Analog Devices AD7877 a Datasheet

A

3

A

2

A

Touch Screen Controller

FEATURES

4-wire touch screen interface
LCD noise reduction feature (STOPACQ pin)
Automatic conversion sequencer and timer
User-programmable conversion parameters
On-chip temperature sensor: −40°C to +85°C
On-chip 2.5 V reference
On-chip 8-bit DAC
3 auxiliary analog inputs
1 dedicated and 3 optional GPIOs
2 direct battery measurement channels (0.5 V to 5 V)
3 interrupt outputs
Touch-pressure measurement
Wake up on touch function
Specified throughput rate of 125 kSPS
Single supply, V
Separate V
Shutdown mode: 1 µA maximum
32-lead LFCSP 5 mm x 5 mm package

APPLICATIONS

Personal digital assistants
Smart hand-held devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones
Pagers

GENERAL DESCRIPTION

The AD7877 is a 12-bit successive approximation ADC with a
synchronous serial interface and low on resistance switches for
driving touch screens. The AD7877 operates from a single 2.7 V
to 5.25 V power supply (functional operation to 2.2V), and
features throughput rates of 125 kSPS. The AD7877 features
direct battery measurement on two inputs, temperature and
touch-pressure measurement.

The AD7877 also has an on-board reference of 2.5 V. When not
in use, it can be shut down to conserve power. An external
reference can also be applied and can be varied from 1 V to

, while the analog input range is from 0 V to V

+V

CC

device includes a shutdown mode, which reduces its current
consumption to less than 1 µA.

of 2.7 V to 5.25 V

CC

level for serial interface

DRIVE

REF

. The

AD7877

FUNCTIONAL BLOCK DIAGRAM

V

CC
7

MUX

ADC DATA

LIMIT

COMPARATOR

GPIO1-3

ON TOUCH

AD7877

REF

STOP

ACQ

LOGIC

ALERT
LOGIC

TO

CLOCK

20

STOPACQ

14

AGND

15

DGND

22

ALERT

21

GPIO4

17

PENIRQ

12

X+

10

X–

13

Y+

11

UX1/GPIO1
UX2/GPIO
UX3/GPIO

BAT1

BAT2

V

REF

AOUT
ARNG

Y–

6
5
4

3

BATTERY
MONITOR

2

BATTERY
MONITOR

TEMPERATURE

SENSOR

31

2.5V
REF

30

8-BIT

DAC

29

CONTROL LOGIC AND SERIAL PORT

19

18

DIN26DCLK27DOUT28V

CS23DAV

9 TO 1

BUF

DAC

REGISTER

I/P

MUX

Y– GND X+ Y+ V

X–

DUAL 3-1

REF–INREF+

12-BIT SUCCESSIVE

APPROXIMATION ADC

WITH TRACK-AND-HOLD

SEQUENCER

RESULTS

REGISTERS

LIMIT

REGISTERS

ALERT STATUS/

MASK REGISTER

GPIO

REGISTERS

CONTROL

REGISTERS

PEN INTERRUPT

AND WAKE-UP

DRIVE

Figure 1.

To reduce the effects of noise from LCDs, the acquisition phase
of the on-board ADC can be controlled via the STOPACQ pin.
User-programmable conversion controls include variable
acquisition time and first conversion delay. Up to 16 averages
can be taken per conversion. There is also an on-board DAC for
LCD backlight or contrast control. The AD7877 can run in
either slave or master mode, using a conversion sequencer and
timer. It is ideal for battery-powered systems such as personal
digital assistants with resistive touch screens and other portable
equipment.

The part is available in a 32-lead lead frame chip scale package
(LFCSP).

