ANALOG DEVICES AD7879W Service Manual

Low Voltage Controller for Touch Screens

AD7879W

Rev. 0

Trademarks and registered trademarks are the property of their respective owners.

Fax: [phone] ©2011 Analog Devices, Inc. All rights reserved.

REF+

X– Y– X+ Y+

FIL

TERING

RESULT

REGISTERS

CONTROL

REGISTERS

SEQUENCER

AND TIMER

REF–

TEMPERATURE

SENSOR

TO

RESULT

REGISTERS

6-TO-1 MUX

AD7879W/

AD7879-1W

VCC/REF

X+
X–

Y+
Y–

GND

AUX/VBAT/GPIO

PENIRQ/INT/DAV

SERIAL PORT

DIN/

ADD1

DOUT/

SDA

SCL

CS/

ADD0

12-BIT

SAR ADC

REF–

10408-001

Data Sheet

FEATURES

4-wire touch screen interface
Qualified for automotive applications

1.6 V to 3.6 V operation
Median and averaging filter to reduce noise
Automatic conversion sequencer and timer
User-programmable conversion parameters
Auxiliary analog input/battery monitor (0.5 V to 5 V)
1 optional GPIO
Interrupt outputs (

INT, PENIRQ
Touch-pressure measurement
Wake-up on touch function
Shutdown mode: 6 µA maximum
16-lead, 4.4 mm × 5 mm TSSOP
16-lead, 4 mm × 4 mm LFCSP

APPLICATIONS

Automotive applications
Personal digital assistants
Smart handheld devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones

)

FUNCTIONAL BLOCK DIAGRAM

Figure 1.

GENERAL DESCRIPTION

The AD7879W is a 12-bit successive approximation analog-to­digital converters (SAR ADCs) with a synchronous serial
interface and low on-resistance switches for driving 4-wire
resistive touch screens. The AD7879W works with a very low
power supply—a single 1.6 V to 3.6 V supply—and feature
throughput rates of 105 kSPS. The devices include a shutdown
mode that reduces current consumption to less than 6 µA.

To reduce the effects of noise from LCDs and other sources, the

AD7879W contains a preprocessing block. The preprocessing

function consists of a median filter and an averaging filter. The
combination of these two filters provides a more robust solution,
discarding the spurious noise in the signal and keeping only the
data of interest. The size of both filters is programmable. Other
user-programmable conversion controls include variable
acquisition time and first conversion delay; up to 16 averages
can be taken per conversion. The AD7879W can run in slave

Information furnishe d 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.

mode or standalone (master) mode, using an automatic
conversion sequencer and timer.

The AD7879W has a programmable pin that can operate as an
auxiliary input to the ADC, as a battery monitor, or as a GPIO.
In addition, a programmable interrupt output can operate in
three modes: as a general-purpose interrupt to signal when new
data is available (

are exceeded (
screen is touched (

DAV

), as an interrupt to indicate when limits

INT

), or as a pen-down interrupt when the

PENIRQ

). The AD7879W offers temperature

measurement and touch-pressure measurement.

The AD7879W is available in a 16-lead, 4.4 mm × 5.0 mm
TSSOP and 16-lead 4 mm × 4 mm LFCSP. Both packages
support an SPI interface (AD7879W) or an I

2

C® interface

(AD7879-1W).

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]

www.analog.com

AD7879W Data Sheet

TABLE OF CONTENTS

Features .............................................................................................. 1

Applications ....................................................................................... 1

Functional Block Diagram .............................................................. 1

General Description ......................................................................... 1

Revision History ............................................................................... 2

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

SPI Timing Specifications (AD7879W) .................................... 4

I2C Timing Specifications (AD7879-1W) .................................. 5

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

Thermal Resistance ...................................................................... 6

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

Pin Configurations and Function Descriptions ........................... 7

Typical Performance Characteristics ............................................. 9

Terminology .................................................................................... 12

Theory of Operation ...................................................................... 13

Touch Screen Principles ............................................................ 13

Measuring Touch Screen Inputs ............................................... 14

Touch -Pressure Measurement .................................................. 15

Temperature Measurement ....................................................... 15

Median and Averaging Filters ....................................................... 17

AUX/VBAT/GPIO Pin ................................................................... 18

Auxiliary Input ............................................................................ 18

