A Review of Implementing ADC in RFID Sensor

7/25/2019 A Review of Implementing ADC in RFID Sensor

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Review ArticleA Review of Implementing ADC in RFID Sensor

M. Zurita,1 R. C. S. Freire,1 S. Tedjini,2 and S. A. Moshkalev3

Federal University of Campina Grande, Rua Aprigo Veloso , - Campina Grande, PB, BrazilUniversite Grenoble Alpes, LCIS, rue de Laffemas, BP , Valence Cedex, Rhone-Alpes, FranceCenter for Semiconductor Components, UNICAMP, P.O. Box , - Campinas, SP, Brazil

Correspondence should be addressed to S. edjini; [email protected]

Received September ; Revised January ; Accepted February

Academic Editor: Andrea Cusano

Copyright M. Zurita et al. Tis is an openaccess article distributed underthe Creative Commons AttributionLicense, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Te general considerations to design a sensor interace or passive RFID tags are discussed. Tis way, power and timing constraintsimposed by ISO/IEC andISO/IEC standardsto HF RFID tags are explored.A generic multisensor interace is proposedand a survey analysis on the most suitable analog-to-digital converters or passive RFID sensing applications is reported. Te mostappropriate converter type and architecture are suggested. At the end, a specic sensor interace or carbon nanotube gas sensorsis proposed and a brie discussion about its implemented circuits and preliminary results is made.

1. Introduction

In the last years, many technological developments havedramatically expanded the unctionality o RFID. Advancesin microelectronics, embedded sofware, and RF/microwave-circuit integration are making new RFID applications pos-sible []. Among these new applications, the use o passiveRFID tags as environmental sensors has a huge number opossibilities [], includingthe Interneto Tings (Io) []andhuman care assisting solutions [].

Tere are too many ways to transorm a RFID taginto a RFID sensor. Some o them exploit the sensitivity

o the tag antenna to the physical characteristics o itssurrounding environment or its inuence on RFID chipresponse as reported in []. However, i the addition osensing capabilities to passive RFID transponder is made byintegrating a sensor interace to the digital core o tags chip, aullset o components should be developed:more specically,an analog-to-digital converter (ADC), a signal conditioningcircuit to the sensors, and a multiplexing circuit to allowinteracing multiple sensors.

Te key component or these augmented tags is clearlythe analog-to-digital converter. When looking or possiblesolutions, the designer willace a huge number o architecturetopologies and different approaches to implement it. Tis

reality brings exibility to design stage but also some incer-titude about the rightest choice. In this context we present asurvey analysis on the most suitable ADCs or RFID sensingapplications.

2. Sensors Connected through the Digital Side

RFID tags with sensors connected through the digital sidedo not have any modication in the analog ront-end circuitor antenna. Te sensing data is usually transmitted to thereader upon a specic requesting command according tothe communication protocol. Te main advantages o thissolution are itscompatibility with the current RFID standardsand its capability to interace multiple sensors.

A sensing tag compliant with an existing RFID standardwill not need any modication on the corresponding readershardware or rmware to be read. It means that all existingcommercial readers are potentially capable to get the sensingdata o these improved RFID tags.

Te capability to interace multiple sensors, in spite o theincreased complexity on the tag circuitry, is also a notableadvantage. Different kinds o sensors can be embedded onthe same tag and almost any kind o sensor can be adopted(limited by the power consumption) making it a powerulsensing device.

Hindawi Publishing CorporationJournal of SensorsVolume 2016, Article ID 8952947, 14 pageshttp://dx.doi.org/10.1155/2016/8952947


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Te main challenge to design the components o thesensors interace is to meet the time and power constraints.Since all energy available in a given transponder is deliveredby the reader, all steps o sensing, signal conditioning, andanalog-to-digital conversion must be done as ast as possibleto allow storing or transmitting the digital result beore the

end o communication and consequently end o poweringenergy. In other words, the protocol timeout constraintsprohibit minimizing speed rates to save power.

3. Power Constraints

Te major constraint or all passive RFID transponders isthe limited amount o available power. A study on the mainconcerning about power constraints o HF RFID systemsoperating at ree space is covered by this section. Systemsembedded in different medium or in mixed medium needsa more complex analysis [].

Near the eld, the magnitude o the magnetic eld alongthe central axis o a circular loop antenna can be calculatedby

||= 222 + 23 , ()where is number o windings, is the current through theantenna,is the antenna radius, andis the distance romthe center o antenna plane []. Far rom the eld, that is,or > /2, the magnitude o the magnetic eld can becalculated by []

||=2

4132

22

+ 22

. ()For the .MHz ISM band (. MHz kHz)

European regulations limit the magnetic eld strength todBA/m at m rom the reader [], while currentAmerican FCC rules limit to dBA/m at m rom thereader. Tis difference should be harmonized in the nearuture []. Considering both regulations, () and () canbe used to calculate the magnetic eld strength generatedby a reader operating at maximum power limit as depictedin Figure . Te magnetic eld strength generated by atypical HF reader along its central axis is also represented asreerential.