03796-001

Rev. A

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 that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]
Fax: [phone] © 2004 Analog Devices, Inc. All rights reserved.

www.analog.com

AD7877

TABLE OF CONTENTS

Specifications..................................................................................... 3

Sequencer Registers ................................................................... 22

Timing Specifications....................................................................... 5

Absolute Maximum Ratings............................................................ 6

ESD Caution.................................................................................. 6

Pin Configuration and Function Descriptions............................. 7

Te r m in o l o g y ...................................................................................... 9

Typical Performance Characteristics ...........................................10

Circuit Information........................................................................ 14

Tou c h S cre en P ri nci p le s ............................................................ 14

Measuring Touch Screen Inputs............................................... 15

Touch-Pressure Measurement .................................................. 16

STOPACQ Pin ............................................................................ 16

Temperature Measurement ....................................................... 17

Battery Measurement................................................................. 18

Auxiliary Inputs .......................................................................... 19

Limit Comparison...................................................................... 19

Interrupts ..................................................................................... 24

Syncronizing the AD7877 to the Host CPU ........................... 25

8-Bit DAC ........................................................................................ 26

Serial Interface................................................................................ 28

Writ i ng D a t a ............................................................................... 28

Write T i min g ............................................................................... 29

Reading Data............................................................................... 29

V

Pin..................................................................................... 29

DRIVE

General-Purpose I/O Pins............................................................. 30

GPIO Configuration .................................................................. 30

Grounding and LayouT ................................................................. 32

PCB Design Guidelines for Chip Scale Packages................... 32

Register Maps.................................................................................. 33

Detailed Register Descriptions..................................................... 35

GPIO Registers ........................................................................... 41

Control Registers............................................................................ 20

Control Register 1.......................................................................20

Control Register 2.......................................................................21

REVISION HISTORY

11/04—Changed from Rev. 0 to Rev. A

Changes to Absolute Maximum Ratings ...................................... 6

Changes to Figure 4.......................................................................... 7

Changes to Table 4............................................................................ 7

Changes to Grounding and Layout section ................................ 32

Changes to Figure 42...................................................................... 32

Changes to Ordering Guide.......................................................... 43

7/04—Revision 0: Initial Version

Outline Dimensions....................................................................... 43

Ordering Guide .......................................................................... 43

Rev. A | Page 2 of 44

AD7877

SPECIFICATIONS

VCC = 2.7 V to 3.6 V, V

Table 1.

Parameter Min Typ Max Unit Test Conditions/Comments
ADC

DC ACCURACY

Resolution 12 Bits
No Missing Codes 11 12 Bits
Integral Nonlinearity
Differential Nonlinearity1
Offset Error1 ±2 ±6 LSB VCC = 2.7 V
Gain Error1
Noise 70 µV rms
Power Supply Rejection 70 dB
Internal Clock Ffrequency 2 MHz

SWITCH DRIVERS

On Resistance

1

Y+, X+ 14 Ω
Y−, X− 14 Ω
ANALOG INPUTS
Input Voltage Ranges 0 V
DC Leakage Current ±0.1 µA
Input Capacitance 30 pF
Accuracy 0.3 % All channels, internal V

REFERENCE INPUT/OUTPUT

Internal Reference Voltage 2.44 2.55 V
Internal Reference Tempco ±50 ppm/°C
V

Input Voltage Range 1 VCC V

REF

DC Leakage Current ±1 µA
V

Input Impedance 1 GΩ

REF

TEMPERATURE MEASUREMENT

Temperature Range −40 +85 °C
Resolution

Differential Method2
Single Conversion Method3

Accuracy

Differential Method2
Single Conversion Method3

BATTERY MONITOR

Input Voltage Range 0.5 5 V @V

Input Impedance 14 kΩ Sampling, 1 GΩ when battery monitor off

Accuracy 1 3.2 % External/internal reference, see Figure 25

= 2.5 V internal or external, f

REF

1

= 2 MHz, TA = −40°C to +85°C, unless otherwise noted.

