REV. 0

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

AD7843

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 2000

Touch Screen Digitizer

FUNCTIONAL BLOCK DIAGRAM

PEN

INTERRUPT

DCLK

GND

BUSY

PENIRQ

SAR + ADC

CONTROL LOGIC

SPORT

DOUT

DIN

4-TO-1

I/P

MUX

AD7843

IN3

REF

IN4

Y+

X+

T/H

CHARGE

REDISTRIBUTION

DAC

COMP

GENERAL DESCRIPTION

The AD7843 is a 12-bit successive-approximation ADC with a
synchronous serial interface and low on resistance switches for
driving touch screens. The part operates from a single 2.2 V to
5.25 V power supply and features throughput rates greater than
125 kSPS.

The external reference applied to the AD7843 can be varied
from 1 V to +V

, while the analog input range is from 0 V to

REF

. The device includes a shutdown mode that reduces the

current consumption to less than 1

µA.

The AD7843 features on-board switches. This coupled with low
power and high-speed operation make this device ideal for
battery-powered systems such as personal digital assistants with
resistive touch screens and other portable equipment. The part
is available in a 16-lead 0.15" Quarter Size Outline (QSOP) pack-
age and a 16-lead Thin Shrink Small Outline (TSSOP) package.

FEATURES
4-Wire Touch Screen Interface
Specified Throughput Rate of 125 kSPS
Low Power Consumption:

1.37 mW Max at 125 kSPS with V

= 3.6 V

Single Supply, V

of 2.2 V to 5.25 V

Ratiometric Conversion
High-Speed Serial Interface
Programmable 8- or 12-Bit Resolution
Two Auxiliary Analog Inputs
Shutdown Mode: 1 A max
16-Lead QSOP and TSSOP Packages

APPLICATIONS
Personal Digital Assistants
Smart Hand-Held Devices
Touch Screen Monitors
Point-of-Sales Terminals
Pagers

PRODUCT HIGHLIGHTS

1. Ratiometric conversion mode available eliminating errors

due to on-board switch resistances.

2. Maximum current consumption of 380

µA while operating at

125 kSPS.

3. Power-down options available.

4. Analog input range from 0 V to V

REF

5. Versatile serial I/O port.

REV. 0

AD7843SPECIFICATIONS

= 2.7 V to 3.6 V, V

REF

= 2.5 V, f

SCLK

= 2 MHz unless otherwise noted; T

40 C to +85 C, unless otherwise noted.)

Parameter

AD7843A

Unit

Test Conditions/Comments

DC ACCURACY

Resolution

Bits

No Missing Codes

Bits min

Integral Nonlinearity

±2

LSB max

Offset Error

±6

LSB max

= 2.7 V

Offset Error Match

LSB max

0.1

LSB typ

Gain Error

±4

LSB max

Gain Error Match

LSB max

0.1

LSB typ

Power Supply Rejection

dB typ

SWITCH DRIVERS

On-Resistance

Y+, X+

typ

Y, X

typ

ANALOG INPUT

Input Voltage Ranges

0 to V

REF

Volts

DC Leakage Current

±0.1

µA typ

Input Capacitance

pF typ

REFERENCE INPUT

REF

Input Voltage Range

1.0/+V

V min/max

DC Leakage Current

±1

µA max

REF

Input Impedance

typ

CS = GND or +V

REF

Input Current

µA max

µA typ

SAMPLE

= 12.5 kHz

µA max

CS = +V

; 0.001

µA typ

LOGIC INPUTS

Input High Voltage, V

INH

2.4

V min

Input Low Voltage, V

INL

0.4

V max

Input Current, I

±1

µA max

Typically 10 nA, V

= 0 V or +V

Input Capacitance, C

pF max

LOGIC OUTPUTS

Output High Voltage, V

0.2

V min

SOURCE

= 250

µA; V

= 2.2 V to 5.25 V

Output Low Voltage, V

0.4

V max

SINK

= 250

µA

PENIRQ Output Low Voltage, V

0.4

V max

SINK

= 250

µA; 100 k Pull-Up

Floating-State Leakage Current

±10

µA max

Floating-State Output Capacitance

pF max

Output Coding

Straight (Natural) Binary

CONVERSION RATE

Conversion Time

DCLK Cycles max

Track/Hold Acquisition Time

DCLK Cycles min

Throughput Rate

125

kSPS max

POWER REQUIREMENTS

(Specified Performance)

2.7/3.6

V min/max

Functional from 2.2 V to 5.25 V

Digital I/Ps = 0 V or V

Normal Mode (f

SAMPLE

= 125 kSPS)

380

µA max

= 3.6 V, 240

µA typ

Normal Mode (f

SAMPLE

= 12.5 kSPS)

170

µA typ

= 2.7 V, f

DCLK

= 2 00 kHz

Normal Mode (Static)

150

µA typ

= 3.6 V

Shutdown Mode (Static)

µA max

Power Dissipation

Normal Mode (f

SAMPLE

= 125 kSPS)

1.368

mW max

= 3.6 V

Shutdown

3.6

µW max

= 3.6 V

NOTES

Temperature range as follows: A Version: 40

°C to +85°C.

See Terminology.

Guaranteed by design.

Sample tested @ 25

°C to ensure compliance.

See Power vs. Throughput Rate section.

Specifications subject to change without notice.