According to the ISO/IEC - [] the RFIDtransponders should be able to operate at a magnetic eld o mA/m (. dBA/m). Tis eld strength correspondsto a maximum operating distance o approximately meterby European regulations or centimeters by American FCCrules. However, RFID tags eaturing a power consummationbelow the standard dened limit can operate at longerdistances. Teoretically the maximum operating distance willbe the limit o ar eld to near eld, where the magneticcoupling is no more possible. Tis corresponds to approx-imately . meters or . MHz. In other words, RFIDHF tag consuming less than . dBA/m or . dBA/mwill be able to operate at the maximum distance o . m

190

170

150

130

110

90

7050

30

10

0.01 0.1 1 10

Distance (m)

Magnetice

ldstrength(dBA/m)

EuropeAmerica

Typical reader

F : Maximum allowed magnetic eld strength or . MHzISM band according to European and American regulations(dBA/m@manddBA/m @ m, resp.).Te lower curverepresents the magnetic eld strength o a typical HF reader (square

coil, cm, turns, mA).under European and American regulations, respectively.When the ISO/IEC - (NFC standard) [] is adopted,the minimum magnetic eld required or a transponderoperation is . A/m (. dBA/m), which corresponds toa maximum communicating distance o centimeters byEuropean regulations or centimeters by American FCCrules.

A recent NF NFC tag chip, developed by EM Microelec-tronic, requires only . A/m (. dBA/m) or activation[] (less than a hal o the minimum required by ISO/IEC

-), being able to operate at approximately cen-timeters o distance rom the reader (considering Europeanregulations). aking such commercial solution as the state othe art, it is possible to conclude that similar tags includingsome sensing circuitry will operate below centimeters,since the sensor interace and the sensor itsel will draw moreenergy, requiring a strongest eld to be powered. Evidently,active RFID sensor tags can operate at longer distances, sincetheir energy is not provided by readers communication eld.

4. Timing Constraints

Besides the above explained power constraints, sensor inter-

ace circuits and sensor itsel should comply with timingconstraints imposed by standardized communication pro-tocol. Te time constraint related to the chosen protocolshould be taken into account or circuit design and choice osensors. Analog-to-digital converter and signal conditionerare usually the most time consuming circuits:

AFE+ Core+ ADC+ SCC+ MUX+ Sens 1, ()whereAFE is the response time o the analog ront-endcircuit or a transmission operation,Core is the time needby the digital tags core to complete all operations relatedto command decoding and sensor interace control,

ADCis


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Read REQ

Response

Reader

Tag

AFE+core

MUXSens

SCC

ADC

T1

F : ime diagram o a ull sensor reading sequence. Onlycrosshatch slices are taken into account or total 1time internal.the time interval o one complete analog-to-digital conver-sion (including sample holding time), andSCC andMUXare the time consumed by signal conditioning circuit andmultiplexer, respectively, beore to deliver the sensor signal to

ADC. Sensis the response time o the sensor, that is, the timebetween its powering and a correct output. Finally,1is themaximum allowed time or a tag to reply on a reading alikerequest. Te time diagram o a ull sensor reading sequenceperormed by a RFID sensing tag is depicted in Figure .

For HF tags complaint with ISO/IEC protocol, thetimeout constraint or 1is dened as []

1=

4192 , minimum,4224 , nominal,4256 , maximum.

()

For NFC complaint tags, the ISO/IEC protocoldenes the timeout constraint or 1as []

1=

409620 , minimum,409624 , nominal,

4096214

, maximum

.

()

Solving both () and () or the nominal carrier re-quency (. MHz), the minimum and maximum time-outs are . s/.s or ISO/IEC protocol and.s/ms or ISO/IEC protocol. Tis way,a sensing system targeting both standards should considerthe most restrictive timing limits, that is, . s as theminimum timeout and . s as the maximum timeout.5. Cost and Size Constraints

Te size o passive RFID tags is usually constrained by itsantenna size since all other components are integrated on

a tiny silicon chip plus a small tuning capacitor (sometimeseven the tuning capacitor is integrated on the chip), takingan area o no more than some ew square millimeters. Fur-thermore, the antenna type and size have also a huge impacton tag costs. Tis way, aspects o cost and size are treatedtogether, departing rom the tag antenna characteristics.

Tere are basically three types o antennas currentlyadopted or HF RFID tags:

(i) Ferrite core coil antenna.

(ii) Planar spiral coil antenna.

(iii) Air core copper wire coil antenna.

Ferrite core coil antennas presents a small aperture butallows a very good read range when well aligned with theinterrogator reading eld. In spite o its small orm actor,it is still being a not exible and thicker component, whichmakes it not convenient or integration with stickers, productlabels, or other common applications where a at element isdesirable. Moreover, its cost is typically ten times greater than

the equivalent printed coil in high volume [].Planar spiral antenna is one o the most common types

currently in use. Tey can be abricated with copper overFR (or similar substrate) by etching process or printed withsome conductive ink over paper or a polymer substrate. Telast one is the most cost effective, allowing mass production,but is also the less robust solution. Its low cost and physicalcharacteristics makes this type o antenna the best suited ormass consumer applications.

