26128-modern sensors greatly enhance consumer electronic system performance pdf (1)

8/3/2019 26128-Modern Sensors Greatly Enhance Consumer Electronic System Performance PDF (1)

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You can expect a largeevolutionary trend insensors over the nextfew years. Demand isstrong for more precisenavigation technology,especially for indoor asset and global

tracking. This technology will significantly affect theinertial- and motion-sensor consumer markets, whichinclude accelerometers, gyroscopes, and magnetome-ters in mobile phones, tablets, game stations, laptops,and other devices. Addressing this trend, this article isthe first of a two-part series on sensors in consumer sys-tems (see sidebar Perspectives on modern sensors). Itexplores the various sensor options and how to properlylink the analog world of sensors and their conditioningcircuits to the digital world of processing data and addingintelligence. It also examines the inherent trade-offs for

some popular options and discusses discrete approachesversus highly integrated approaches. A one-size-fits-allapproach may sometimes fail to attain the performanceparameters that some systems need. Although designersmay in many cases gravitate to more highly integratedapproaches due to time-to-market constraints, theseapproaches may not lead to a design that consumers willflock to the stores to purchase. Designers should balanceperformance and features with size and cost to make asuccessful product for the consumer market.

BY Steve taranovichcontriButing technical editor

Modern sensors

greatly enhanceconsumer-electronic-systemperformance

images:istockphoto

[www.edn.com]38 eDn | SEPTEMBER 22, 2011


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With the aDoption of sensors anD connectivity, consumer

Devices have unDergone a revolution: they are no longer

one-Way, isolateD islanDs.they noW incluDe user anD

environmental aWareness anD interactivity, as Well as

connectivity With surrounDing systems anD the internet.

SEPTEMBER 22, 2011 | eDn 39[www.edn.com]


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Sensors live in the analog world oftouch, temperature, image, light, posi-tion, motion, and pressure, in whichmobile phones, smartphones, note-books, MP3 players, and game consolesare ubiquitous. The most popular typesof sensors help to enliven your world ofentertainment, information technology,communication, home appliances, andother product markets. Most touch sen-sor technologies in todays designs areeither capacitive or resistive. Capacitivesensors are suitable for a range of sensingapplications, such as keypads, rotators,or buttons, whereas resistive-touch-screen sensors use a four-wire resistivetechnology, usually with built-in ADCs,to offer both ease of design and greaterflexibility to touchscreen applications.

capacitive touchscreens

A capacitive-touchscreen panel com-prises an insulator, such as glass, coatedwith a transparent conductor, such asITO (indium tin oxide). Because thehuman body is also an electrical conduc-tor, touching the surface of the screenresults in a distortion of the screenselectrostatic field, which is measurableas a change in capacitance.

When choosing a touchscreen con-troller, you should consider severalaspects, including the accuracy of theCDC (capacitance-to-digital convert-er); the units noise-handling ability,which the digital filter of the ADC usu-ally handles; its environmental compen-sation; and the advanced algorithms in

its host processor or CDC chip, whichprovide WinCE (Windows CompactEdition) or Linux driver and softwarecapabilities. These features allow you todevelop a system that accurately detectsfinger presence, motion, and intendedactivity on the touchscreen.

STMicroelectronics offers highlyintegrated capacitive and resistive,multiple-channel touchscreen solutionswith controllers that interface seamless-ly to a host processor using the S-Touchseries of controllers for touch-key andtouchscreen applications. The com-panys portfolio of sensors also includesMEMS (microelectromechanical-system) motion sensors for measuringmotion, acceleration, inclination, andvibration; proximity detectors for met-al-body-proximity sensing; and analogand digital temperature sensors.

Touchscreen controllers typically usea 24-bit converter because they need todigitize only small levels of capacitance.One such converter is Analog Devices24-bit, two-channel AD7746 CDC.The device measures capacitance thatconnects between the on-chip exci-tation source and the on-chip sigma-delta modulators input. An on-chipsquare-wave excitation signal is appliedon the capacitance of the touchscreenduring the conversion, and the modu-lator continuously samples the charge

through the capacitor. The digital filterprocesses the modulators output, whichis a stream of zeros and ones contain-ing the information in zero and onedensity. The CDC then scales the datafrom the digital filter, applying the cali-bration coefficients. You can read thefinal result through the serial interface,which sends it to an external host pro-cessor through a serial bus (Figure 1).