DCLK

±2 LSB LSB size = 610 µV

−0.99/+2 LSB LSB size = 610 µV

±4 LSB External reference

V

REF

CS

= GND or VCC; typically 25 Ω when on-board

reference enabled

1.6 °C

0.3 °C

±4 °C
±2 °C Calibrated at 25°C

= 2.5 V

REF

REF

Rev. A | Page 3 of 44

AD7877

Parameter Min Typ Max Unit Test Conditions/Comments

DAC

Resolution 8 Bits
Integral Nonlinearity ±1 Bits
Differential Nonlinearity ±1 Guaranteed monotonic by design
Voltage Mode

Output Voltage Range 0 − VCC/2 V DAC register Bit 2 = 0, Bit 0 = 0
0 − V

CC

Slew Rate −0.4, +0.5 V/µs
Output Settling Time 12 15 µs 0 to 3/4 scale, R
Capacitive Load Stability 50 100 pF R

Output Impedance 75 kΩ Power-down mode

Short Circuit Current 21 mA

Current Mode

Output Current Range 0 1000 µA DAC register Bit 2 = 1, full-scale current is set by R
Output Impedance Open Power-down mode

LOGIC INPUTS

Input High Voltage, V
Input Low Voltage, V

0.7 V

INH

0.3 V

INL

V

DRIVE

Input Current, IIN ±1 µA Typically 10 nA, VIN = 0 V or V
Input Capacitance, C

4

10 pF

IN

LOGIC OUTPUTS

Output High Voltage, VOH V

− 0.2 V I

DRIVE

Output Low Voltage, VOL 0.4 V I
Floating-State Leakage Current ±10 µA
Floating-State Output Capacitance4 10 pF
Output Coding Straight (natural) binary

CONVERSION RATE

Conversion Time 8 µs
Throughput Rate 125 kSPS

POWER REQUIREMENTS

VCC (Specified Performance) 2.7 3.6 V Functional from 2.2 V to 5.25 V
V

1.65 V

DRIVE

ICC Digital I/Ps = 0 V or V

Converting Mode 240 380 µA ADC on, internal reference off, V
650 900 µA ADC on, internal reference on, VCC = 3.6 V
900 µA ADC on, internal reference on, DAC on

Static 150 µA

Shutdown Mode 1 µA

1

See the section. Terminology

2

Difference between Temp0 and Temp1 measurement. No calibration necessary.

3

Temperature drift is −2.1 mV/°C.

4

Sample tested @ 25°C to ensure compliance.

V DAC register Bit 2 = 0, Bit 0 = 1

= 10 kΩ, C

LOAD

= 10 kΩ

LOAD

LOAD

= 50 pF

V

DRIVE

CC

= 250 µA, VCC/V

SOURCE

= 250 µA

SINK

CS high to DAV low

CC

V

ADC on, but not converting, internal reference off,

= 3.6 V

V

CC

= 2.7 V to 5.25 V

DRIVE

CC

= 3.6 V

CC

RNG

Rev. A | Page 4 of 44

AD7877

TIMING SPECIFICATIONS

TA = T

are specified with t

Table 2.

Parameter Limit at T

f

DCLK

t

1

t

2

t

20 ns min DCLK low pulse width

3

t

12 ns min DIN setup time

4

t

12 ns min DIN hold time

5

2

t

6

2

t

7

3

t

8

t

0 ns min

9

1

Mark/space ratio for the DCLK input is 40/60 to 60/40.

2

Measured with the load circuit of and defined as the time required for the output to cross 0.4 V or 2.0 V. Figure 3

3

t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of 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
time of the part and is independent of the bus loading.

to T

MIN

1

, unless other wise noted; VCC = 2.7 V to 5.25 V, V

MAX

= tF = 5 ns (10% to 90% of VCC) and timed from a voltage level of 1.6 V.

R

MIN

, T

MAX

Unit Description

10 kHz min
20 MHz max
16 ns min

= 2.5 V. Sample tested at 25°C to ensure compliance. All input signals

REF

CS falling edge to first DCLK rising edge

20 ns min DCLK high pulse width

16 ns max

CS falling edge to DOUT, three-state disabled
16 ns max DCLK falling edge to DOUT valid
16 ns max

CS rising edge to DOUT high impedance

CS rising edge to DCLK ignored

Figure 3.