Air core copper wire coil antenna is an alternative orplanar spiral coils. Similar coil shapes implemented in planarsolutions can be made in air core wire coil but taking thebenet o a lower parasitic series resistance andthe possibility

to stack multiple wire turns to reduce the inner or outercoil dimension. Furthermore, since its terminals are wireends, it can be easily connected to IC pads or other standardelectronic components without dedicated machinery. Teyare usually thicker and more expensive than printed planarspiral ones.

Generally speaking, the minimum area o a given tagantenna is constrained by the power required by its circuitryand the desired operating distance rom the reader. As muchas the power consumption increases, bigger will be the areao antenna needed to harvest this energy rom the readerat the same distance. Longer communication distances willalso require bigger antennas to provide the same amount o

energy.Te impact o the required power over the tag size can

be estimated regarding the elementary model o an HFRFID tag ront-end with a resistive load (Figure ) and itscorresponding actor equation []

= 1/+/ , ()where is the parasitic series resistance o antenna, is theinductance o antenna,is the carrier requency, andisthe load resistance.

For evaluation purposes, two sets o planar square spiralcoil antennas were designed and simulated. Considering that


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: actor and conductive ink area or two sets o planarspiral coils.

Coil area (mm2) Approach Approach Ink area (mm2) Ink area (mm2)

. . . .

. . . .

. . . .

. . . .

. . . .

HF antenna model

Rp

L Cp Ct RL

F : Elementary model o an HF RFID tag ront-end andload.

the amount o conductive ink is an important parameteror the cost o printed antennas, the design approach o therst set o coils took the ink savings as the main priority,regardless actor. For the second set, coils were designedtaking the actoras the main priority, regardless theamounto conductive ink needed. Te resulting actor and the totalink area or each coil is presented on able . For both sets axed resistance o k

was adopted as circuit load.

Te two sets o tag antennas were simulated consideringthe losses o a ull bridge rectiying circuit, coil parasitics andproper tuning capacitors or . MHz carrier requency.All simulations were perormed to obtain the minimummagnetic eld strength needed to deliver . mW to the load.Simulation results are depicted on Figure .

As one can see, the relationship between cost, size, andpower demand is quite evident. As much as the eld strengthdecreases, bigger is the coil area needed to provide the samepower to the load. Cost reduction with conductive ink alsoimpacts the coil area. For all simulations, the magnetic eldstrength was considered uniorm andconstant along o wholecoil area. Evidently, in real case this convenient assumption

will be seldom satised, making necessary bigger coils orpractical operations [].

When RFID tags with sensing capabilities are considered,the cost o integrated sensors and the sensor integration itselmay take values much higher than those involving antennaabrication and chip integration. emperature sensors orusual environment conditions ( to Celsius) can beeasily integrated with the tag circuitry atvery low cost. A lightsensor can be easily integrated onto a chip but needs a specialcare on tag packaging to allow the light entrance. However,other sensors may need a special abrication process whichis not compatible with current CMOS processes used ortag circuit abrication. Its integration requires more complex

Magnetic eld strength (A/m)

Approach 1

Approach 2

Tag

size(mm

2)

700

600

500

400

300

200

100

00.40.60.81.01.21.41.6

F : Minimum HF tag size according to two differentapproaches. Approach : coil tracks were designed with minimalwidth to save conductor costs. Approach : coil tracks wheredesigned with optimal width and number o turns to keep the samequality actor.

and expensive techniques, besides the additional costs o adedicated packaging.

For mass production at low cost the most promisingalternative are ully printed sensors (orthick lmsensors). Itsabrication processes can be easily adequate with the processo printed antennas. Furthermore, some printed and exiblesensors such as photodetectors, temperature sensors or gassensors are transitioning rom R&D to mass production [].

6. Sensor Interface

A generic diagram o a passive RFID tag with a multiplesensor interace integrated to its digital core is shown onFigure . Te sensor interace is composed by three maincomponents: a multiplexer, a signal conditioner, and ananalog-to-digital converter.

Once the energy is the most scarce resource in suchsystems, the role o the multiplexer is not only to select thesensor signal to be processed but also to select the sensorwitch should be powered, keeping others unconnected tosave that power. Even the selected sensor should be switchedoff as soon as the ADC sample-and-hold circuit nishes its

job. Some sensors need a signicant warm-up time beoreto be ready or correct readings. Tese kinds o sensors are

not appropriated or passive RFID solutions and should beavoided i possible.Te signal conditioning circuit is also a very common

need when a sensor signal should be sent to a analog-to-digital converter. Although its clear importance, the signalconditioning circuit should be minimized in order to savepower. Generally speaking, our different solutions are pro-posed or this block, being only one (the last) a real signalconditioning:

(i) Bypass and ADC Input Range Statically Adjusted. Ithe adopted ADC has an adjustable input range andonly one sensor is interaced or multiple sensors withsimilar output voltage range, the input range o A/D


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Antenna

Analog front-end

Rectier

Modulator

DemodulatorNon-volatile

Singal

conditioner

memory Multiplexer

Sensor 0

Sensor 1

Sensor N

ADC

Digital core

Digital core

Voltageregulator

Sensors interface

..