Battery Input ............................................................................... 18

Limit Comparison ...................................................................... 18

GPIO ............................................................................................ 18

Conversion Timing ........................................................................ 20

Register Map ................................................................................... 21

Detailed Register Descriptions ..................................................... 22

Control Registers ............................................................................ 26

Control Register 1 ...................................................................... 26

Control Register 2 ...................................................................... 28

Control Register 3 ...................................................................... 29

Interrupts ..................................................................................... 30

Synchronizing the AD7879W to the Host CPU .................... 31

Serial Interface ................................................................................ 32

SPI Interface ................................................................................ 32

I2C-Compatible Interface .......................................................... 34

Grounding and Layout .................................................................. 37

Lead Frame Chip Scale Packages ............................................. 37

Outline Dimensions ....................................................................... 38

Ordering Guide .......................................................................... 39

Automotive Products ................................................................. 39

REVISION HISTORY

12/11—Revision 0: Initial Version

Rev. 0 | Page 2 of 40

Data Sheet AD7879W

Internal Clock Accuracy

1.8 2.2

MHz

SPECIFICATIONS

VCC = 1.6 V to 3.6 V, TA = −40°C to +85°C, unless otherwise noted.

Table 1.

Parameter Min Typ Max Unit Test Conditions/Comments

DC ACCURACY

Resolution 12 Bits
No Missing Codes 11 12 Bits
Integral Nonlinearity (INL)1 ±3 LSB LSB size = 390 µV.
Differential Nonlinearity (DNL)1 LSB size = 390 µV.

Negative DNL −0.99 LSB

Positive DNL 2 LSB
Offset Error
Gain Error
Noise3 70 µV rms
Power Supply Rejection3 60 dB
Internal Clock Frequency 2 MHz

SWITCH DRIVERS

On Resistance1

Y+, X+ 6 Ω

Y−, X− 5 Ω

ANALOG INPUTS

Input Voltage Range 0 VCC V
DC Leakage Current ±0.1 µA
Input Capacitance 30 pF
Accuracy 0.3 %

TEMPERATURE MEASUREMENT

Temperature Range −40 +85 °C
Resolution 0.3 °C
Accuracy2 ±2 °C Calibrated at 25°C.

BATTERY MONITOR

Input Voltage Range 0.5 5 V
Input Impedance3 16 kΩ
Accuracy 2 5 % Uncalibrated accuracy.

LOGIC INPUTS (DIN, SCL, CS, SDA, GPIO)

Input High Voltage, V
Input Low Voltage, V
Input Current, IIN 0.01 µA VIN = 0 V or VCC.
Input Capacitance, C

LOGIC OUTPUTS (DOUT, GPIO, SCL, SDA,

Output High Voltage, VOH VCC − 0.2 V
Output Low Voltage, VOL 0.4 V
Floating-State Leakage Current ±0.1 µA
Floating-State Output Capacitance2 5 pF

CONVERSION RATE3

Conversion Time 9.5 µs Including 2 µs of acquisition time, MAV

Throughput Rate 105 kSPS

1, 2

±2 ±6 LSB

1, 2

±4 LSB

0.7 × VCC V

INH

0.3 × VCC V

INL

3

10 pF

IN

)

INT

filter off. 2 µs of additional time is required
if MAV filter is on.

Rev. 0 | Page 3 of 40

AD7879W Data Sheet

Static

406 µA

ADC and temperature sensor are off; the

t6

20

ns max

DOUT access time after SCL falling edge

CS

SCL

DIN

DOUT

t

1

1

16

15

MSB

LSB

2 3

MSB

LSB

1 2

15

16

t

2

t

4

t

5

t

3

t

6

t

7

t

8

10408-002

Parameter Min Typ Max Unit Test Conditions/Comments

POWER REQUIREMENTS

VCC 1.6 2.6 3.6 V Specified performance.
ICC Digital inputs = 0 V or VCC.

Converting Mode 480 650 µA ADC on, PM = 10.

reference and oscillator are on; PM = 01
or 11.

Shutdown Mode 0.5 6 µA PM = 00.

1

See the Terminology section.

2

Guaranteed by characterization; not production tested.

3

Sample tested at 25°C to ensure compliance.

