Analog Dialogue 35-04 (2001) 1by Paul Kearney
([email protected])INTRODUCTIONToday,handheldpersonaldigitalassistants(PDAs)arepoppingupjustabouteverywhere.
Theyarefindingfavor,notonlywithbusy executives, but also with
mainstream consumersand evenin the classroom! Their ease of use and
portability have been keystotheirrapidlygrowingsuccess.
Themostpopularexamples*include the Palm Pilot series, the
Handspring Visor, and the
SonyClie.Itisestimatedthatthisyearalonesome15millionPDAunits will
be sold worldwide, with annual sales expected to
growto34millionunitsby2004.AcommonfeatureofallofthesePDAunitsistheirmethodofenteringdataviaastylusandaresistive
touch screen. With it the user can easily enter dates in his/her
diary, send email, keep shorthand minutes of meetings, etc.The
touch screen itself is typically a 4-wire (X and Y)
resistiveelement.(Therearealso5-wireresistivescreensandcapacitorscreens
on the market, but they are typically more expensive.)Thetouch
screen interfaces with the host microprocessor via an A/Dconverter
(ADC), such as the AD7873 and AD7843, which havespecial features
tailored to the application. This article will discussthe common
application issues faced by designers when interfacingan ADC to a
resistive touch screen, and how they are solved withthe
AD7873.Theory of Operation and Details of the
ApplicationThetouchscreenusuallyconsistsoftwolayersoftransparentresistive
materialusually indium tin oxide (ITO) or some otherform of
resistive polyester material, with silver ink for
electrodes.Thetotalresistanceofeachlayervariesfromvendortovendor,but
typical screens are in the 100 to 900 range. The two layersare
stacked on an insulating layer of glass, separated by tiny
spacerdots. They are interfaced electrically to a controller A/D
converter.Figures 1 and 2 show, in simplified form, how a
controller ADCmight interface with a 4-wire resistive
screen.HOSTMICRO-PROCESSORPROTECTIVEHARD COATY-PLANE RESISTIVE
FILMX-PLANE RESISTIVE FILMPROTECTIVEHARD BACKINGADCPEN APPLIES
PRESSURETO TOUCH SCREENY+YXX+TOUCH SCREEN CONTROLLER ADC
POWERSSCREENS AND SENSES POSITION FROM THEM.Figure 1. The touch
screen uses a multilayer sandwich ofresistive films and protective
coatings.ADCY PLANER YPLUS R YMINUSX PLANE R XMINUSR XPLUSYX++VE
SUPPLYGROUNDVOLTAGE SENSED ~ POSITIONSWITCHES INTERNAL TO
THETOUCH-SCREEN CONTROLLERWHICH POWER THE SCREENXY+STYLUSY NODE
FLOATSSPACERDOTS TOSEPARATELAYERSPEN (STYLUS) PRESSURE FORCESTHE X
PLANE TO SHORT TO THEY PLANE AT LOCATION (X, Y)ADC I/P SENSED @
Y+PIN FOR X COORDINATEFigure 2. Stylus interfacing with a 4-wire
resistive touchscreen. The X-coordinate measurement is
depicted.Duringmeasurementofagivencoordinate,oneoftheresistiveplanes
is powered along its axis through switches on the controllerADC
chip, and the other plane is used to sense the location of
thecoordinate on the powered plane. For X coordinate
measurement,theXplaneispowered. The
Yplaneisusedtosensewherethepenislocatedonthepoweredplaneasfollows:
Atthelocationwhere the pen depresses the touch screen, the planes
are shorted.The voltage picked up on the sensed plane is
proportional to thelocation of the touch on the powered plane. This
voltage is thenconverted using the controllers ADC.For a
Y-coordinate measurement, power is applied to the Y plane,the X
plane is used to sense the position, and the voltage is
digitized.The digital code corresponding to the X and Y coordinates
is
thenoperatedonbythehostmicroprocessor,andthecommand,information,orinstructionintendedbythestyluslocationisregistered.Basic
AD7873 touch-screen interfaceFigure 3 illustrates a simplified
block diagram of the AD7873.