, quoted in the timing characteristics is the true bus relinquish

8

CS

t

DCLK

DIN

DOUT

1

1 2 3 15 16

MSB LSB

t

6

MSB LSB

t

2

t

4

t

3

t

5

t

7

t

9

t

8

03796-004

Figure 2. Detailed Timing Diagram

Rev. A | Page 5 of 44

AD7877

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 3.

Parameter Rating

VCC to GND −0.3 V to +7 V
Analog Input Voltage to GND −0.3 V to VCC + 0.3 V
Digital Input Voltage to GND −0.3 V to VCC + 0.3 V
Digital Output Voltage to GND −0.3 V to VCC + 0.3 V
V

to GND −0.3 V to VCC + 0.3 V

REF

Input Current to Any Pin Except Supplies110 mA
ESD Rating 2.5 kV
Operating Temperature Range −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Junction Temperature 150°C
LFCSP Package

Power Dissipation 450 mW

θJA Thermal Impedance 135.7°C/W
IR Reflow Peak Temperature 220°C
Pb-Free Parts Only 260°C (±0.5°C)
Lead Temperature (Soldering 10 s) 300°C

1

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

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only and 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.

200µAI

TO OUTPUT

PIN

C

L

50pF

200µAI

Figure 3. Load Circuit for Digital Output Timing Specifications

OL

1.6V

OH

03796-003

ESD 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 this product 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.

Rev. A | Page 6 of 44

AD7877

A

A

A

E

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

1

NC

2

BAT2

3

BAT1
UX3/GPIO3
UX2/GPIO2
UX1/GPIO1

NC = NO CONNECT

4
5
6
7

V

CC

8

NC

REF

NC

V

32 31 30 29 28 27 26 25

9 10 11 12 13 14 15 16

X–

NC

DRIV

AOUT

ARNG

V

AD7877

TOP VIEW

(Not to Scale)

Y–

X+

Y+

DOUT

AGND

DCLK

DGND

NC

NC

24

NC

23

DAV

22

ALERT

21

GPIO4

20

STOPACQ

19

DIN

18

CS

17

PENIRQ

03796-002

Figure 4. Pin Configuration

Table 4. Pin Function Descriptions

Pin No. Mnemonic Description

1 NC No Connect.
2 BAT2 Battery Monitor Input. ADC Input Channel 7.
3 BAT1 Battery Monitor Input. ADC Input Channel 6.
4 AUX3/GPIO3 Auxiliary Analog Input. ADC Input Channel 5. Can be reconfigured as GPIO pin.
5 AUX2/GPIO2 Auxiliary Analog Input. ADC Input Channel 4. Can be reconfigured as GPIO pin.
6 AUX1/GPIO1 Auxiliary Analog Input. ADC Input Channel 3. Can be reconfigured as GPIO pin.
7 V

CC

Power Supply Input. The VCC range for the AD7877 is from 2.2 V to 5.25 V.
8–9 NC No Connect.
10 X− Touch Screen Position Input.
11 Y− Touch Screen Position Input. ADC Input Channel 2.
12 X+ Touch Screen Position Input. ADC Input Channel 0.
13 Y+ Touch Screen Position Input. ADC Input Channel 1.
14 AGND

Analog Ground. Ground reference point for all analog circuitry on the AD7877. All analog input signals and any

external reference signal should be referred to this voltage.
15 DGND

Digital Ground. Ground reference for all digital circuitry on the AD7877. All digital input signals should be

referred to this voltage.
16, 32 NC No Connect.
17

18

PENIRQ
CS Chip Select Input. Active low logic input. This input provides the dual function of initiating conversions on the

Pen Interrupt. Digital active low output (has 50 kΩ internal pull-up resistor).

