage output with the ADCand store the calculated gain values into the μC electricallyerasable programmable read-only memory. These values aresubsequently used in the acquisition firmware while acquiringwith the μTCD. Due to the feedback network described inSection III, the differential signal of the two INAs featuresa zero CM voltage referred to Vref . Therefore, this INA2

amplifies only a differential voltage, even if asymmetries of itsdiscrete components could cause a CM rejection ratio differentfrom zero.

The two INAs and the presence of a μC allowed us torealize an AGC algorithm, described in Fig. 6. The presentalgorithm has been created as a threshold algorithm, whichchanges the gain to hold the information signal in two op-timal ranges between 0.3–1.25 and 3.75–4.7 V, for negativeand positive imbalances, respectively. The algorithm featuressix thresholds (th#), with 0V < th1 < th2 < th3 < Vref <th4 < th5 < th6 < 5 V. Gain1 and Gain2 are the gains ofINA1 and INA2, respectively. The thresholds are symmetricalwith respect to Vref and have been chosen in such a way toassure maximum amplification and, at the same time, to avoid


Fig. 7. μTCD equipped with the NIC circuitry.

saturation of the amplifier chain. Moreover, owing to the fastacquiring rate of the algorithm, applying the μTCD downstreamof a gas-chromatographic column, we are able to reveal veryshort signal impulses of gas, down to 100 ms.

In order to filter the residual noise from the poweringcircuitry (i.e., 100-Hz noise after the full-wave rectificationand smoothing and linear regulation, Section III), the systemacquires eight samples every 20 ms in one mains semiperiod,makes an average, sets the gain for the next step, and sends theacquired data with the corresponding gain through the serialinterface. It was experimentally verified that all these operationsof the μC, with a clock frequency of 20 MHz, last about 7 msand are sufficiently less than a 50-Hz semiperiod; thus, thisaveraging technique can be adapted to operate with a 60-Hzmains frequency, too. The 120-Hz noise frequency could alsobe filtered provided some firmware modifications of systemparameters are accomplished. However, a noise generated bya full-wave rectification, even in the case of an average op-eration, could introduce a systematic error since a full-wavesmoothed waveform includes a nonzero offset. However, theNIC (Section III-C) adopted in our system removes all possiblesystematic errors, provided they remain constant during thewhole measurement cycle.

C. NIC

To avoid saturation of the INA1 output signal due to thelarge values of amplification and in order to remove a pos-sible systematic error introduced by the power supply noisegenerated by the full-wave rectification (Section III), it wasnecessary to introduce a NIC circuitry and algorithm. Asdepicted in Fig. 7, we make it digitally by using anotherAD5293, setting the values of RBW2 and RAW2 to move alittle higher or a little lower the voltage at SIGN−, in orderto obtain ΔV compensated

native = VSIGN+ − V compensatedSIGN− = 0. This

is accomplished by changing the pair RBW2 and RAW2 whilemeasuring the output of INA1 with the ADC, until Vout isequal to Vref with no unknown gas entering the μTCD analyticchannel (Fig. 3) and with only the feeding current I (Fig. 4) forthe couple μTCD–NIC to hold the set point.

The maximum value of the adjustable offset depends on thedefined set point, because it changes the resistance values of theμTCD. Considering the physical resistance of a single sensibleelement of the μTCD of about 300 Ω at ambient temperature

(20 ◦C), with an effective temperature coefficient of resistance(TCReff) of 0.00237 K−1, the maximum operating resistanceof every single element created with our technology, whichassures the best sensitivity and long-term reliability, is around570 Ω at a temperature of about 400 ◦C. Let us suppose toascribe the imbalance of the bridge to only one resistor (e.g.,R4) which deviates from the theoretical value of ΔR. With thesimplifying assumption that R3 and R4 do not change duringthe balancing procedure and with software limits on the digitaltrimmer of Rmax

BW2 = 20 kΩ−RminWA2 = 17.5 kΩ (RBW2 =

896× 20 kΩ/1024) and RminBW2 = 20 kΩ−Rmax

WA2 = 2.5 kΩ(RBW2 = 128× 20 kΩ/1024) to limit the sensitivity variationand the current in the potentiometer IC, the NIC can com-pensate a maximum positive deviation of ΔR = +88 Ω anda maximum negative deviation of ΔR = −67 Ω, in both casesmore than the 11.8% of the operative value. This value is higherthan the resistance tolerance due to the fabrication process ofthe whole Wheatstone bridge, which is lower than 5%.

The presence of the NIC affects only slightly the Wheatstonebridge sensitivity (i.e., ΔV/ΔR, not to be confused with thesystem sensitivity that will be defined in Section IV). As anexample, considering the same operative conditions used tocalculate the maximum adjustable ΔR with the presence of agas concentration which gives an operative relative resistancevariation of 0.1% in the analytical branch and considering theworst case with a compensated ΔR = 28 Ω, the Wheatstonebridge sensitivity decreases to about 4%. The decrease is lessthan 4.5% even in the case of a relative resistance variation of50% in the analytical branch.