Inadditiontoservingasacoordinate-measurementtransducer,itprovides a
number of peripheral functions important for the
PDAsfunction,includingmeasurementsoftemperatureandbatterycondition,
touch detection, and pressure measurementplus anon-chip 2.5-V
voltage reference.Figure 4 illustrates a typical application
diagram for the AD7873interfacing with a 4-wire resistive touch
screen. First lets discussthe basic functionalitystylus coordinate
measurement. The setof touch-screen switches at the left in Figure
3 drive the resistivescreenviatheso-calledtabletpins,X+, Y+,X,and
Y. Thetablet pins also serve as sensors of position voltage from
the screenwhen not active, in applying the method of interfacing
describedbriefly above.A number of things were considered in the
implementation of thedesign. First, the switches must be able to
source and sink currentto/from the low impedance screen. Say, for
example, the supply is5 V and the screen impedance is 200 . The
p-MOS switches tothepositivesupply(ontheX+and
Y+pins)mustbeabletosource25mAwhenthescreenispoweredforcoordinatemeasurement.
Similarly the n-MOS switches to ground (these areThe PDA
ChallengeMet by the AD7873Resistive-Touch-ScreenController
ADC*Allbrandorproductnamesmentionedaretrademarksorregisteredtrade-marks
of their respective holders.2 Analog Dialogue 35-04 (2001)on the Y
and X pins) must be capable of sinking this 25
mA.Theswitchesdrainandsourceconnectionsmustbecarefullydesignedatthelayoutstagetoavoidelectromigrationproblems(due
to the large current density in these nodes). The
conservativelydesigned AD7873 switches are capable of supplying a
70 screen,using a 5-V supply.SUPPLYDECOUPLING1615141312119
81234765AD7873CSDINDCLKVREFVCCGNDDOUTX+VBATAUX10BUSYVCCPENIRQXY+YSERIAL/CONVERSION
CLOCKCHIP SELECTSERIAL DATA INCONVERTER STATUSSERIAL DATA OUTPEN
INTERRUPTREF DECOUPLING0.1FTOUCHSCREENVOLTAGEREGULATORTO
BATTERY10k100k PULL-UPFigure 4. Typical application circuit using
the
AD7873.Theon-resistanceoftheswitches,about6,alsoposedachallenge.
For example, when used with screen impedances as lowas 100 and a
minimum supply voltage of 2.7 V, the voltage dropacross the
switches is appreciable and greatly reduces the
dynamicrangeofsignalsappliedtotheconverterinput.
Toavoidacorrespondingreductionofconversionaccuracyandeffectiveresolution,andbecausetheONresistanceoftheswitchesmaynot
track the resistance of the screen over temperature and supply,the
converter must be able to operate in a ratiometric mode. Figures5
and 6 illustrate respectively how the AD7873 can be
configuredinbothratiometricandsingle-endedmodesofoperationforcoordinate
measurements.Usingtheratiometricmode,theactual
ADCreferenceistakenfromthedrainnodesoftheswitchespoweringthescreen.Forexample,
if a Y coordinate is being measured, the Y plane is poweredand the
REF+ and REF of the ADC are taken from the Y+ andY pins,
respectively. The A/D conversion will be ratiometric;
i.e.,theresultoftheconversionwillbeequaltotheratiooftouch-screen
resistance at the measuring point to the total
touch-screenresistance,irrespectiveofthevoltagedropintheAD7873'sswitches.
This is the best way of ensuring the accuracy of
coordinatemeasurements.ADCREF+REFGROUND+VE
SUPPLYX+XYY+RX+RXRYRY+TOUCH SCREENMODELLED ASRESISTOR MESHADC
REFERENCE ISEFFECTIVELY THEPOTENTIAL ACROSSTHE SCREENAIN = VSCREEN
RXRX + RX+VSCREEN = VX+ VXFigure 5. Ratiometric operation of the
ADC is independentof switch on-resistance voltage
drop.ADCREF+REFGROUND+VE SUPPLYX+XYY+RX+RXRYRY+TOUCH SCREENMODELLED
ASRESISTOR MESHTO GROUNDTO INT/EXT REFSOURCE***CAN NEVER ACHIEVE
TRUE FULLSCALE OF ZEROSCALEBECAUSE OF FINITE ON RESISTANCE OF THE
SWITCHES.Figure 6. Single-ended operation of the ADC.INPUTMUX 6 T0
1INVBATYMINUSXMINUSXPLUSYPLUSPENINTERRUPTSARCAPDACADCREF+ADCREFBATINAUXINYY+X+TEMPINBATTERYMONITORGNDVCCTEMPERATURESENSORTOUCH-SCREENSWITCHESVINXPLUS+DUAL3
TO 1 MUXYPLUSXMINUSYMINUSGNDREFINT/EXTVREF
REFBUFFBANDGAPREFVDACCOMPVCOMSPORTDINCSDCLKBUSYDOUTPENIRQFigure 3.