AD7877 and enabling the serial input/output register.
19 DIN

SPI® Serial Data Input. Data to be written to the AD7877’s registers should be provided on this input and is

clocked into the register on the rising edge of DCLK.
20 STOPACQ

Stop Acquisition Pin. A signal applied to this pin can be monitored by the AD7877, so that acquisition of new

data by the ADC is halted while the signal is active. Used to reduce the effect of noise from an LCD screen on

the touch screen measurements.
21 GPIO4 Dedicated general-purpose logic input/output pin.
22

ALERT Digital Active Low Output. Interrupt output, which goes low if a GPIO data bit is set, or if the AUX1, TEMP1,

BAT1, or BAT2 measurements are out of range.
23

DAV Data Available Output. Active low logic output. Asserts low when new data is available in the AD7877 results

registers. This output is high impedance when

CS is high.

24–25 NC No Connect.
26 DCLK External Clock Input. Logic input. DCLK provides the serial clock for accessing data from the part.
27 DOUT

Serial Data Output. Logic output. The conversion result from the AD7877 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

CS is high.

when
28 V

DRIVE

Logic Power Supply Input. The voltage supplied at this pin determines the operating voltage for the serial

interface of the AD7877.

Rev. A | Page 7 of 44

AD7877

Pin No. Mnemonic Description

29 ARNG When the DAC is in current output mode, a resistor from ARNG to GND sets the output range.
30 AOUT Analog Output Voltage or Current from DAC.
31 V

REF

Reference output for the AD7877. The internal 2.5 V reference is available on this pin for use external to the
device. The reference output must be buffered before it is applied elsewhere in a system. A capacitor of 100nF
is strongly recommended between the V

pin and GND to reduce system noise effects.

REF

Alternatively, an external reference can be applied to this input. The voltage range for the external reference is

1.0 V to V

. For the specified performance, it is 2.5 V on the AD7877.

CC

Rev. A | Page 8 of 44

AD7877

TERMINOLOGY

Integral Nonlinearity

The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function. The endpoints 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

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

Offset Error

The deviation of the first code transition (00…000) to
(00…001) from the ideal (AGND + 1 LSB).

Gain Error

The deviation of the last code transition (111…110) to
(111…111) from the ideal (V
has been adjusted out.

On Resistance

A measure of the ohmic resistance between the drain and the
source of the switch drivers.

− 1 LSB) after the offset error

REF

Rev. A | Page 9 of 44

AD7877

TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C, VCC = 2.7 V, V

800

= 2.5 V, f

REF

SAMPLE

= 125 kHz, f

DCLK

= 16 × f

SAMPLE

= 2 MHz, unless otherwise noted.

200

ADC, REF, AND DAC

700

ADC AND REF

CURRENT (µA)

600

500

–50 –30 –10 0 30 50 70 90

TEMPERATURE (°C)

Figure 5. Supply Current vs. Temperature

1000

900

800

700

CURRENT (µA)

600

500

400

2.0 2.3 2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5.0

Figure 6. Supply Current vs. V

ADC, REF, AND DAC

ADC AND REF

VCC (V)

CC

0.6

0.5

0.4

0.3

0.2

0.1
0

–0.1
–0.2

DELTA FROM 25°C (LSB)

–0.3
–0.4
–0.5
–0.6

–50–30–101030507090

TEMPERATURE (°C)

Figure 7. Change in ADC Gain vs. Temperature

03796-030

03796-031

03796-039

180

160

140

CURRENT (nA)

120

100

80

–50–30–101030507090

Figure 8. Full Power-Down I

TEMPERATURE (°C)

vs. Temperature

DD

0.6

0.5

0.4

0.3

0.2

0.1
0

–0.1
–0.2

DELTA FROM 25°C (LSB)

–0.3
–0.4
–0.5
–0.6

–50–30–101030507090

TEMPERATURE (°C)

Figure 9. Change in ADC Offset vs. Temperature

1.0

0.8

0.6

0.4

0.2

0

INL (LSB)