When considering a devices envi-ronmental compensation, keep in mindthat ambient-temperature changes can

cause fluctuations in stray capacitance.You can use an external temperaturesensor for compensation, such as the

at a GL ance

A on-siz-fits-a approach

may somtims fai to attain th pr-

formanc paramtrs that som

systms nd.

Th most popuar typs of sn-

sors hp to nivn your word of

ntrtainmnt, information tchnoo-

gy, communication, hom appianc-s, and othr product markts.

Capacitiv snsors ar suita

for a rang of snsing appications,

such as kypads, rotators, or

uttons.

Rsistiv-touchscrn snsors

us a four-wir rsistiv tchnoogy,

usuay with uit-in ADCs, to

offr oth as of dsign and

gratr fxiiity to touchscrn

appications.

24-BITSIGMA-DELTAMODULATOR

DIGITALFILTER

DATA

CLOCK

GENERATOR

EXCITATION

CX

CIN

EXC

CDC

Figur 1Anaog Dvics 24-it, two-chann AD7746 CDC masurs touchscrn

capacitanc that conncts twn th on-chip xcitation sourc and th on-chip

sigma-dta moduators input. An on-chip squar-wav xcitation signa is appid on

th capacitanc of th touchscrn during th convrsion, and th moduator continu-ousy samps th charg through th capacitor.

TAble 1 Passive and active temPerature sensors

thercule RtD therr secnducr

Tmpratur

rang (C)

184 to +2300 200 to +850 0 to 100 55 to +150

Accuracy/

inarity

High accuracy

and rpataiity

Fair inarity Poor inarity 1C inarity,

1C accuracy

Commnts Nds cod-junction compn-

sation; has ow-votag output

Rquirs xcitation;

is ow cost

Rquirs xcitation;

has high snsitivity

Rquirs xcitation; 10-mV/K,

20-mV/K, or 1-A/K typica output

40 eDn | SEPTEMBER 22, 2011 [www.edn.com]


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base-to-emitter voltage junction of atransistor; however, you must be carefulto compensate for the PCBs (printed-circuit boards) resistance from the tran-sistor to the chip input.

resistive touchscreens

A resistive-touchscreen panel includesseveral layers, the most important ofwhich are two thin, electrically conduc-tive layers separated by a narrow gap.When an object, such as a finger, presses

on a point on the panels outer surface,the two metallic layers connect at thatpoint. The panel then behaves as a pairof voltage dividers with connected out-puts, causing a change in the electricalcurrent, which the panel registers as atouch event and sends to the controllerfor processing.

When choosing a resistive-touch-screen controller, use many of the samecriteria you would use in selecting thecapacitive-touchscreen controller.

These criteria include the accuracy ofthe ADC; the noise-handling abilityfor which digital filtering can occur onthe controller chip, the host proces-sor, or both; environmental compensa-tion; and the ability to use advancedalgorithms, which, in resistive unitscase, usually reside in the host processorand enable WinCE or Linux driver andsoftware features. A 12-bit ADC is typi-cally accurate enough, and some devicesprovide ratiometric-conversion tech-

perspectives on modern sensors

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referenceA Rako, Pau, Sing-op-amp San-Ky fitrs, fitr fanatics, EDN, Apri 20, 2011, http://it.y/nCea3n.



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niques, which provide noise immunityand tolerance of long leads. The binaryresult is free of reference drift errorsbecause the excitation source is drivenby the ADC reference. To implementenvironmental compensation for resis-

tive touchscreens, you can use eitheron-chip measurement to sense fluctua-tions in temperature that can affect thedata converter or off-chip temperaturemonitoring to compensate for resistivechanges with ambient temperature.

One example of a resistive-touch-screen controller, Maxims low-powerMAX11811, works with power-sen-sitive, handheld systems employingadvanced low-voltage processors. Thedevice contains a 12-bit SAR (succes-sive-approximation-register) ADC anda multiplexer to interface with a resis-tive-touchscreen panel. A digital serialinterface provides communications.The MAX11811 includes digital pre-processing of the touchscreens meas-urements, reducing bus-loading andapplication-processor requirements.

temperature sensors

Passive temperature sensors includeRTDs (resistance-temperature detec-tors), thermocouples, and thermistors.You can usually interface RTDs andthermistors to an amplifier circuit toprovide a voltage that is proportional totemperature. Table 1 compares passiveand active temperature sensors.