.

F : A generic diagram or a passive RFID tag integrated with a multiple sensor interace.

converter can be adjusted to t the sensor signal rangeand the signal conditioning circuit eliminated. Tissolution is very light in terms o power consumptionbut in practice can only be adopted in some casesbecause ultralow power ADCs usually have a limited

capability o input range adjusting, specially or smalldifferences between minimum and maximum volt-ages.

(ii)ADC Input Range Dynamically Adjusted. I theadopted ADC has an adjustable input range andmultiple sensors with nonsimilar output voltagesare interaced, the input range o the analog-to-digital converter can be dynamically adjusted to tthe signal range o the selected sensor according toprevious established values stored into tags memoryor physically dened component values. Once more,the major problem is the limited capability o ADCinput range adjusting.

(iii)Bypass and High Resolution ADC. Tis solution con-sists in adopting an ADC with a resolution muchhigher than the needs o target application in orderto avoid spending power with analog signal con-ditioning. Although the consummation o a highresolution ADC is higher than the similar one with aminimum adequate resolution, the additional powercan be smaller than the necessary to support ananalog conditioning circuit.

(iv)Signal Conditioning. Depending on the sensor, di-erent signal treatments can be necessary. Te most

common treatments considering a voltage signal areoffset and gain adjusts. Instrumentation ampliers(IAs) are usually employed in conventional sensorsignal conditioning circuits. However, conventionalIAs are too much power consuming or passiveRFIDs. Even the best current commercial low powerdevices [, ] consumes about o W which isalmost twelve times the whole power requested by thebest commercial UHF tag. In other hand, applicationspecic IAs have been reported at much lower powerconsuming like W [], . W [] and evennW [], indicating that its use is possible orRFID sensing tags at HF and UHF as well.

Notwithstanding, sensing RFID tags or some specicapplications can dispense the ADC, the multiplexer and evenhavea conditioning circuit as simple as a RC. Te detection oproduct seal violation [] is a good example o sensing RFIDtags without an analog-to-digital converter. Te threshold

detection o a given quantity can also be implemented usinga simple Schmitt trigger gate and some ew components.In this case, the sensing tag will not be able to provide aprecise inormation about the quantity magnitude but onlyits placement below or above a previous dened threshold.

Although the ADC is nota mandatory component to readsensors through the digital side o a RFID transponder, ageneric sensor interace should have at least one, speciallyi the inormation about the quantity magnitude should beacquired. Since the ADC is usually the most complex andcritical component o the sensor interace, its design tends tobe the major challenge. Its characteristics also have inuenceover the signal conditioning circuit or are inuenced by it.A right choice on the most appropriated converter type andimplementation approach is very important at this point. Amore careul analysis about analog-to-digital converters orpassive RFID sensing applications will be carried out on thenext section.

7. ADCs for RFID Sensing Applications

A good indication o the most appropriated ADCs or RFIDsensing applications can be ound looking or the mostenergy-efficient implementations reported on VLSI and ISSCconerences. aking the power consumption as the mainguideline, thegureo merit (FOM) o more than analog-to-digital converters reported on VLSI and ISSC [] over

the last ten years were classied according to its type andrepresented on Figure .

Since the most efficient converter year afer year isthe successive approximation register (SAR) [], ollowedby pipeline and Delta-Sigma types. More specically, exceptby the converter proposed by Patil et al. [], all ADCseaturing a FOM below J/Conversion-step are SAR typewith a capacitive DAC implementing some variation o thecharge redistribution approach, rst proposed by [].

When the gure o merit o all the reported CMOS ADCssince to the present are organized by the technologynode, the result is a clear advantage o SAR type or all nodesbelow nm as can be seen in Figure .


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: Te most efficient converters reported on VLSI and ISSCC since .

Re. ype FOM (J/Conv-step) s (MHz) (uW) ENOB ech. (nm)[] SAR . . . .

[] SAR . . . .

[] SAR . . . .

[] SAR . . . . [] SAR . . . .

[] SAR . . . .

[] SAR . . . .

[] SAR . . . .

[] SAR . . . .

[] Async. . . . .

[] SAR . . . .

[] SAR . . . .

[] SAR . . . .

[] SAR . . . .

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

Year

SAR

Delta-Sigma

Pipe

Flash

Others

FOM(fJ/Conversion-step)

1E + 06

1E + 05

1E + 04

1E + 03

1E + 02

1E + 01

1E + 00

1E 01

F : Figure o merit (FOM) o the reported ADCs on ISSCCand VLSI over the last ten years.

Te most efficient converters reported on VLSI andISSCC since are listed with more details on able . Telimit o J/Conversion-step was arbitrary chosen.