SPI TIMING SPECIFICATIONS (AD7879W)

VCC = 1.6 V to 3.6 V, TA = −40°C to +85°C, unless otherwise noted. Sample tested at 25°C to ensure compliance. All input signals are
specified with t

Table 2.

Parameter1 Limit Unit Description

f

5 MHz max

SCL

t1 5 ns min
t2 20 ns min SCL high pulse width

t3 20 ns min SCL low pulse width
t4 15 ns min DIN setup time
t5 15 ns min DIN hold time

= tF = 5 ns (10% to 90% of VCC) and timed from a voltage level of 1.4 V.

R

falling edge to first SCL falling edge

CS

t7 16 ns max
t8 15 ns min SCL rising edge to CS high

1

Guaranteed by design; not production tested.

rising edge to DOUT high impedance

CS

Figure 2. Detailed SPI Timing Diagram

Rev. 0 | Page 4 of 40

Data Sheet AD7879W

S

I2C TIMING SPECIFICATIONS (AD7879-1W)

VCC = 1.6 V to 3.6 V, TA = −40°C to +85°C, unless otherwise noted. Sample tested at 25°C to ensure compliance. All input signals are
timed from a voltage level of 1.4 V.

Table 3.

Parameter1 Limit Unit Description

f

400 kHz max

SCL

t1 0.6 μs min Start condition hold time, t
t2 1.3 μs min Clock low period, t
t3 0.6 μs min Clock high period, t
t4 100 ns min Data setup time, t
t5 300 ns min Data hold time, t

LOW

SU; DAT

HD; DAT

HIGH

t6 0.6 μs min Stop condition setup time, t
t7 0.6 μs min Start condition setup time, t
t8 1.3 μs min Bus-free time between stop and start conditions, t
tR 300 ns max Clock/data rise time
tF 300 ns max Clock/data fall time

1

Guaranteed by design; not production tested.

SCL

DA

t

t

2

t

1

t

8

R

t

5

t

F

t

3

t

4

HD; STA

SU; STO

SU; STA

t

7

BUF

t

1

t

6

STOP START STOPSTART

Figure 3. Detailed I

2

C Timing Diagram

10408-003

Rev. 0 | Page 5 of 40

AD7879W Data Sheet

ESD Rating (X+, Y+, X−, Y−)

16-Lead LFCSP

30.4

°C/W

200µA I

OL

200µA I

OH

1.4V

TO OUTPUT

PIN

C

L

50pF

10408-004

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 4.

Parameter Rating

VCC to GND −0.3 V to +3.6 V
Analog Input Voltage to GND −0.3 V to VCC + 0.3 V
AUX/VBAT to GND −0.3 V to +5 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
Input Current to Any Pin Except Supplies1 10 mA

Air Discharge Human Body Model 15 kV
Contact Human Body Model 10 kV

ESD Rating (All Other Pins)

Human Body Discharge 4 kV
Field-Induced Charged Device Model 1 kV

Machine Model 0.2 kV
Operating Temperature Range −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Junction Temperature 150°C
Power Dissipation

TSSOP (4-Layer Board) 577.2 mW

LFCSP (4-Layer Board) 2.138 W
IR Reflow Peak Temperature 260°C (±0.5°C)
Lead Temperature (Soldering 10 sec) 300°C

1

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

THERMAL RESISTANCE

θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.

Table 5. Thermal Resistance

Package Type1 θJA Unit

16-Lead TSSOP 112.6 °C/W

1

4-layer board.

Figure 4. Circuit Used for Digital Timing

ESD CAUTION

Stresses above those listed under Absolute Maximum Ratings
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 indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

Rev. 0 | Page 6 of 40

Data Sheet AD7879W

5 5 X−

Touch Screen Input Channel.

NC = NO CONNECT

1

2

3

4

5

6

7
8

NC

X+
Y+

NC

Y–

X–

V

CC

/REF

DIN

16

15

14

13
12

11

10

9

NC
AUX/VBAT/GPIO
PENIRQ/INT/DAV

NC
GND

SCL

DOUT

CS

AD7879W

TOP VIEW

(Not to S cale)

10408-005

NC = NO CONNECT

1

2

3

4

5

6

7
8

NC

X+
Y+

NC

Y–

X–

V

CC

/REF

ADD1

16

15

14

13
12

11

10

9

NC
AUX/VBAT/GPIO
PENIRQ/INT/DAV

NC
GND

SCL

SDA

ADD0

AD7879W

TOP VIEW

(Not to S cale)

10408-006

PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

Figure 5. AD7879W TSSOP Pin Configuration

Figure 6. AD7879-1W TSSOP Pin Configuration

Table 6. Pin Function Descriptions, TSSOP

Pin No.