REV. 0

AD7843

Parameter

Limit at T

MIN

, T

MAX

Unit

Description

DCLK

kHz min

MHz max

ACQ

1.5

µs min

Acquisition Time

ns min

CS Falling Edge to First DCLK Rising Edge

ns max

CS Falling Edge to BUSY Three-State Disabled

ns max

CS Falling Edge to DOUT Three-State Disabled

200

ns min

DCLK High Pulsewidth

200

ns min

DCLK Low Pulsewidth

ns max

DCLK Falling Edge to BUSY Rising Edge

ns min

Data Setup Time Prior to DCLK Rising Edge

ns min

Data Valid to DCLK Hold Time

200

ns max

Data Access Time after DCLK Falling Edge

ns min

CS Rising Edge to DCLK Ignored

200

ns max

CS Rising Edge to BUSY High Impedance

200

ns max

CS Rising Edge to DOUT High Impedance

NOTES

Sample tested at 25

°C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of V

) and timed from a voltage level of 1.6 V.

Mark/Space ratio for the SCLK input is 40/60 to 60/40.

Measured with the load circuit of Figure 1 and defined as the time required for the output to cross 0.4 V or 2.0 V.

is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then extrapolated

back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t

, quoted in the timing characteristics is the true bus relinquish

time of the part and is independent of the bus loading.

Specifications subject to change without notice.

TIMING SPECIFICATIONS

= T

MIN

to T

MAX

, unless otherwise noted; V

= 2.7 V to 3.6 V, V

REF

= 2.5 V)

OUTPUT

PIN

50pF

1.6V

200 A

Figure 1. Load Circuit for Digital Output Timing Specifications

REV. 0

AD7843

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD7843 features proprietary ESD protection circuitry, permanent damage may occur on
devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS

= 25

°C unless otherwise noted)

to GND . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to +7 V

Analog Input Voltage to GND . . . . . . . 0.3 V to V

+ 0.3 V

Digital Input Voltage to GND . . . . . . . 0.3 V to V

+ 0.3 V

Digital Output Voltage to GND . . . . . 0.3 V to V

+ 0.3 V

REF

to GND . . . . . . . . . . . . . . . . . . . . 0.3 V to V

+ 0.3 V

Input Current to Any Pin Except Supplies

. . . . . . .

±10 mA

Operating Temperature Range

Commercial . . . . . . . . . . . . . . . . . . . . . . . . 40

°C to +85°C

Storage Temperature Range . . . . . . . . . . . 65

°C to +150°C

Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 150

°C

QSOP, TSSOP Package, Power Dissipation . . . . . . . 450 mW

Thermal Impedance . . . . . . . . . . . 149.97

°C/W (QSOP)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150.4

°C/W (TSSOP)

Thermal Impedance . . . . . . . . . . . . . 38.8

°C/W (QSOP)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27.6

°C/W (TSSOP)

Lead Temperature, Soldering

Vapor Phase (60 secs) . . . . . . . . . . . . . . . . . . . . . . . 215

°C

Infrared (15 secs) . . . . . . . . . . . . . . . . . . . . . . . . . . 220

°C

NOTES

Stresses above those listed under Absolute Maximum Rating may cause permanent

damage to the device. This is a stress rating only; functional operation of the device
at these or any other conditions above those listed in the operational sections of this
specification is not implied. Exposure to absolute maximum rating conditions for
extended periods may affect device reliability.

Transient currents of up to 100 mA will not cause SCR latch-up.

ORDERING GUIDE

Temperature

Linearity

Package

Branding

Model

Range

Error (LSB)

Option

Description

Information

AD7843ARQ

°C to +85°C

±2

RQ-16

QSOP

AD7843ARQ

AD7843ARQ-REEL

°C to +85°C

±2

RQ-16

QSOP

AD7843ARQ

AD7843ARQ-REEL7

°C to +85°C

±2

RQ-16

QSOP

AD7843ARQ

AD7843ARU

°C to +85°C

±2

RU-16

TSSOP

AD7843ARU

AD7843ARU-REEL

°C to +85°C

±2

RU-16

TSSOP

AD7843ARU

AD7843ARU-REEL7

°C to +85°C

±2

RU-16

TSSOP

AD7843ARU

EVAL-AD7843CB

Evaluation Board

EVAL-CONTROL BRD2

Controller Board

NOTES

Linearity error here refers to integral linearity error.

RQ = 0.15" Quarter Size Outline Package.

This can be used as a stand-alone evaluation board or in conjunction with the EVALUATION BOARD CONTROLLER for evaluation/demonstration purposes.

This EVALUATION BOARD CONTROLLER is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the

CB designators.

PIN CONFIGURATION QSOP/TSSOP

TOP VIEW

(Not to Scale)

X+

Y+

GND

IN3

IN4

DCLK

DIN

BUSY

DOUT

PENIRQ

VREF

AD7843

REV. 0

AD7843

PIN FUNCTION DESCRIPTIONS

Pin
No.

Mnemonic

Function

1, 10

Power Supply Input. The +V

range for the AD7843 is from 2.2 V to 5.25 V. Both +V

pins should

be connected directly together.

X+

X+ Position Input. ADC Input Channel 1.

Y+

Y+ Position Input. ADC Input Channel 2.

X Position Input.

Y Position Input.