Almost all these converters were implemented in CMOS

nm or below. A remarkable exception was proposed byJeong et al. []. A . ENOB SAR ADC achieving a very lowFOM and very low power consumption (about o nW)using a relatively old technology ( nm). In act, this isonly reported ADC above nm eaturing a FOM below J/Conversion-step. Te converter uses a charge recyclingtechnique which saves previous sample MSB voltage andreuses it throughout subsequent conversions, preventingunnecessary switching o large MSB capacitors as well asconversion cycles.

It is important to note that a good analog-to-digitalconverter or passive RFID sensing applications should havenot only a low FOM but also very low power consumption.

>350

350

250

180

130

90

65

45

40

32

28

14

1E + 06

1E + 05

1E + 04

1E + 03

1E + 02

1E + 01

1E + 00

1E 01

FOM(fJ/Conversion-step)

Technology node (nm)

SAR

Delta-Sigma

Pipe

Flash

Others

F : Figure o merit (FOM) o the reported ADCs on ISSCCversus technology node. Only designs on CMOS processes wereconsidered.

Some solutions as the asynchronous converter proposed in[] or the pipeline converter proposed in [] achieves a verylow FOM but under high sampling rates and higher overall

power consumption than other similar solutions with lowersampling rates.

In spite o the the very good results achieved by recentADCs on able , there are other aspects to be consideredbeore to choose a specic design or a given application. Tetarget technology node, silicon area, operating temperatureand calibration procedures are only some o most commonaspects. All the things considered, the choice can all on anADC which the power consumption is not exactly the lowestone. Fortunately, depending on the operating distance andthe size o tag antenna, the power available to an HF RFIDtag can be quite high. For instance, the power induced ontoa tag load by the typical HF RFID reader reereed on Figure


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Journal o Sensors

1E + 5

1E + 4

1E + 3

1E + 2

1E + 1

1E + 0

00 2 4 6 8 10 12 14 16 2018

Distance (cm)

Pow

erattagload(W)

F : Powerinducedonto a tagloadby a typical HF RFID readeraccording to its distance rom the reader. Te reader antenna is asquare coil, cm, turns, and mA. Te tag is equippedwith a square spiral planar coil, mm, turns.

was simulated according to its distance rom the reader. Forthis simulation a tag equipped with a small square spiralplanar coil ( mm, turns) connected to a k loadwas assumed. Simulation results are presented in Figure .

As one can see, there is virtually no power issues orshort distances. On the other hand, or communicationdistances above cm the available power is only some ewW, requiring a more careul choice o the analog-to-digitalconverter. At cm, the available power to the tag load isabout o W. Te standard tag circuitry consumes between and W. It means that the power portion lef orthe whole sensing circuitry (ADC + signal conditioner +sensor multiplexer) is about o to W. Since the Signalconditioner uses to demand a signicant power, is reasonableassume that the the power lef to the ADC will not be morethan a hal o this total, that is, W.

According to the previous timing analysis (Section ),the maximum and minimum timeouts or HF RFID tagscomplaint with both ISO/IEC and standards, arebetween .s and .s. Considering that part o thistime will be consumed by the tags logic core, analog ront-end and sensor interace component delays, the time lef orone analog-to-digital conversion will be slightly smaller. Inpractice it will correspond to conversion data rates between. and kSample/s.

Summarizing, in order to be appropriate or passiveHF RFID sensing at a reasonable distance rom reader,

an ADC should have a maximum power consumption oW, assuming the worst case and leaving some powermargin to other sensor interace blocks and the sensor itsel.Concerning the timing constraints, a minimum sample rateo . kSample/s should be satised. Only three ADCs listedon able do not ulll these conditions. All others are SARtype and satisy both constraints with a good margin.

In act, the charge redistribution SAR ADC is a verysuitable solution or passive RFID sensing as demonstratedby Brenk et al. in []. An ultra-low power -bit SARADC has been designed and used to implement a passivetemperature sensor UHF RFID tag able to establish a stablecommunication up to . m rom the reader.

8. Proposed Sensor Interface

Recent researches in nanotechnology eld demands or smallsized gas reactors called micro- and nanoreactors [].Essentially, these reactors are small volume chambers, typi-cally less than mL, equipped with gas inputand output chan-nels andmicro-sensors inside. Te design andconstruction othese systems aims at a comprehensive control o actors suchas size, conguration, surace quality, integration, and costs.Works on high perormance mass and pressure sensors basedon carbon nanotubes have already been reported [, ]. Temicro- and nanoreactors are at the base o high perormancenanodevices, allowing a more detailed assessment o physicaland chemical processes occurring inside and on the suraceo the nanostructures as compared with more conventionalsensors.

A common issue with these micro-reactors is connectingthe inside sensors to outside world where all measuringinstruments are placed. In conventional approaches theseconnections pass through the chamber walls by mean o

metal pins electrically insulated rom the body using somehard material like borosilicate sealing glass. Te problem withthat solution is theinconvenient gasleakage causedby ailuresin insulation material due to the stress they are submitted.Tis problem can be eliminated replacing pins and insulatorby some kind o wireless communication. For that, an RFIDsolution is proposed.