AD7879W AD7879-1W

Mnemonic Description

1 1 VCC/REF Power Supply Input and ADC Reference.
2, 7, 10, 15 2, 7, 10, 15 NC No Connect.
3 3 X+ Touch Screen Input Channel.
4 4 Y+ Touch Screen Input Channel.

6 6 Y− Touch Screen Input Channel.
8 N/A DIN SPI Serial Data Input to the AD7879W.
N/A 8 ADD1 I2C Address Bit 1 for the AD7879-1W. This pin can be tied high or low to determine an

address for the AD7879-1W (see Table 25).

9 9 GND Ground. Ground reference point for all circuitry on the AD7879W. All analog input signals

and any external reference signal should be referred to this voltage.
11 11 SCL Serial Interface Clock Input.
12 N/A DOUT SPI Serial Data Output for the AD7879W.
N/A 12 SDA I2C Serial Data Input and Output for the AD7879-1W.
13 13

PENIRQ/INT
DAV

/

Interrupt Output. This pin is asserted when the screen is touched (

ment exceeds the preprogrammed limits (

(

). Active low, internal 50 kΩ pull-up resistor.

DAV

), or when new data is available in the registers

INT

), when a measure-

PENIRQ

14 14 AUX/VBAT/GPIO This pin can be programmed as an auxiliary input to the ADC (AUX), as a battery measure-

ment input to the ADC (VBAT), or as a general-purpose digital input/output (GPIO).
16 N/A

Chip Select for the SPI Serial Interface on the AD7879W. Active low.

CS

N/A 16 ADD0 I2C Address Bit 0 for the AD7879-1W. This pin can be tied high or low to determine an

address for the AD7879-1W (see Table 25).

Rev. 0 | Page 7 of 40

AD7879W Data Sheet

16

16

X+

Touch Screen Input Channel.

PIN 1
INDICATOR

1Y+
2NC
3NC
4X–

11 NC

12

10 NC
9 DOUT

5
Y–

6
DIN

7

GND

8

SCL

15

V

CC

/REF

16

X+

14

13

AUX/VBAT/GPIO

AD7879W

TOP VIEW

(Not to S cale)

PENIRQ/INT/DAV

CS

NOTES

1. NC = NO CONNECT

2. THE EXPOSED PAD IS NOT CONNECTED INTERNALLY.
FOR INCREAS E D RE LIABILI TY OF THE SOLDER JOINTS

AND MAXIMUM T HE RM AL CAPABILITY, IT IS RECOMMENDED

THAT THE PAD BE SOLDERED TO T HE GROUND PLANE.

10408-007

PIN 1
INDICATOR

1Y+
2NC
3NC
4X–

11 NC

12

10 NC
9 SDA

5
Y–

6

ADD1

7

GND

8

SCL

15

V

CC

/REF

16

X+

14

13

AUX/VBAT/GPIO

AD7879-1W

TOP VIEW

(Not to S cale)

PENIRQ/INT/DAV

ADD0

NOTES

1. NC = NO CONNECT

2. THE EXPOSED PAD IS NOT CONNECTED INTERNALLY.
FOR INCREAS E D RE LIABILI TY OF THE SOLDER JOINTS

AND MAXIMUM T HE RM AL CAPABILITY, IT IS RECOMMENDED

THAT THE PAD BE SOLDERED TO T HE GROUND PLANE.

10408-008

Figure 7. AD7879W LFCSP Pin Configuration

Figure 8. AD7879-1W LFCSP Pin Configuration

Table 7. Pin Function Descriptions, LFCSP

Pin No.

AD7879W AD7879-1W

Mnemonic Description

1 1 Y+ Touch Screen Input Channel.
2, 3, 10, 11 2, 3, 10, 11 NC No Connect.
4 4 X− Touch Screen Input Channel.
5 5 Y− Touch Screen Input Channel.
6 N/A DIN SPI Serial Data Input to the AD7879W.
N/A 6 ADD1 I2C Address Bit 1 for the AD7879-1W. This pin can be tied high or low to determine an

address for the AD7879-1W (see Table 25).