GND

Analog Ground. Ground reference point for all circuitry on the AD7843. All analog input signals and
any external reference signal should be referred to this GND voltage.

IN3

Auxiliary Input 1. ADC Input Channel 3.

IN4

Auxiliary Input 2. ADC Input Channel 4.

REF

Reference Input for the AD7843. An external reference must be applied to this input. The voltage
range for the external reference is 1.0 V to +V

. For specified performance it is 2.5 V.

PENIRQ

Pen Interrupt. CMOS Logic open drain output (requires 10 k

to 100 k pull-up resistor externally).

DOUT

Data Out. Logic Output. The conversion result from the AD7843 is provided on this output as a
serial data stream. The bits are clocked out on the falling edge of the DCLK input. This output is
high impedance when

CS is high.

BUSY

BUSY Output. Logic Output. This output is high impedance when

CS is high.

DIN

Data In. Logic Input. Data to be written to the AD7843's Control Register is provided on this input
and is clocked into the register on the rising edge of DCLK (see Control Register section).

Chip Select Input. Active Low Logic Input. This input provides the dual function of initiating con-
versions on the AD7843 and also enables the serial input/output register.

DCLK

External Clock Input. Logic Input. DCLK provides the serial clock for accessing data from the part.
This clock input is also used as the clock source for the AD7843's conversion process.

TERMINOLOGY
Integral Nonlinearity

This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The end-
points of the transfer function are zero scale, a point 1 LSB
below the first code transition, and full scale, a point 1 LSB
above the last code transition.

Differential Nonlinearity

This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.

Offset Error

This is the deviation of the first code transition (00 . . . 000) to
(00 . . . 001) from the ideal, i.e., AGND + 1 LSB.

Gain Error

This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (i.e., V

REF

1 LSB) after the offset

error has been adjusted out.

Track/Hold Acquisition Time

The track/hold amplifier enters the acquisition phase on the fifth
falling edge of DCLK after the START bit has been detected.
Three DCLK cycles are allowed for the Track/Hold acquisition
time and the input signal will be fully acquired to the 12-bit
level within this time even with the maximum specified DCLK
frequency. See Analog Input section for more details.

On-Resistance

This is a measure of the ohmic resistance between the drain and
source of the switch drivers.

REV. 0

TEMPERATURE C

207

SUPPLY CURRENT

100

206

205

204

203

202

201

200

199

198

20

40

TPC 1. Supply Current vs. Temperature

 V

230

SUPPLY CURRENT

5.0

190

180

170

160

150

4.6

4.2

3.8

3.4

3.0

2.6

2.2

210

220

200

SAMPLE

= 12.5kHz

REF

= +V

TPC 2. Supply Current vs. +V

TEMPERATURE C

0.20

DELTA FROM +25

LSB

100

0.00

0.05

0.10

0.15

0.20

20

40

0.10

0.15

0.05

TPC 3. Change in Gain vs. Temperature

TEMPERATURE C

141

SUPPLY CURRENT

100

138

137

136

135

134

20

40

140

139

TPC 4. Power-Down Supply Current vs. Temperature

 V

1000

100

2.2

5.2

3.7

SAMPLE RATE

kSPS

4.7

4.2

3.2

2.7

REF

= +V

TPC 5. Maximum Sample Rate vs. +V

TEMPERATURE C

0.6

DELTA FROM +25

LSB

100

0.0

0.2

0.4

0.6

20

40

0.4

0.2

TPC 6. Change in Offset vs. Temperature

AD7843Typical Performance Characteristics

REV. 0

AD7843

SAMPLE RATE kHz

7.5

REFERENCE CURRENT

130

3.5

2.5

1.5

0.5

115

6.5

5.5

4.5

100

TPC 7. Reference Current vs. Sample Rate

5.5

4.5

4.0

3.5

3.0

2.5

2.0

5.0

Y+

X+

TPC 8. Switch-On-Resistance vs. +V

(X+, Y+: +V

Pin; X, Y: Pin to GND)

SAMPLING RATE kSPS

2.0

ERROR

LSB

195

1.0

0.2

175

115

1.4

1.2

135

155

0.4

0.6

0.8

1.6

1.8

INL: R = 2k

INL: R = 500

DNL: R = 500

DNL: R = 2k

TPC 9. Maximum Sampling Rate vs. R

TEMPERATURE C

REFERENCE CURRENT

20

40

TPC 10. Reference Current vs. Temperature

TEMPERATURE

 C

100

20

40

Y+

X+

TPC 11. Switch-On-Resistance vs. Temperature (X+, Y+:
+V

to Pin; X, Y: Pin to GND)

FREQUENCY kHz

SNR

60.0

100

120

52.5

45.0

30.0

22.5

15.0

7.50

37.5

SAMPLE

= 125kHz

= 15kHz

SNR = 68.34dB

TPC 12. Auxiliary Channel Dynamic Performance

REV. 0

AD7843

CIRCUIT INFORMATION

The AD7843 is a fast, low-power, 12-bit, single supply, A/D
converter. The AD7843 can be operated from a 2.2 V to 5.25 V
supply. When operated from either a 5 V supply or a 3 V supply,
the AD7843 is capable of throughput rates of 125 kSPS when
provided with a 2 MHz clock.