AD7873 block diagram.Analog Dialogue 35-04 (2001)
3Butthereisatrade-off:Inratiometricmode,thescreenmustremain powered
during the actual conversion process, in
additiontotheacquisitionphase,sincethescreenvoltageisusedastheADC
reference. Alternatively, during a single-ended conversion,
thescreenneedonlybepoweredduringtheacquisitionoftheinputsignal. For
the AD7873 this requires 3 clock cycles, at worst 20% ofthe overall
conversion time (assuming a 15
clocks-per-conversionmodewhichisthedevicesfastestvalidthroughputrate).However,
used ratiometrically, at this fastest rate, the screen will
bepermanently powered while the device is on. If the screen
resistanceis 100 and a 5-V supply is used, the screen will draw 50
mAanobvious consideration in battery-operated handheld devices.With
this dilemma in mind, the AD7873 has been designed to offerthe
designer a choice of four power-down options to better managethe
overall power consumption of the touch-screen controller, usingthe
devices 2-bit selective power-down option control.Power-down bit
setting00. Fullpowerdownbetweenconversions.Boththe ADCandthe
internal reference are powered down between conversions.If the
ratiometric mode is selected, the touch-screen driverswill be
turned off between conversions (for cycles longer than15
clocks).01. The ADC remains on between conversions, but the
referencepowersdown.(Anexternalreferencemaybeused.) Thetouch screen
power is switched off between conversions, asin mode 00.10.
TheADCpowersdownbetweenconversions,whiletheinternal reference
remains on. This is useful, as the referencerequires about 7 s to
power up. So when doing
single-endedmeasurements(suchasbatteryandtemperaturereadings)with
the internal reference, there is no need to allow a
delayforthereferencetopowerup. Thetouchscreenpowerisswitched off
between conversions, as in mode 00.11. The ADC and internal
reference remain powered on betweenconversions. In this mode, the
switch drivers to the touch screenwill remain on until the selected
input channel or power-downmode is changed, or until CS (convert
start) is brought high.Whatever option is used, the designer must
realize that the bulk ofthe power will be dissipated in the
external touch screen when it ispowered. The AD7873 itself will
dissipate only about 2.4 mW withthe internal reference enabled, a
2-MHz data clock, throughputof125kSPSandwitha3.6-Vsupplywhilea100
screenisdissipating 129.6 mW! So the converter dissipates less than
2% ofthe power dissipated in the screen. However, it should be
notedthatinthefirst3ofthe4caseslistedabove,thescreen-driveswitchescanbeturnedoffbetweenconversions.
Thisfeatureisusefulforprovidingaconsiderablereductioninaveragescreenpower
if the application can allow for a reduced throughput rate(with the
clock frequency held at 2 MHz).The first three power-down options
above will be especially helpful.For example, using those options,
a throughput rate of 20 kSPSwould reduce the screen power to an
average of 20 mW under
theconditionsdescribedabove.Forcharacterrecognitionfromthescreen,
throughput rates of 20 kSPS may be acceptable withoutany
discernible reduction in performance. In general, however,
aresponse-speedtradeoffshouldbeanticipated.
Thescreensthemselvesmayhavelargeparasiticcapacitanceassociatedwiththem10
nF is not uncommon. This may result in a non-negligibleRC time
constant, so that time must be allowed for settling
beforescreenmeasurementscanbetaken.Forthisreason,inmanycircumstances,poweringdownthescreensbetweenconversionsmight
not be such a good ideaunless steps are taken to accountfor the
large screen R-C time constant (this will be discussed
later).Resolution of the converter itself needs to be in the 10- to
12-bitrangefortypicalscreens.
TheAD7873provides12bitsofresolution.The B grade guarantees 12-bit
no-missing-code (NMC)performance, while the lower-cost A version
provides 11-bit
NMC.Peripheralfunctionsbattery,temperature,andpressuremeasurement:
TheAD7873supportsanumberofimportantperipheral functions vital to
the PDA:AdedicatedbatterymonitoringADCchannelconsistsofanattenuator
circuit that divides down the unregulated battery supplyvoltage by
4 and digitizes it. A supply of up to 6 volts can be appliedto the
VBAT input pin. The accuracy of the function is important
inapplications that use battery supplies with a very shallow
dischargeslope and a very sharp knee (e.g., Li-ion batteries), so
it's importantfor the system to know where the battery is in its
discharge profileat any given time. Usually, the microprocessor
instructs the batterychannel to digitize the battery voltage every
few seconds. Shouldthe battery enter the sharp discharge part of
its curve (Figure 7),it could be damaged and not recover from such
a 'deep discharge'event. In practice, typical PDAs will flag a
condition well beforethe knee is reached and respond appropriately.