–0.2

–0.4

–0.6

–0.8

–1.0

0 500 1000 1500 2000 2500 3000 3500 4000

CODE

Figure 10. ACD INL Plot

03796-032

03796-040

03796-044

Rev. A | Page 10 of 44

AD7877

1.0

0.8

0.6

0.4

0.2

0

DNL (LSB)

–0.2

–0.4

–0.6

–0.8

–1.0

0 500 1000 1500 2000 2500 3000 3500 4000

CODE

Figure 11. ADC DNL Plot

03796-045

16

14

12

10

8

6

4

REFERENCE CURRENT (µA)

2

0

–50 9070503010–10–30

TEMPERATURE (°C)

Figure 14. External Reference Current vs. Temperature

03796-046

22

20

18

16

(Ω)

ON

14

R

12

10

8

2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5

X+ TO V

Y+ TO V

DD

VDD (V)

Figure 12. Switch On Resistance vs. V
(X+, Y+: V

22

20

18

16

(Ω)

ON

14

R

12

to Pin; X−, Y−: Pin to GND)

CC

Y+ TO V

X– TO GND

Y– TO GND

DD

CC

X– TO GND

Y– TO GND

DD

2.520

2.515

2.510

2.505

2.500

(V)

REF

2.495

V

2.490

2.485

2.480

03796-048

2.475
–50–30–101030507090

Figure 15. Internal V

2.508

2.506

2.504

(V)

2.502

REF

V

2.500

TEMPERATURE (°C)

vs. Temperature

REF

03796-033

10

X+ TO V

8

–40–200 20406080

DD

TEMPERATURE (°C)

Figure 13. Switch On Resistance vs. Temperature

(X+, Y+: V

to Pin; X−, Y−: Pin to GND)

CC

03796-049

Rev. A | Page 11 of 44

2.498

2.496

2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5.0

Figure 16. Internal V

VCC (V)

vs. V

REF

CC

03796-034

AD7877

3145

3135

3125

3115

3105

3095

3085

3075

ADC CODE (Decimal)

3065

3055

3045

–50–30–101030507090

Figure 17. ADC Code vs. Temperature (2.7 V Supply)

1183

TEMPERATURE (°C)

03796-041

(V)

REF

INTERNAL V

10

6

NO CAP

0.711µs SETTLING TIME

3

0

Figure 20. Internal V

20 40 60 80 100 120–20 0

TURN-ON TIME (µs)

100nF CAP

54.64µs SETTLING TIME

vs. Turn-On Time

REF

03796-047

1182

1181

1180

1179

TEMP1 CODE

1178

1177

1176

2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6

Figure 18. Temp1 vs. V

982

981

980

979

978

TEMP0 CODE

977

976

975

2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6

VCC (V)

CC

VCC (V)

03796-042

03796-043

–10

–30

–50

–70

–90

–110

INPUT TONE AMPLITUDE (dB)

–130

–150

0 10k 20k 30k

SNR 70.25dB
THD 78.11dB

FREQUENCY

40k

Figure 21. Typical FFT Plot for the Auxiliary Channels of the AD7877

at 90 kHz Sample Rate and 10 kHz Input Frequency

3.50

3.25

3.00

2.75

2.50

2.25

2.00

1.75

1.50

1.25

DAC O/P LEVEL (V)

1.00

0.75

0.50
DAC O/P SINK ABILITY

0.25

0

012345678910

SOURCE/SINK CURRENT (mA)

DAC O/P SOURCE ABILITY

03796-035

03796-036

Figure 19. Temp0 vs. V

CC

Figure 22. DAC Source and Sink Current Capability

Rev. A | Page 12 of 44

AD7877

∆

: 144mV

@: 1.296V

VDD = 3V
TEMPERATURE = 25°C

1

CH1 200mV CH2 100mV M2.00µs CH1 780mV

Figure 23. DAC O/P Settling Time (Zero Scale to Half-Scale)

600

03796-037

–2 –1 0 1 2

ERROR (%)