The AD7843 provides the user with an on-chip track/hold,
multiplexer, A/D converter, and serial interface housed in a tiny
16-lead QSOP or TSSOP package, which offers the user consid-
erable space-saving advantages over alternative solutions. The
serial clock input (DCLK) accesses data from the part but also
provides the clock source for the successive-approximation A/D

converter. The analog input range is 0 V to V

REF

(where the

externally-applied V

REF

can be between 1 V and V

The analog input to the ADC is provided via an on-chip multi-
plexer. This analog input may be any one of the X and Y panel
coordinates. The multiplexer is configured with low resistance
switches that allow an unselected ADC input channel to provide
power and an accompanying pin to provide ground for an exter-
nal device. For some measurements the on-resistance of the
switches may present a source of error. However, with a dif-
ferential input to the converter and a differential reference
architecture this error can be negated.

ADC TRANSFER FUNCTION

The output coding of the AD7843 is straight binary. The
designed code transitions occur at successive integer LSB values
(i.e., 1 LSB, 2 LSBs, etc.). The LSB size is = V

REF

/4096. The ideal

transfer characteristic for the AD7843 is shown in Figure 2 below.

000...000

ADC CODE

ANALOG INPUT

111...111

000...001

000...010

111...110

111...000

011...111

1LSB

REF

1LSB

1LSB = V

REF

/4096

Figure 2. AD7843 Transfer Characteristic

TYPICAL CONNECTION DIAGRAM

Figure 3 shows a typical connection diagram for the AD7843 in
a touch screen control application. The AD7843 requires an exter-
nal reference and an external clock. The external reference can
be any voltage between 1 V and V

. The value of the reference

voltage will set the input range of the converter. The conversion
result is output MSB first followed by the remaining eleven bits
and three trailing zeroes depending on the number of clocks used
per conversion, see the Serial Interface section. For applications
where power consumption is of concern, the power management
option should be used to improve power performance. See
Table III for the available power management options.

AD7843

DIN

DCLK

REF

GND

DOUT

X+

IN3

IN4

BUSY

PENIRQ

Y+

SERIAL/CONVERSION CLOCK

CHIP SELECT

SERIAL DATA IN

CONVERTER STATUS

SERIAL DATA OUT

PEN INTERRUPT

AUXILIARY INPUTS

0.1 F

1 F TO 10 F

(OPTIONAL)

2.2V TO 5V

100k
(OPTIONAL)

0.1 F

TOUCH

SCREEN

Figure 3. Typical Application Circuit

TPC 12 shows a typical FFT plot for the auxiliary channels of
the AD7843 at 125 kHz sample rate and 15 kHz input frequency.

TPC 13 shows the power supply rejection ratio versus V

supply frequency for the AD7843. The power supply rejection
ratio is defined as the ratio of the power in the ADC output at
full-scale frequency f, to the power of a 100 mV sine wave applied
to the ADC V

supply of frequency f

PSRR (dB) = 10 log (Pf/Pfs)

Pf = Power at frequency f in ADC output, Pfs = power at fre-
quency f

coupled onto the ADC V

supply. Here a 100 mV

peak-to-peak sine wave is coupled onto the V

supply. Decou-

pling capacitors of 10

µF and 0.1 µF were used on the supply.

RIPPLE FREQUENCY

 kHz

PSRR

100

60

80

100

120

20

40

= 3V, V

REF

= 2.5V

100mV p-p SINEWAVE ON +V

SAMPLE

= 125kHz, f

= 20kHz

TPC 13. AC PSRR vs. Supply Ripple Frequency

REV. 0

AD7843

ANALOG INPUT

Figure 4 shows an equivalent circuit of the analog input struc-
ture of the AD7843 which contains a block diagram of the input
multiplexer, the differential input of the A/D converter and the
differential reference.

Table I shows the multiplexer address corresponding to each
analog input, both for the SER/

DFR bit in the control register

set high and low. The control bits are provided serially to the
device via the DIN pin. For more information on the control
register see the Control Register section.

When the converter enters the hold mode, the voltage difference
between the +IN and IN inputs (see Figure 4) is captured on
the internal capacitor array. The input current on the analog
inputs depends on the conversion rate of the device. During the
sample period, the source must charge the internal sampling
capacitor (typically 37 pF). Once the capacitor has been fully
charged, there is no further input current. The rate of charge
transfer from the analog source to the converter is a function of
conversion rate.

Acquisition Time

The track and hold amplifier enters its tracking mode on the
falling edge of the fifth DCLK after the START bit has been
detected (see Figure 13). The time required for the track and
hold amplifier to acquire an input signal will depend how
quickly the 37 pF input capacitance is charged. With zero
source impedance on the analog input three DCLK cycles will
always be sufficient to acquire the signal to the 12-bit level.
With a source impedance R

on the analog input, the actual

acquisition time required is calculated using the formula:

ACQ

= 8.4

× (R

+100

) × 37 pF

where R

is the source impedance of the input signal, and

100

, 37 pF is the input RC value. Depending on the frequency

of DCLK used, three DCLK cycles may or may not be suffi-
cient to acquire the analog input signal with various source
impedance values.