RTDs are accurate and repeatable,have low drift error, and have a tem-perature range of200 to +850C.Due to nonlinearities, these sensors

XTR105

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tr has two prcision currnt sourcs. Possi choics for Q2

incud 2N4922, TIP29C,

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Figur 3A diffrntia ampifir ampifis th thrmocoup votag, driving a sing-ndd ADC (a). Th ngativ ias gnrator ()

and v-shift gnrator (c) ar optiona (courtsy Nationa Smiconductor).

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6/7

also need linearization, which you canimplement using a look-up table in amicrocontroller. These sensors are usu-ally in a bridge configuration with dif-ferential outputs to help minimize lead-resistance errors. A difference amplifieror an instrumentation amplifier can beused to amplify and filter these sensors.

Texas Instruments, however, offersan integrated approach in its mono-lithic, 4- to 20-mA, two-wire XTR105current transmitter, which has two pre-cision current sources. It provides com-plete current excitation for platinum-RTD temperature sensors and bridges,instrumentation amplifiers, and current

output circuitry on one IC. Engineersusually run the 4- to 20-mA standarddifferential output, which is popularfor noisy environments, across long dis-tances using twisted-pair wires to a 4- to20-mA differential receiver, such as TIsRCV420. Versatile linearization circuit-

ry provides a second-order correction tothe RTD, typically achieving a 40-foldimprovement in linearity (Figure 2).

Thermocouples are rugged, have atemperature range of184 to +2300C,and are inexpensive. On the downside,they are highly nonlinear and typicallyneed significant linearization algo-rithms. Their voltage output is alsorelatively low, so they require analogamplifier-gain stages and cold-junctioncompensation (Figure 3).

Rather than measure absolute tem-perature, thermocouples measure thetemperature difference between twopoints. To measure a single tempera-ture, thermocouples maintain one ofthe junctionsnormally the coldjunctionat a known reference tem-perature and the other junction atthe temperature they want to sense.Although having a junction of knowntemperature is useful for laboratorycalibration, it is inconvenient for mostmeasurement-and-control applica-tions. Thermocouples instead incor-porate artificial cold junctions usingthermally sensitive devices, such asthermistors or diodes, to measure thetemperature of the input connectionsat the instrument. Special care mustbe taken to minimize any temperaturegradient between terminals. Hence, youcan simulate the voltage from a known

cold junction and apply the appropriatecorrection.Thermistors, whose temperatures

range from 0 to 100C, are inexpensiveand come in small packages; however,they require temperature compensa-tion in the form of a look-up table

and also need an excitation current,as do RTDs. Thermistors have eitheran NTC (negative temperature coef-ficient) or a PTC (positive temperaturecoefficient). Most PTC thermistors areswitching devices, meaning that theirresistance rises suddenly at certaincritical temperatures. For this reason,you can use them as current-limitingdevices for circuit protection. You canuse NTC thermistors for temperaturemeasurement as resistance thermome-ters, but calibration is necessary if theapplication operates over large changesin temperature. For applications operat-ing over small changes in temperature,the resistance of the material is linearlyproportional to the temperatureif youselect the appropriate semiconductor(Reference 1 andFigure 4). You canalso use thermistors in a bridge configu-ration in the same way that you do tosolve lead resistance in RTDs and feedthe differential output into an amplifier.

The most common active tempera-ture sensors, digital temperature sen-sors, yield high accuracy from the sen-sor to the microcontroller. Their datasheets specify both drift and repeatabil-ity; they need no calibration, and theyare highly integrated. Examples of thesedevices include On SemiconductorsADT7484A and Texas InstrumentsTMP006, an example of miniaturiza-

tion, integration, and innovation. Thisdevice is the first in a series that meas-ures the temperature of an object with-out making contact with the object.The high level of integration allows theoutput to include a digital serial SMBus(system-management bus) and a chip-

Figur 4 You can us NTC thrmistors for

tmpratur masurmnt as rsistanc

thrmomtrs, ut cairation is ncs-

sary if th appication oprats ovr arg

changs in tmpratur (courtsy Maxim

Intgratd Products).

REF

VSSVSS

R1

VT

RT

VREF

VREF

ADC

AFE/ADC

DIGITAL BITS

DAVINCI DSPAND SITARA

SYSTEM ON CHIP

ANALOG E-PACKETS

IMAGE SENSORCCD OR CMOS

Figur 5 Txas Instrumnts VSP2582 anaog front nd

conditions and digitizs th snsor output and snds th

data to a DSP.

Figur 6 Frscas tiny, ow-powr, handhd

MMA8450Q motion snsor has an onoard DSP for

motion appications.



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scale package. Atmels temperature-sensor portfolio also includes a varietyof options.