Since the main goal is eliminating all sensor pins, thedistance between tag and reader is not a big issue. Moreover,since all intended gas reactions can be well monitoredtaking samples per second, a HF RFID system can nicelysatisy all requirements. In addition, an integrated solutioncontaining sensors and all the measuring circuitry could help

the adoption o these reactors or non-scientic applications.Although the possibility o operate close to the reader

antenna, where the magnetic eld strength is high, theavailable energy to the tag should not be too high due tothe small communication inductor (antenna) which areais very limited by internal micro-reactor dimensions. Onthe other hand, the accuracy required by such scienticapplications demands a good signal conditioning circuitwhich prohibits some power saving alternatives.

Te proposed interace and sensors to be integrated ona passive HF RFID tag are briey explained on the nextsections.

.. emperature Sensor. Te temperature is an importantplayer on most chemical reactions. Additionally, the behavioro carbon nanotube gas sensors is also inuenced by environ-ment temperature. Te capacity to measure it simultaneouslyto gas measurements gives to the system valuable inormationto improve its accuracy.

A low power temperature sensor was designed based onthe Zero emperature Coefficient (ZC) concept. Accordingto [], most o current CMOS technologies present a specicbiasing point where temperature effect over the carriermobility and voltage threshold cancel each other or a widerange o temperatures. I the ZC point o a given transistoris known, it is possible to design a temperature independent


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M1

M2

R1

R2

Vbias

VDD

Vsen

GND

F : Low power temperature sensor based on ZC character-istic o MOS devices.

constant current source in s very simple way. I such constantcurrent is applied through a diode connected MOS, itsGSwill be given by []GS= GSF+ V1 DF , ()

whereGSFandDFare the gate-to-source voltage and draincurrent o the current source MOS operating at ZC pointandV is the threshold voltage temperature coefficient.So, under constant drain current the gate-source voltage isproportional to the temperature.

Te implemented low power temperature sensor isdepicted in Figure .1 is the constant currentsource biasedat ZC point by1 and2.2is the sensing device. Tecircuit was designed to operate under a V power supply,the current through1 was about nA and1, 2 totalingabout o M. Te total power consumption o wholesensor is about o nW.

Te sensor was abricated in nm CMOS process(Figure ) and tested or a temperature range rom toC ( to F). Te simulated and measured responseo the designed temperature sensor are shown in Figure .

As one can see, the empirical response o the temperaturesensor is even more linear than the simulated one. Since theobserved deviation o the simulated output rom the ideal

linear behavior is mostly related to the variation o the ZCoperation point o1(Figure ), this improved response isprobably due to a small variation in threshold voltages oabricated MOS transistors or due to a ortunate combinationo mutual canceling effects in designed layout which placeshigh sized biasing resistors and a very large pMOS transistor.

Te temperature error between the measured sensoroutput and an ideal linear response within the entire testedtemperature range is represented on Figure . Te highestabsolute error occurs in C, being approximately .C,or .%. Te absolute error remains smaller than .Cor temperatures bellow C and smaller than .C ortemperatures between C and C.

Temperature (C)

Outputvoltage(mV)

Measured points

Ideal linear responseSimulated response

1401201008060402002040

340

320

300

280

260

240

220

200

180

160

140

F : Measured and simulated response o abricated temper-ature sensor.

Temperature (C)

14012010080604020020400.5

2.0

1.5

1.0

0.5

0.0Measurederror(C)

F : emperature error between the measured sensor outputand an ideal linear response within a to C temperaturerange.

.. Carbon Nanotube Gas Sensor. Carbon nanotubes (CNs)present a high surace-to-volume ratio which makes them

very suitable or gas sensing applications []. Its main advan-tage over conventional gas sensors, based on semiconductingmetal oxides (MOX), is its low energy consumption. Inaddition, CN based gas sensors are very sensitive and can bedecorated by nanoparticles sensitive to the gases o interest,

achieving an improved perormance on gas sensing.Tereare basically two differentcongurations or carbon

nanotube sensors: eld effect transistors and chemiresistors.Field effect transistors based on single-wall carbon nanotubesare very sensitive but need a complex abrication process[]. Chemiresistive CN sensors do not needs a so complexabrication process and can be made in two different ways[]:

(i) Tin lms o mixed metallic and semiconductingCNs deposited between two electrodes.

(ii) Microsensor ormed by a small quantity o CNs con-necting two electrodes separated by ew micrometers.


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1

2

3

4

F : Schematic drawing o the CN gas microsensor. Oxi-dized silicon substrates (), gold patterned electrodes (), wirebonding connections (), and the gap with DEP deposited carbonnanotubes () compose the device. Te inset shows a closer view othe CNs deposited between the gold electrodes [].

Tin lm sensors are usually simpler to build when

compared with micro-sensors. Common manuacturingapproaches are chemical deposition and jet printing [, ].In spite o its simplicity, thin lm CN sensors get easily con-taminated by gases or other substances. Tat contaminationchanges the sensor response and usually requires a specialtreatment to be removed.