03796-050

Figure 25. Typical Accuracy for Battery Channel (25°C)

500

400

DAC SINK CURRENT

300

200

DAC SINK CURRENT (µA)

100

0

0 25 50 75 100 125 150 175 200 225 250

INPUT CODE (Decimal)

03796-038

Figure 24. DAC Sink Current vs. Input Code

Rev. A | Page 13 of 44

AD7877

CIRCUIT INFORMATION

The AD7877 is a complete, 12-bit data acquisition system for
digitizing positional inputs from a touch screen in PDAs and
other devices. In addition, it can monitor two battery voltages,
ambient temperature, and three auxiliary analog voltages, with
high and low limit comparisons on three of the inputs, and has
up to four general-purpose logic I/O pins.

The core of the AD7877 is a high speed, low power, 12-bit
analog-to-digital converter (ADC) with input multiplexer,
on-chip track-and-hold, and on-chip clock. The results of
conversions are stored in 11 results registers, and the results
from one auxiliary input and two battery inputs can be
compared with high and low limits stored in limit registers to

ALERT

generate an out-of-limit
resistance analog switches to switch the X and Y excitation
voltages to the touch screen, a STOPACQ pin to control the
ADC acquisition period, 2.5 V reference, on-chip temperature
sensor, and 8-bit DAC to control LCD contrast. The high speed
SPI serial bus provides control of, and communication with, the
device.

Operating from a single supply from 2.2 V to 5 V, the AD7877
offers throughput rates of up to 125 kHz. The device is available
in a 5 mm by 5 mm 32-lead lead frame chip scale package.

The data acquisition system of the AD7877 has a number of
advanced features:

• Input channel sequenced automatically or selected by

the host

• STOPACQ feature to reduce noise from LCD

• Averaging of from 1 to 16 conversions for noise

reduction

• Programmable acquisition time

• Power management

• Programmable ADC power-up delay before first

conversion

• Choice of internal or external reference

• Conversion at preprogrammed intervals

TOUCH SCREEN PRINCIPLES

A 4-wire touch screen consists of two flexible, transparent,
resistive-coated layers that are normally separated by a small air
gap. The X layer has conductive electrodes running down the
left and right edges, allowing the application of an excitation
voltage across the X layer from left to right.

. The AD7877 also contains low

PLASTIC FILM WITH

TRANSPARENT, RESISTIVE

CONDUCTIVE ELECTRODE

ON BOTTOM SIDE

CONDUCTIVE ELECTRODE

ON TOP SIDE

Figure 26. Basic Construction of a Touch Screen

Y+

LCD SCREEN

COATING ON BOTTOM SIDE

Y–

X+

PLASTIC FILM WITH

TRANSPARENT, RESISTIVE

COATING ON TOP SIDE

X–

03796-005

The Y layer has conductive electrodes running along the top
and bottom edges, allowing the application of an excitation
voltage down the layer from top to bottom.

Provided that the layers are of uniform resistivity, the voltage at
any point between the two electrodes is proportional to the
horizontal position for the X layer and the vertical position for
the Y layer.

When the screen is touched, the two layers make contact. If only
the X layer is excited, the voltage at the point of contact, and
therefore the horizontal position, can be sensed at one of the
Y layer electrodes. Similarly, if only the Y layer is excited, the
voltage, and therefore the vertical position, can be sensed at one
of the X electrodes. By switching alternately between X and
Y excitation and measuring the voltages, the X and Y coordi­nates of the contact point can be found.

In addition to measuring the X and Y coordinates, it is also
possible to estimate the touch pressure by measuring the
contact resistance between the X and Y layers. The AD7877 is
designed to facilitate this measurement.

Figure 28 shows an equivalent circuit of the analog input
structure of the AD7877, showing the touch screen switches, the
main analog multiplexer, the ADC with analog and differential
reference inputs, and the dual 3-to-1 multiplexer that selects the
reference source for the ADC.

Rev. A | Page 14 of 44