4-TO-1

MUX

X+

Y+

IN3

IN4

DATA OUT

3-TO-1

MUX

3-TO-1

MUX

X+ Y+

REF
EXT

X Y GND

X+
X

Y+
Y

ON-CHIP SWITCHES

REF

ADC CORE

REF+

IN+

IN

Figure 4. Equivalent Analog Input Circuit

REV. 0

AD7843

Touch Screen Settling

In some applications, external capacitors may be required across
the touch screen to filter noise associated with it, e.g., noise
generated by the LCD panel or backlight circuitry. The value of
these capacitors will cause a settling time requirement when the
panel is touched. The settling time will typically show up as a
gain error. There are several methods for minimizing or elimi-
nating this issue. The problem may be that the input signal, or
reference, or both, have not settled to their final value before the
sampling instant of the ADC. Additionally, the reference voltage
may still be changing during the conversion cycle. One option is
to stop, or slow down the DCLK for the required touch screen
settling time. This will allow the input and reference to stabilize
for the acquisition time. This will resolve the issue for both
single-ended and differential modes.

The other option is to operate the AD7843 in differential mode
only for the touch screen, and program the AD7843 to keep the
touch screen drivers ON and not go into power-down (PD0 =
PD1 = 1). Several conversions may be required depending on
the settling time required and the AD7843 data rate. Once the
required number of conversions have been made, the AD7843
can then be placed in a power-down state on the last measure-
ment. The last method is to use the 15 DCLK cycle mode, which
maintains the touch screen drivers ON until it is commanded to
stop by the processor.

Reference Input

The voltage difference between +REF and REF (see Figure 4)
sets the analog input range. The AD7843 will operate with a
reference input in the range of 1 V to V

. The voltage into

the V

REF

input is not buffered and directly drives the capaci-

tor DAC portion of the AD7843. Figure 5 shows the reference
input circuitry. Typically, the input current is 8

µA with V

REF

2.5 V and f

SAMPLE

= 125 kHz. This value will vary by a few

microamps, depending on the result of the conversion. The
reference current diminishes directly with both conversion rate
and reference voltage. As the current from the reference is
drawn on each bit decision, clocking the converter more quickly
during a given conversion period will not reduce the overall
current drain from the reference.

REF

ADC

3-TO-1

MUX

X+
Y+

Figure 5. Reference Input Circuitry

When making touch screen measurements, conversions can be
made in the differential (ratiometric) mode or the single-ended
mode. If the SER/

DFR bit is set to 1 in the control register, a

single-ended conversion will be performed. Figure 6 shows the
configuration for a single-ended Y coordinate measurement.
The X+ input is connected to the analog to digital converter,
the Y+ and Y drivers are turned on and the voltage on X+ is
digitized. The conversion is performed with the ADC refer-
enced from GND to V

REF

. The advantage of this mode is that

the switches that supply the external touch screen can be turned
off once the acquisition is complete, resulting in a power saving.
However, the on-resistance of the Y drivers will affect the input
voltage that can be acquired. The full touch screen resistance
may be in the order of 200

to 900 , depending on the manu-

facturer. Thus if the on-resistance of the switches is approximately
6

, true full-scale and zero-scale voltages cannot be acquired

regardless of where the pen/stylus is on the touch screen.

Note: The minimum touch screen resistance recommended for
use with the AD7843 is approximately 70

X+

Y+

GND

IN REF

IN+ ADC CORE

REF+

REF

IN+

Figure 6. Single-Ended Reference Mode (SER/

DFR = 1)

In this mode of operation, therefore, some voltage is likely to be
lost across the internal switches and, in addition to this, it is
unlikely that the internal switch resistance will track the resis-
tance of the touch screen over temperature and supply, providing
an additional source of error.

The alternative to this situation is to set the SER/

DFR bit low.

If one again considers making a Y coordinate measurement,
but now the +REF and REF nodes of the ADC are connected
directly to the Y+ and Y pins, this means the analog to digital
conversion will be ratiometric. The result of the conversion will
always be a percentage of the external resistance, independent of
how it may change with respect to the on-resistance of the internal
switches. Figure 7 shows the configuration for a ratiometric Y
coordinate measurement. It should be noted that the differential

Table I. Analog Input, Reference, and Touch Screen Control

SER/

DFR

Analog In

X Switches

Y Switches

+REF

REF

X+

OFF

REF

GND

IN3

OFF

REF

GND

Y+

OFF

REF

GND

IN4

OFF

REF

GND

X+

OFF

Y+

OFF

X+

Outputs Identity Code, 1000 0000 0000

NOTES

All remaining configurations are invalid addresses.

Internal node not directly accessible by the user.

REV. 0

AD7843

reference mode can only be used with +V

as the source of the

+REF voltage and cannot be used with V

REF

The disadvantage of this mode of operation is that during both
the acquisition phase and conversion process, the external touch
screen must remain powered. This will result in additional sup-
ply current for the duration of the conversion.

X+

Y+

IN+

GND

IN

REF

IN+ ADC CORE

REF+

Figure 7. Differential Reference Mode (SER/

DFR = 0)

CONTROL REGISTER

The control word provided to the ADC via the DIN pin is
shown in Table II. This provides the conversion start, channel
addressing, ADC conversion resolution, configuration and
power-down of the AD7843. Table II provides detailed infor-
mation on the order and description of these control bits within
the control word.

Initiate START

The first bit, the "S" bit, must always be set to 1 to initiate the
start of the control word. The AD7843 will ignore any inputs on
the DIN line until the start bit is detected.