The discrete nature of the NIC causes an imbalance com-pensation that, in general, yields ΔV compensated

native �= 0, due todifferent values of the operative resistance and the quantizationerror of the NIC. Moving the trimmer position by one bit,the maximum imbalance that can be obtained with the ΔRpreviously considered is about (ΔV compensated

native )max = 3 mV,which is also the worst case that we can consider. This limits themaximum usable value of the gain of the amplification chain toabout 410.

IV. EXPERIMENTS

We made a session of two experiments to check and testthe μTCD and the electronic architecture. In the first one, wetested the system featuring only the AD8555, and in the secondexperiment, we exploited the gain stage evolution with both theINAs (the AD8555 and the discrete INA2).

In a first experiment (Fig. 8), the analytical branch of theμTCD was connected to the outlet of a commercial GC system,i.e., Thermo Scientific Focus GC, equipped with a 5-m-longFAST-GC column with a 50-μm inner diameter (ID), coatedwith a dimethylpolysiloxane stationary phase. The column wasoperated at a constant He carrier pressure of 400 kPa. An emptyfused silica capillary was used as reference branch, which is 1 mlong and has 50-μm ID, at a constant pressure of 135 kPa. Inthese conditions, the flow inside both columns is estimated tobe the same, nominally 0.086 sccm. The columns were installedinside the same GC oven, which was operated in isothermalmode at 60 ◦C. The μTCD temperature set point was 300 ◦C.


Fig. 8. Sensitivity tests on several toluene masses. The vertical scale showsthe Wheatstone bridge imbalance.

Fig. 9. AGC algorithm operating in the acquisition of 574 ng of toluene. TheA/D line represents the as-received signal, i.e., the real acquisition.

Liquid toluene was injected at very high split ratios between1 : 1500 and 1 : 4500, resulting in masses below 1 μg. As can bedisclosed from Fig. 8, the signal shape is very well defined overthe baseline with these amounts of toluene, and the detectionlimit is expected to be much lower and will be determined oncethe final specifications for the GC column geometry and for themicrofluidic section will be defined. Given that the responseof a TCD is a function of the analyte concentration, the goodresponse to masses in the range of nanograms is due to theextremely low volume of the sensing chamber of the proposedμTCD, as enabled by the micro electro-mechanical systems(MEMS) technology. The 191 ng of toluene ideally eluted intothe 0.3-μL μ-chamber results in an equivalent concentration ashigh as 6.37 · 105 μg/L. The peak area, as typically used in GCto quantify the amount of analytes, is also reported in the plot.The trend of the peak area as a function of the analyte mass islinear, showing a system sensitivity of 15.2± 0.6 μVs/ng. Theoperation of the AGC algorithm in the acquisition of 574 ngof toluene is depicted in Fig. 9, which highlights that the onlyINA1 was operating in this experiment.

In a second experiment, we analyzed a natural gas samplein order to test both the gain stages (INA1 and INA2) in asingle acquisition run (Fig. 10 and its zoom in Fig. 11). TheμTCD has been connected downstream of a commercial GC

Fig. 10. GC analysis of a natural gas sample with our μTCD and the doublestage of gain. The gain curve represents only the gain of the active INA duringthe acquisition, showing both the gain changes and the transitions betweenINA1 and INA2. We also observe that, when INA1 is active, INA2 gain (notused) is set to its maximum value (i.e., 30) whereas, when INA2 is active,INA1 gain (not used) is minimum (i.e., 70).

Fig. 11. Zoom of the first two acquired peaks of the GC analysis in Fig. 10.

system with a 3.2-m-long and 75-μm-ID analytical column,coated with a 1-μm MEGA-1 FAST stationary phase, while thereference column was 1.5 m long and has 50-μm ID, with nostationary phase.

The chromatogram was acquired in isothermal mode at 35 ◦Cat a constant He carrier pressure of 125 kPa and a flow rateof 0.11 L/min. A natural gas sample was injected at the veryhigh split ratio of 1 : 1820, resulting in 0.55 μL in column. Theacquired signal showed a good sensitivity, good operation ofthe coupled gain stages, and a very wide dynamic range, whichforced us to use a special vertical scale in Fig. 10 to highlightall the features of the chromatogram.

The data associated with the revealed compounds are shownin Table II. The complete physical modeling of a TCD responseis rather complex and still under investigation. As a first-orderapproximation, the signal of a GC peak on a TCD detectoris generated by the difference between the TC of the mixtureof the peak sample gas and carrier gas flowing through theanalytical branch of the detector and the TC of the pure carriergas flowing in the reference branch. Second-order signals aregenerated by several other physical parameters (heat capacity,convection, etc.) and depend on the operation conditions of theGC/TCD system (pressure, flow rate, detector geometry and


TABLE IICONCENTRATIONS OF THE COMPOUNDS REVEALED

IN THE EXPERIMENT IN FIG. 10

temperature, etc.). For this reason, usually in GC applications,the GC/TCD system is periodically calibrated against certifiedgas mixtures, and the peak areas of the analyzed samples arecorrelated to the peak areas of the certified mixture.