The AD7873 batterychannelwillgivetypicalaccuracyperformanceinthe0to
1%range with worst case error of 3%, using the internal
reference.Where better accuracy is required, the user may need to
calibratethis reading, depending on the battery's discharge
characteristic.VOLTAGETIMESHALLOW DISCHARGESHARP
DISCHARGEKNEEIMPORTANT FOR CONTROLLERTO DETECT KNEETO PREVENTDEEP
DISCHARGEFigure 7. Typical battery discharge
curve.Temperaturemeasurementisthesecondimportantperipheralfunction
performed by the AD7873. Temperature within the
PDA'scaseisanespeciallyimportantparametertomonitorindevicescontainingrechargeablebatteries.
Typically,ifthetemperatureexceeds45Cduringcharging,themicroprocessorneedstobeflaggedandprovideanappropriateactiontoavoidpermanentdamage
to the PDA due to overheating. Figure 8 shows the schemeused to
measure temperature.4 Analog Dialogue 35-04
(2001)ADCTEMP12STEMP120SFROM TEMP 0CURRENT SOURCEFROM TEMP 1CURRENT
SOURCELOGIC SIGNALS TO SWITCHIN/OUT DIFFERENT CURRENT SOURCESLOGIC
SIGNALS TO SWITCHIN/OUT DIFFERENT CURRENT SOURCES20SDIODECONNECTED
PNPFigure 8. Temperature-measurement block diagram.The AD7873
provides two modes of temperature
measurement.Thefirst,thesingle-conversionmethod,performsasimpleconversionofthejunctionvoltageofadiode-connectedPNPtransistor,
biased with a constant current. The diode voltage willvary with
temperature by about 2.1 mV/C. Typically the
readingwouldbefactory-calibratedat25CduringmanufactureofthePDA.
This method will typically provide a resolution of about 0.3Cwith
an accuracy of
2C.ToavoidcalibrationbythePDAmanufacturer,aseconddifferentialmethod
is provided on the AD7873. Two conversionsare required, Temp 0 and
Temp 1. Temp 0 is performed with a
lowvalueofdiodebiascurrent,I0,and Temp1isperformedwithabias
current, I1=105 I0. From the Ebers-Moll equation and usingsome
simple math, we can show that:Vbe kT q nN = ( / ) ( ) 1wherek =
Boltzmann's constant, 1.38054 1023 eV/Kq = Charge on the electron,
1.602189 1019 CT = Absolute temperature in kelvinsN= I1/I0,
typically 105, as suggested for the AD7873.HenceT C Vbe K ( ) =
2490 273 Because Vbe is limited to about 142 mV, the resolution of
thedifferentialmethodissignificantlylessthenthatofthesingle-conversion
method. Typical resolution of about 1.6C is achieved.The accuracy
of both methods is typically the same, about
2C.Figure9plotsatypicalaccuracycomparisonforbothmethodsover the
temperature range, 0C to 70C. The main advantage ofthe differential
temperature method is that it eliminates the needfor calibration by
the PDA manufacturer.Pressure (or more precisely touch resistance)
can be
calculatedviaanumberofsimplearithmeticmanipulationsusingtheAD7873.
One can determine whether the touch response is
beinggeneratedbythestylusorafingerorsomeotherobjectbymeasuring the
contact resistance between the X and Y plates.
Thisprovidesanindicationofthesizeofthedepressedareaandtheapplied
pressure. The area of the spot touched is proportional tothe touch
resistance. Two methods can be used. The first
requirestheusertoknowthetotalresistanceoftheXplanemembrane.ThesecondmethodrequiresthatthetotalresistanceofbothXand
Y membranes be known. Equations and diagrams can be foundon the
data sheet.TEMPERATURE C3.00 20 40 60
80ERROR2.02.52.01.51.00.500.51.01.5VBE ERRORVBE vs. SINGLE DIODE
METHODSSINGLE DIODEFigure 9. Comparative errors of differential-
and single-conversion temperature measurement.Waking up with a
touchThe pen-interrupt
functionFigure10illustratesthepeninterruptfunction,whichprovidesan
active-low open-drain output signal to the host microprocessor.When
the part is powered down in a mode where the pen
interruptisenabled,atouchonthescreenwillcausethevoltageonthePENIRQpintobepulledtoground.Inthismodethe
Ypinswitchdrivertogroundisturnedonduringpowerdown;whenthe two
screens touch, the X+ pin is pulled to ground via the Ypinthus
initiating an interrupt. The touch-resistance
thresholdistypically