On the other hand, CN microsensors are abricatedby micromanuacturing techniques, which are clearly morecomplex than chemical deposition or jet printing but are lesssensitive to contamination since the contaminants can beevaporated rom the surace o carbon nanotubes by a pulse ocurrent. Additionally, the reduced size o these microsensors,allied with their high sensitivity due to strong sel-heating by

Joule effect [], makes them very attractive to be used inmicroreactors. A schematic drawing o the CN gas micro-sensor is depicted on Figure .

A previous microreactor containing one CN microsen-sor inside has already been abricated ollowing the samedirections explained, but without any embedded electronics,being all measurements made rom outside [, ].

.. Multiplexer. Following the previous explained guide-lines, a ve-input sensor multiplexer was conceived as shownin Figure . Primary selection is made by : digital MUXcommanded by the digital tags core (lines0 to2). Teour rst addresses correspond to our external CN sensors

(Sens0 to Sens3) and the last address corresponds to theinternal temperature sensor (not shown). Te selection o anexternal sensor connects its lef side to ground by the corre-sponding switch (0to3) and its right side to the output(

out) through the corresponding analog transmission gate

(SW to SW).All external sensors have two biasing lines, Vread and

Vclear, controlled by read sens and clear sens inputs. TeVread line is intended to reading operations, so it brings asmall current to the selected carbon nanotube sensor throughthe corresponding biasing resistor (0 to3) orminga simple voltage divider proportional to sensor response.Te Vclear line is intended to clear the selected sensor by

mean o a short duration current pulse (about o Aduring ew microseconds). For that, appropriated values oresistance should be used or0to3resistors. Since theexact resistance value o each CN microsensor is not welldetermined (depending on the amount and characteristicso deposited nanotubes), both

and

resistors are placed

externally to the chip.

.. Signal Conditioning Circuit. Te CN gas microsensorsdesigned or this system are planned to present a gas reeresistance between kand k. Previous experimentsindicate a resistance variation between.% and +%under gas exposure []. Ia A biasing currentis applied tothe sensor, the minimum resistance variation will correspondto a . V variation on the sensor signal. Such small

voltage signal requires a dB gain stage in order to t ina mV input rage ADC, so a signal amplication stage ismandatory or this specic application.

o achieve the desired gain keeping a minimal inter-

erence on the signal, an instrumentation amplier (IA)is required. Very low power implementations have beenreported with capacitively-coupled topologies [, ] whichmake them attractive or passive RFID sensing. However, dueto the need or a high linearity device with a programmablegain, a traditional resistive eedback IA has been chosen.Moreover, the short distance between tag and reader ensuresenough power to eed this device. Te proposed signalconditioning circuit is represented in Figure .

Te gain adjustment, needed to adequate the voltagerange o different CN microsensors, is dened by a -bitword (Gain) which controls the programmable gain amplier(PGA). Te zero adjustis obtained by an -bit capacitive DACconnected to the positive IA input and controlled by ZeroAdjdigital input. Both Gain and ZeroAdj words are inormedby the digital core o the tag. Te power consumption othe complete signal conditioning circuit designed in nmCMOS technology, with a . V supply voltage, achieves abouto W in simulation... Analog-to-Digital Converter. As demonstrated inSection , the charge redistribution SAR ADC is a verysuitable solution or passive RFID sensing. For this reason,an -bit capacitive SAR ADC was chosen or this application.Its DAC working principle is based on the charge sharingbetween binary weighted capacitors, using only CMOS

controlled switches as depicted in Figure . Its conversionsequence can be resumed in steps as ollows:

(i)1: switch 1is turned to inand switches 0to 7areturned to1 node connecting all lower capacitanceplates toinpotential. At the same time, switches2and 2are closed discharging any residual voltage onthe nodes and OUand setting OUto zero.

(ii)2: switches2 and2 are opened and switch1is turned toREF setting the voltage on the uppercapacitance plates to REF in.

(iii)3: switch7 is turned toREF+setting and nodeto

(REF+

REF

)/2 in. Since the Cs capacitor


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Journal o Sensors

Biasing network

Sensors

Multiplexer

Rr0 Rr1 Rr2 Rr3

Rc0 Rc1 Rc2 R

M4

M5

M3M2M1M0

s2 s1 s0

c3

_read

sens0

sens1

sens2

sens3

Sens_0

Sens_1

Sens_2

Sens_3

_clear

SW0

SW1

SW2

SW3

CMD

CMD

CMD

CMD

sels_0..sels_3

MUXout in0..out3

read_sens

clear_sens

Vout

s0..s2

VDD

VDD

sel_0

sel_

0

sel_

1sel_1

sel_2

s

el_

2

sel_

3sel_3

F : Schematic diagram o CN sensors connected to a biasing network and multiplexing circuitry.

was discharged during1 phase, this same potentialwill appear onOU. At this point, the voltage signalon

OU node will indicate the value o

7 output.

A negative voltage means that the sampled voltage(in) is less than hal the reerence voltage, resulting as7= 0 or 7= 1 otherwise. I the resulting bitwas zero,the SAR controller should turn the7switch back toREF.

(iv)4to10: the testing sequence described in previousstep should be repeated or switches6 down to0.At the end o last step, all output bits (0, . . . , 7) willbe known, completing the analog-to-digital conver-sion.