Channel Addressing

The next three bits in the control register, A2, A1 and A0, select
the active input channel(s) of the input multiplexer (see Table I
and Figure 4), touch screen drivers, and the reference inputs.

MODE

The MODE bit sets the resolution of the analog to digital con-
verter. With a 0 in this bit the following conversion will have
12 bits of resolution. With a 1 in this bit the following conver-
sion will have 8 bits of resolution.

SER/

DFR

The SER/

DFR bit controls the reference mode which can be

either single ended or differential if a 1 or a 0 is written to this
bit respectively. The differential mode is also referred to as
the ratiometric conversion mode. This mode is optimum for
X-Position and Y-Position measurements. The reference is
derived from the voltage at the switch drivers, which is almost
the same as the voltage to the touch screen. In this case a sepa-
rate reference voltage is not needed as the reference voltage to
the analog to digital converter is the voltage across the touch
screen. In the single-ended mode, the reference voltage to the
converter is always the difference between the V

REF

and GND pins.

See Table I and Figures 4 through 7 for further information.

As the supply current required by the device is so low, a preci-
sion reference can be used as the supply source to the AD7843.
It may also be necessary to power the touch screen from the
reference, which may require 5 mA to 10 mA. A REF19x volt-
age reference can source up to 30 mA and, as such, could supply
both the ADC and the touch screen. Care must be taken, how-
ever, to ensure that the input voltage applied to the ADC does
not exceed the reference voltage and hence the supply voltage.
See Maximum Ratings section.

NOTE: The differential mode can only be used for X-Position
and Y-Position measurements All other measurements require
the single-ended mode.

PD0 and PD1

The power management options are selected by programming
the power management bits, PD0 and PD1, in the control regis-
ter. Table III summarizes the available options.

Table II. Control Register Bit Function Description

MSB

LSB

MODE

SER/

DFR

PD1

PD0

Bit

Mnemonic

Comment

Start Bit. The Control word starts with the first high bit on DIN. A new control word can start every fifteenth
DCLK cycle when in the 12-bit conversion mode or every eleventh DCLK cycle when in 8-bit conversion mode.

A2A0

Channel Select Bits. These three address bits along with the SER/

DFR bit control the setting of the multi-

plexer input, switches, and reference inputs, as detailed in Table I.

MODE

12-Bit/8-Bit Conversion Select Bit. This bit controls the resolution of the following conversion. With a 0 in
this bit the conversion will have 12-bit resolution or with a 1 in this bit, 8-bit resolution.

SER/

DFR

Single-Ended/Differential Reference Select Bit. Along with bits A2A0, this bit controls the setting of the
multiplexer input, switches, and reference inputs as described in Table I.

1, 0

PD1, PD0

Power Management Bits. These two bits decode the power-down mode of the AD7843 as shown in Table III.

REV. 0

AD7843

POWER VS. THROUGHPUT RATE

By using the power-down options on the AD7843 when not con-
verting, the average power consumption of the device decreases at
lower throughput rates. Figure 8 shows how, as the through-
put rate is reduced while maintaining the DCLK frequency at
2 MHz, the device remains in its power-down state longer and
the average current consumption over time drops accordingly.

For example, if the AD7843 is operated in a 24 DCLK continu-
ous sampling mode, with a throughput rate of 10 kSPS and a
SCLK of 2 MHz, and the device is placed in the power-down
mode between conversions, (PD0, PD1 = 0, 0), the current
consumption is calculated as follows. The power dissipation
during normal operation is typically 210

µA (V

= 2.7 V). The

power-up time of the ADC is instantaneous, so when the part
is converting it will consume 210

µA. In this mode of operation

the part powers up on the 4th falling edge of DCLK after the
start bit has been recognized. It goes back into power-down at
the end of conversion on the 20th falling edge of DCLK. This
means the part will consume 210

µA for 16 DCLK cycles only,

µs, during each conversion cycle. With a throughput rate of

10 kSPS, the cycle time is 100

µs and the average power dissi-

pated during each cycle is (8/100)

× (210 µA) = 16.8 µA.

THROUGHPUT

 kSPS

1000

SUPPLY CURRENT

120

100

DCLK

= 16 f

SAMPLE

DCLK

= 2MHz

= 2.7V

= 40 C to +85 C

Figure 8. Supply Current vs. Throughput (

µA)

SERIAL INTERFACE

Figure 9 shows the typical operation of the serial interface of the
AD7843. The serial clock provides the conversion clock and
also controls the transfer of information to and from the AD7843.
One complete conversion can be achieved with twenty-four
DCLK cycles.

The

CS signal initiates the data transfer and conversion process.

The falling edge of

CS takes the BUSY output and the serial

bus out of three-state. The first eight DCLK cycles are used to
write to the Control Register via the DIN pin. The Control
Register is updated in stages as each bit is clocked in and once
the converter has enough information about the following con-
version to set the input multiplexer and switches appropriately,
the converter enters the acquisition mode and if required, the
internal switches are turned on. During the acquisition mode
the reference input data is updated. After the three DCLK
cycles of acquisition, the control word is complete (the power
management bits are now updated) and the converter enters the
conversion mode. At this point the track and hold goes into hold
mode and the input signal is sampled and the BUSY output
goes high (BUSY will return low on the next falling edge of
DCLK). The internal switches may also turn off at this point if
in single-ended mode.