Nevertheless, in Table II, the measured concentration ofevery single peak was computed by normalizing the peak areato the square root of the ratio between the TC of the peak sampleand the TC of the carrier gas, as suggested in [20]. Consideringthe wide range of the gas concentrations, the results are in verygood agreement with the nominal concentrations reported bythe manufacturer of the test gas mixture.

The aim of this work was the development of a readoutcircuitry capable of measuring signals generated by a μTCDWheatstone bridge with a very high gain range, and the resultsin Table II, together with the plot in Fig. 10, are a verypromising result of a preliminary complex sample analysis.

V. CONCLUSION

The subsystems for the control and the acquisition of aμTCD have been described, depicting the constant resistancecontrol, the NIC of the bridge, and the amplification network.Measurements on the μTCD have been presented. Severaltests on masses of toluene revealed a good system sensitivityof 15.2± 0.6 μVs/ng: it makes the μTCD suitable for manyportable applications like in-line quality control, and industrialsecurity and safety.

A second experiment on a natural gas sample (mixture ofmethane, ethane, propane and butane) showed the good opera-tion of the double gain stage and the AGC algorithm, with anacquisition signal featuring a very wide dynamic range and verygood concentration measurements.

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Fabio Rastrello received the Ph.D. degree in information engineering from theUniversity of Perugia, Perugia, Italy, in 2012.

Since January 2009, he has been with the Department of Electronic andInformation Engineering, University of Perugia, where he is working onembedded systems and innovative gas sensing systems (ultralow power, thermalconductivity detectors, ion mobility spectrometry, etc.).

Pisana Placidi (M’96) received the Ph.D. degree in electronic engineering fromthe University of Perugia, Perugia, Italy, in 2000.

Since January 2005, she has been a Research Assistant with the Departmentof Electronic and Information Engineering, University of Perugia. Her currentresearch interests include the very large scale integration design of CMOSactive pixel sensors, interfacing of sensors with programmable system-on-chip,and the design and fabrication of the readout circuits of capacitive biosensors.


Andrea Scorzoni (M’90) received the Ph.D. degree in electronic engineeringfrom the University of Bologna, Bologna, Italy, in 1989.

From 1983 to 1998, he was a Grant Holder and then a Research Fellow withthe Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delleRicerche, Bologna. Since 1998, he has been a Professor of electronics with theDepartment of Electronic and Information Engineering, University of Perugia,Perugia, Italy. Since 1989, he has been a Local Scientist responsible for a num-ber of national and international projects. His research interests include electrondevices and test structures on silicon and silicon carbide, micromachinedgas sensors and solid-state radiation sensors, electromigration, microsys-tems, embedded systems, radio-frequency identification, and brain–computerinterfaces.

Enrico Cozzani received the Ph.D. degree in electronics and computer sciencefrom the University of Bologna, Bologna, Italy, in 2011.

Since 2006, he has been working as a grant student at the Istituto per laMicroelettronica e Microsistemi, Consiglio Nazionale delle Ricerche, Bologna,on gas sensing microsystem design, characterization, and electrothermal finite-element simulations.

Marco Messina received the Ph.D. degree in mechanical engineering fromImperial College London, London, U.K., in 2004.

He is currently a Researcher with the Istituto per la Microelettronica eMicrosistemi, Consiglio Nazionale delle Ricerche, Bologna, Italy. His researchinterests include numerical simulations of microfluidic conjugate heat transferand piezoelectric actuators.

Ivan Elmi received the M.S. degree in physics from the University of Bologna,Bologna, Italy, in 1998.

Since April 1999, he has been a grant student with the Istituto per laMicroelettronica e Microsistemi, Consiglio Nazionale delle Ricerche, Bologna,developing a system for environmental monitoring and characterizing gassensors and miniaturized gas-chromatography systems. His recent activitiesinclude design and fabrication processes for microelectromechanical systems.His recent research projects include FP5 CleanAir and FP6 Goodfood, as wellas Italian national and regional research projects and industrial collaborations.

Stefano Zampolli received the M.S. degree in physics from the University ofBologna, Bologna, Italy, in 2000.

Since April 2000, he has been a grant student with the Istituto per laMicroelettronica e Microsistemi, Consiglio Nazionale delle Ricerche, Bologna,where he has been a Researcher in the Sensors and Microsystem Group since2005, designing and developing gas sensors and gas sensing microsystems forenvironmental monitoring and agrofood applications. He has participated to theFP5 Clean-Air and FP6 GoodFood projects as well as to several national andregional research projects and industrial collaborations.

Gian Carlo Cardinali received the M.S. degree in electronic engineering fromthe University of Bologna, Bologna, Italy, in 1979.

Since 1982, he has been with the Istituto per la Microelettronica e Micro-sistemi, Consiglio Nazionale delle Ricerche, Bologna, where he is currentlythe Head. Since 1996, he has been involved in research projects dealing withthe implementation of systems for air quality monitoring based on micro gassensors. His scientific interests are in the areas of design, fabrication, andtesting of electronic devices and microsystems, and development of dedicatedmicroelectronic processes suitable for integration into microsystems (e.g., ink-jet printheads).

thermal conductivity detector for gas chromatography: very wide gain range acquisition system and...

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