Te successive approximation register in charge o allcontrol signals was implemented by a nite state machine

ully customized to save power. Moreover, once the con-version nishes the SAR cuts off its eeding clock to avoidunnecessary power consumption with gate switching.

With the view to improve the converter accuracy withoutincrease the energy demand, a low power and low offsettime domain comparator (DC) [] was used. Te layout oall DAC capacitors and DC main components were madeaccording to common centroid techniques.

Te ADC was abricated in IBM nm CMOS process.A chip photograph is shown in Figure . Te total chip

occupies . .mm2 and the ADC core area is m2, including a Daisy Chain testing structure.

Static tests were perormed in order to measure thedifferential non-linearity (DNL) and the integral nonlinearity(INL) which gives +./. and +./. LSB, respec-tively. High values are due a parasitic-nonlinear capacitances


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Journal o Sensors

+

Gain

Gain PGA

Vout

Vref+

V

4

8

ref+

Vref

DAC

b0 b1 b2 b3 b4 b5 b6 b7ZeroAdj

Vin

Vout

F : Proposed signal conditioning circuit.

Vx

C C/2 C/4 C/8 C/8 C/8C C/2 C/4

S1

VRE

Cs

F

VREF+

S2a S2b

VOUT

Vin

b7 b6 b5 b4 b3 b2 b1 b0

F : Capacitive DAC adopted or A/D converter.

F : Micrograph o chip containing the proposed low powercharge redistribution SAR ADC (at center) and the low powertemperature sensor (at lef).

which were underestimated during the design stage. Temeasured DNL and INL are presented in Figure .

Te FF spectrum was measured using points with dB normalized sinusoid inputs at . kHz and . kHz asshown in Figures and . Te measured SNDR is . dBand the SFDR is . dB. Results indicate ENOB equals. bits. Te total power consumption with V o supply

voltage is . W at kS/s which corresponds to a gureo merit (FoM) equals to J/Conversion-step. Preliminaryrened measurements indicate that about o % o wholeconsumption is only in successive approximation register(SAR) unit which is slightly greater than expected.

.. Overall Results. Te overall simulated and measuredresults or power consumption and time delay are summa-rized in able . Te timedelay o whole systemis much lowerthan the maximum allowed by ISO and ISO


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Journal o Sensors

Output code

1.00.5

0.00.5

1.0

1.00.0

2.01.0

3.0

DNL[LSB]

INL[LSB]

250200150100500

Output code

250200150100500

F : Measured DNL and INL error o abricated ADC.

0

20

40

60

80

100

Pout

(dB)

2nd harmonic

3rd harmonic5th harmonic

Frequency (kHz)

50403020100

42.8 dB

53.1 dB50.8 dB

Output spectrum withfIN = 2.92 kHz

F : Output spectrum o designed ADC or an input re-quency o . kHz.

0

20

40

60

80

100

Pout

(dB)

2nd harmonic

3rd harmonic

5th harmonic

42.8 dB

62.7 dB

50.8 dB

Output spectrum withfIN = 44.8 kHz

Frequency (kHz)

50403020100

F : Output spectrum o designed ADC or an input re-quency o . kHz.

standards, as discussed on Section . Although the analog-to-digital converters consume more energy than expected,its power consumption is still within the limits previously

discussed. Moreover, the total power consumption is stillcompatible with a NFC system or a to cm distancerange, according to the simulation results o Section .

Since the proposed system is a sensor interace or passiveHF RFID tags, it could be integrated to a standard tag circuitas a single chip solution or connected with a RFID interacechip, like the NF- rom EM Microelectronic [], with thehelp o a simple glue logic circuit.

9. Conclusion

Te general considerations to design a sensor interace orpassive RFID tags was discussed. Te power and timing

: Power and time consumption o sensor and sensorinterace according to simulation and testing results.

Device Power consumption ime delay

MUX . W .sSignal conditioning circuit .W . sADC .

W .

s

CN sensor . W . semperature sensor .W Overall results . W .s

constraints concerning HF tags was presented and a genericmultisensor interace was proposed. A survey analysis onanalog to digital converters was made over more than reported converters. Analysis shows that, over the last eightyears, the successive approximation register A/D based oncapacitive DAC is the most appropriated solution achiev-ing the best results in technology nodes below nm. A

specic sensor interace or carbon nanotube gas sensorswas proposed. Simulation and measured results indicate thatthe designed system is more than ten times aster than themaximum time acceptable by the corresponding standard,leaving a wide time margin or core processing or systemadjusts. Finally, the overall estimated and measured powerconsumption are compatible with the target application,which should be capable o operating at ew centimeters romthe reader antenna as intended.

Conflict of Interests

Te authors declare that there is no conict o interests

regarding the publication o this paper.

Acknowledgments

Te authors acknowledge the support rom Region Rhone-Alpes (France), the CNPq (Brazil), and the INC/NAMIEC(Brazil) or the nancial support and MOSIS or chip abrica-tion.

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A Review of Implementing ADC in RFID Sensor