The next 12 DCLK cycles are used to perform the conversion
and to clock out the conversion result. If the conversion is
ratiometric (SER/

DFR set low) the internal switches are on

during the conversion. A thirteenth DCLK cycle is needed to
allow the DSP/micro to clock in the LSB. Three more DCLK
cycles will clock out the three trailing zeroes and complete the
twenty four DCLK transfer. The twenty-four DCLK cycles may
be provided from a DSP or via three bursts of eight clock cycles
from a microcontroller.

Table III. Power Management Options

PD1

PD0

PENIRQ

Description

Enabled

This configuration will result in power-down of the device between conversions. The AD7843
will only power down between conversions. Once PD1 and PD0 have been set to 0, 0, the
conversion will be performed first and the AD7843 will power down upon completion of that
conversion. At the start of the next conversion, the ADC instantly powers up to full power. This
means there is no need for additional delays to ensure full operation and the very first conversion
is valid. The Y switch is ON while in power-down.

Disabled

This configuration will result in the same behavior as when PD1 and PD0 have been programmed
with 0, 0, except that

PENIRQ is disabled. The Y switch is OFF while in power-down.

Enabled

This configuration will result in keeping the AD7843 permanently powered up with the

PENIRQ

enabled.

Disabled

This configuration will result in keeping the AD7843 always powered up with the

PENIRQ

disabled.

REV. 0

AD7843

Detailed Serial Interface Timing

Figure 10 shows the detailed timing diagram for serial interfacing
to the AD7843. Writing of information to the Control Register
takes place on the first eight rising edges of DCLK in a data
transfer. The Control Register is only written to if a START bit
is detected (see Control Register section) on DIN and the initia-
tion of the following conversion is also dependent on the presence
of the START bit. Throughout the eight DCLK cycles when
data is being written to the part, the DOUT line will be driven
low. The MSB of the conversion result is clocked out on the
falling edge of the ninth DCLK cycle and is valid on the rising
edge of the tenth DCLK cycle, therefore nine leading zeros may
be clocked out prior to the MSB. This means the data seen on
the DOUT line in the twenty four DCLK conversion cycle, will
be presented in the form of nine leading zeros, twelve bits of
data and three trailing zeros.

The rising edge of

CS will put the bus and the BUSY output

back into three-state, the DIN line will be ignored and if a con-
version is in progress at the time then this will also be aborted.
However, if

CS is not brought high after the completion of the

conversion cycle, then the part will wait for the next START bit
to initiate the next conversion. This means each conversion
need not necessarily be framed by

CS, as once CS goes low the

part will detect each START bit and clock in the control word
after it on DIN. When the AD7843 is in the 12-bit conversion
mode, a second START bit will not be detected until seven DCLK
pulses have elapsed after a control word has been clocked in on
DIN, i.e., another START bit can be clocked in on the eighth
DCLK rising edge after a control word has been written to the
device (see Fifteen Clock Cycle section). If the device is in the
8-bit conversion mode, a second START bit will not be recog-
nized until three DCLK pulses have elapsed after the control
word has been clocked in, i.e., another START bit can be
clocked in on the fourth DCLK rising edge after a control word
has been written to the device.

Because a START bit can be recognized during a conversion, it
means the control word for the next conversion can be clocked
in during the current conversion, enabling the AD7843 to com-
plete a conversion cycle in less than twenty-four DCLKs.

DCLK

DOUT

DIN

THREE-STATE

ACQ

MODE

SER/
DFR

PD1 PD0

BUSY

AQUIRE

IDLE

THREE-STATE

CONVERSION

IDLE

(MSB)

(START)

OFF

(LSB)

ZERO FILLED

OFF

THREE-STATE

X/Y SWITCHES(1)

(SER/

DFR HIGH)

X/Y SWITCHES(1,2)

(SER/

DFR LOW)

NOTES

Y DRIVERS ARE ON WHEN X IS SELECTED INPUT CHANNEL (A2A0 = 001), X DRIVERS ARE ON WHEN Y IS SELECTED INPUT CHANNEL (A2A0 = 101).
WHEN PD1, PD0 = 10 OR 00, Y WILL TURN ON AT END OF CONVERSION.

DRIVERS WILL REMAIN ON IF POWER-DOWN MODE IS 11 (NO POWER-DOWN) UNTIL SELECTED INPUT CHANNEL, REFERENCE MODE,
OR POWER-DOWN MODE IS CHANGED.

Figure 9. Conversion Timing, 24 DCLKS per Conversion Cycle, 8-Bit Bus Interface. No DCLK delay required with dedi-
cated serial port

DB11

DCLK

DOUT

DB10

DIN

PD0

BUSY

Figure 10. Detailed Timing Diagram

REV. 0

AD7843

Sixteen Clocks per Cycle

The control bits for the next conversion can be overlapped with
the current conversion to allow for a conversion every 16 DCLK
cycles, as shown in Figure 11. This timing diagram also allows
for the possibility of communication with other serial peripherals
between each (eight DCLK) byte transfer between the processor
and the converter. However, the conversion must be complete
within a short enough time frame to avoid capacitive droop
effects which may distort the conversion result. It should also
be noted that the AD7843 will be fully powered while other
serial communications may be taking place between byte transfers.