Image sensors typically use ana-log CCD (charge-coupled device) orCMOS technology. A CCD convertslight in the form of photons into anelectrical signal in the form of electronsand requires CDS (correlated double

sampling) to condition the sensor. Anactive-pixel CMOS imaging chip usesa CMOS-semiconductor process andrequires a sample-and-hold device. Extracircuitry next to each photo sensor con-verts the light energy to a voltage. Thechip may include additional circuitry toconvert the voltage to digital data.

Interfacing and signal conditioningcan involve processes as simple as anintegrated, high-speed ADC, such asNXPs TDA8784, or an AFE (analogfront end), such as Texas InstrumentsVSP2582 (Figure 5). The AFE or theADC conditions and digitizes the sensoroutput and sends the data off to a DSP.

Display management is a critical

part of any consumer product, especial-ly battery-dependent devices. Intersiloffers a great selection of light-to-digitaland light-to-analog sensors for back-light control and ambient-light sens-ing for mobile or fixed devices witha display. Intelligent optoelectronicsensors from TAOS (Texas AdvancedOptoelectronic Solutions) simplify themeasurement and analog-to-digital con-version of light and reduce the need forsignal-conditioning or preprocessingcircuitry in light-centric systems.

Many position and motion sensorsare available, including Freescaleslow-g-force-acceleration sensors,which detect orientation, shake, tap,double tap, fall, tilt, motion, position-ing, shock, or vibration. The companyshandheld MMA8450Q has an inte-grated DSP (Figure 6). You can con-figure this intelligent, low-power, low-noise accelerometer to generate inertialwake-up-interrupt signals when a pro-grammable acceleration threshold iscrossed on any of three sensed axes. End

users can program the acceleration andtime thresholds of the interrupt genera-tors. Other tiny, low-power sensors inthe industry have easy serial interfacesto DSPs for motion applications.

MEMS piezoresistive bridge sen-sors are the most common devices formeasuring pressure. Bosch-Sensortecfeatures a variety of such sensors inits product portfolio. Designers canalso use bridge pressure sensors in dis-crete designs with analog condition-ing circuitry for custom applications.Measurement Specialties, for example,offers the MS7301-D series of bridgepiezoresistive devices in die form. A dis-crete bridge pressure sensor needs a con-stant current excitation, which greatlyimproves the temperature dependenceof the bridge over the voltage-sourcedrive, and a differential-amplifier inputconditioner for amplification andcommon-mode-noise rejection can beused because bridge sensors typicallyoutput 2 mV/V full-scale (Figure 7).You can also use an instrumentationamplifier, which has high input imped-ance, for high-impedance bridges. A24-bit sigma-delta ADC has a highdynamic range for better resolution(Reference 2). Good design architec-tures to signal-condition these sensorscouple with DSPs or microcontrollersto provide improved safety and secu-

rity in vehicles, smarter perform-ance in ubiquitous home networks,location data from a gamut of possibleinputs, and more realistic experiencesin gaming electronics.EDN

references1 A Simp Thrmistor Intrfac to an

ADC, Maxim Intgratd Products,

Appication Not 1753, Nov 26, 2002,

http://it.y/qP668.2 Mays, Micha K, and Drk Rd-

mayn, 1- and 2-Chann, No latncy

, 24-bit ADCs easiy Digitiz a Vari-ty of Snsors, Linear Technology

Magazine, Voum 10, No. 1, Fruary

2000, pg 1, http://it.y/q2TIlV.

anlg Devce

www.anaog.com

Bch-senrec

www.oschsnsortc.com

Freecle

www.frsca.com

inerl

www.intrsi.com

Lner

technlgy

www.inar.com

mx inegred

prduc

www.maxim-ic.com

meureen

secle

www.mas-spc.com

Nnl

secnducr

www.nationa.com

NXp

www.nxp.com

on secnducr

www.onsmi.com

stmcrelecrnc

www.st.com

tex advnced

oelecrnc

slun

www.taosinc.com

tex inruen

www.ti.com

for more information

Y

cb

t e

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LT1126

LT1126

10k

10k200

350350

350350

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RP1

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RP4

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IDC0

GND

CH0

CH1

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Figur 7A discrt ridg prssur snsor nds a constant currnt xcitation, which

graty improvs th tmpratur dpndnc of th ridg ovr th votag-sourc

driv, and a diffrntia-ampifir input conditionr for ampification and common-mod

nois rjction (courtsy linar Tchnoogy).


26128-modern sensors greatly enhance consumer electronic system performance pdf (1)

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