Fifteen Clocks per Cycle

Figure 12 shows the fastest way to clock the AD7843. This
scheme will not work with most microcontrollers or DSPs as in
general they are not capable of generating a 15-clock cycle per
serial transfer. However, some DSPs will allow the number of
clocks per cycle to be programmed and this method could also
be used with FPGAs (Field Programmable Gate Arrays) or
ASICs (Application Specific Integrated Circuits). As in the 16-
clocks-per-cycle case, the control bits for the next conversion
are overlapped with the current conversion to allow for a con-
version every 15 DCLK cycles, using 12 DCLKs to perform
the conversion and three DCLKs to acquire the analog input.

This will effectively increase the throughput rate of the AD7843
beyond that used for the specifications which are tested using 16
DCLKs per cycle, and DCLK = 2 MHz.

8-Bit Conversion

The AD7843 can be set up to operate in an 8-bit rather than
12-bit mode, by setting the MODE bit to 1 in the control regis-
ter. This mode allows a faster throughput rate to be achieved,
assuming 8-bit resolution is sufficient. When using the 8-bit
mode a conversion is complete four clock cycles earlier than in
the 12-bit mode. This could be used with serial interfaces that
provide 12 clock transfers, or two conversions could be com-
pleted with three eight-clock transfers. The throughput rate will
increase by 25% as a result of the shorter conversion cycle, but
the conversion itself can occur at a faster clock rate because the
internal settling time of the AD7843 is not as critical because
settling to 8 bits is all that is required. The clock rate can be as
much as 50% faster. The faster clock rate and fewer clock cycles
combine to provide double the conversion rate.

PEN INTERRUPT REQUEST

The pen interrupt equivalent output circuitry is outlined in
Figure 13. By connecting a pull-up resistor (10 k

to 100 k)

between V

and this CMOS Logic open drain output, the

PENIRQ output will remain high normally. If PENIRQ has

DCLK

DOUT

DIN

BUSY

CONTROL BITS

Figure 11. Conversion Timing, 16 DCLKS per Cycle, 8-Bit Bus Interface. No DCLK Delay Required with Dedicated Serial Port

DCLK

DOUT

DIN

BUSY

PD1 PD0

SER/
DFR

PD1 PD0

MODE

SER/

DFR

MODE

Figure 12. Conversion Timing, 15 DCLKS per Cycle, Maximum Throughput Rate

REV. 0

AD7843

been enabled (see Table III), when the touch screen connected
to the AD7843 is touched via a pen or finger, the

PENIRQ output

will go low initiating an interrupt to a microprocessor which may
then instruct a control word to be written to the AD7843 to
initiate a conversion. This output can also be enabled between
conversions during power-down (see Table III) allowing power-
up to be initiated only when the screen is touched. The result of
the first touch screen coordinate conversion after power-up will
be valid assuming any external reference is settled to the 12- or
8-bit level as required.

EXTERNAL
PULL-UP

TOUCH

SCREEN

100k

X+

Y+

PENIRQ
ENABLE

PENIRQ

Figure 13.

PENIRQ Functional Block Diagram

Figure 14 assumes the

PENIRQ function has been enabled in

the last write or the part has just been powered up so

PENIRQ

is enabled by default. Once the screen is touched, the

PENIRQ

output will go low a time t

PEN

later. This delay is approximately

µs, assuming a 10 nF touch screen capacitance, and will vary

with the touch screen resistance actually used. Once the START
bit is detected, the pen interrupt function is disabled and the
PENIRQ will not respond to screen touches. The PENIRQ

DCLK

DIN

PEN

MODE

SER/
DFR

NO RESPONSE TO TOUCH

(START)

PENIRQ

INTERRUPT

PROCESSOR

SCREEN

TOUCHED

HERE

PD1 = 1, PD0 = 0,

PENIRQ

ENABLED AGAIN

Figure 14.

PENIRQ Timing Diagram

output will remain low until the fourth falling edge of DCLK
after the START bit has been clocked in, at which point it will
return high as soon as possible, regardless of the touch screen
capacitance. This does not mean the pen interrupt function is
now enabled again as the power-down bits have not yet been
loaded to the control register. So regardless of whether

PENIRQ

is to be enabled again or not the

PENIRQ output will always

idle high normally. Assuming the

PENIRQ is enabled again as

shown in Figure 14, once the conversion is complete, the
PENIRQ output will respond to a screen touch again. The fact
that

PENIRQ returns high almost immediately after the fourth

falling edge of DCLK, means the user will avoid any spurious
interrupts on the microprocessor or DSP which could occur if
the interrupt request line on the micro/DSP was unmasked
during or toward the end of conversion with the

PENIRQ pin

still low. Once the next START bit is detected by the AD7843
the

PENIRQ function is disabled again.

If the control register write operation will overlap with the data
read, a START bit will always be detected prior to the end of
conversion, meaning that even if the

PENIRQ function has been

enabled in the Control Register it will be disabled by the START
bit again before the end of the conversion is reached, so the
PENIRQ function effectively cannot be used in this mode.
However, as conversions are occurring continuously, the
PENIRQ function is not necessary and, therefore, redundant.

GROUNDING AND LAYOUT

For information on grounding and layout considerations for the
AD7843 refer to the "Layout and Grounding Recommendations
for Touch Screen Digitizers" Technical Note.

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: AD7843A1

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: AD7843A1