7 7 GND Ground. Ground reference point for all circuitry on the AD7879W. All analog input signals

and any external reference signal should be referred to this voltage.
8 8 SCL Serial Interface Clock Input.
9 N/A DOUT SPI Serial Data Output for the AD7879W.
N/A 9 SDA I2C Serial Data Input and Output for the AD7879-1W.
12 12

PENIRQ/INT/DAV

Interrupt Output. This pin is asserted when the screen is touched (

ment exceeds the preprogrammed limits (

(

). Active low, internal 50 kΩ pull-up resistor.

DAV

), or when new data is available in the registers

INT

), when a measure-

PENIRQ

13 13 AUX/VBAT/GPIO This pin can be programmed as an auxiliary input to the ADC (AUX), as a battery measure-

ment input to the ADC (VBAT), or as a general-purpose digital input/output (GPIO).
14 N/A

Chip Select for the SPI Serial Interface on the AD7879W. Active low.

CS

N/A 14 ADD0 I2C Address Bit 0 for the AD7879-1W. This pin can be tied high or low to determine an

address for the AD7879-1W (see Table 25).
15 15 VCC/REF Power Supply Input and ADC Reference.

EP Exposed Pad. The exposed pad is not connected internally. For increased reliability of the

solder joints and maximum thermal capability, it is recommended that the pad be

soldered to the ground plane.

Rev. 0 | Page 8 of 40

Data Sheet AD7879W

475

470

465

460

455

450

445

440

435

430
425

–40 –25 –10 10 25 40 55 70 85

TEMPERATURE (°C)

CURRENT (µA)

10408-009

700

600

500

400

300

200

100

0

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
V

CC

(V)

CURRENT (µA)

10408-010

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

–40 –25 –10 10 25 50 75 100

TEMPERATURE (°C)

CURRENT (µA)

10408-011

1.0

–1.0

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

TEMPERATURE (°C)

GAIN ERROR V ARIATION (LSB)

2.6V

3.6V

1.6V

85–40 –25 –10 10 25 40 55 70

10408-012

1.0

–1.0

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

TEMPERATURE (°C)

OFFSET VARIATION (LSB)

2.6V

3.6V

1.6V

85

–40 –25 –10 10

25 40 55 70

10408-013

2.0

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

0 4096358430722560204815361024512

CODE

INL (LSB)

10408-014

TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C, VCC = 2.6 V, f

= 2 MHz, unless otherwise noted.

SCL

Figure 9. Supply Current vs. Temperature

Figure 10. Supply Current vs. VCC

Figure 12. Change in ADC Gain vs. Temperature

Figure 13. Change in ADC Offset vs. Temperature

Figure 11. Full Power-Down IDD vs. Temperature

Figure 14. ADC INL

Rev. 0 | Page 9 of 40

AD7879W Data Sheet

1.0

0.8

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

–0.8
–1.0

1 501 1001 1501 2001 2501 3001 3501 4001

CODE

DNL (LSB)

10408-015

7

6

5

4

3

2

1

0

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
V

CC

(V)

R

ON

(Ω)

X+ TO V

CC

Y+ TO V

CC

X– TO GND
Y– TO GND

10408-016

6.0

5.5

5.0

4.5

4.0

3.5

3.0
–40 –25 –10 10 25 40 55 70 85

TEMPERATURE (°C)

R

ON

(Ω)

X+ TO V

CC

Y+ TO V

CC

X– TO GND
Y– TO GND

10408-017

2370

2369

2368

2367

2366

2365

2364

2363

2362

2361
2360

–40 –25 –15 –5 5 15 25 35 45 55 65

TEMPERATURE (°C)

ADC CODE (Decimal )

75 85

10408-018

Figure 15. ADC DNL

Figure 16. Switch On Resistance vs. VCC
(X+, Y+: Pin to V

; X−, Y−: Pin to GND)

CC

Figure 17. Switch On Resistance vs. Temperature

(X+, Y+: Pin to V

; X−, Y−: Pin to GND)

CC

Figure 18. ADC Code vs. Temperature (Fixed Analog Input)

Rev. 0 | Page 10 of 40

Data Sheet AD7879W

1400

1200

1000

800

600

400

200

0

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6

V

CC

(V)

TEMPERAT URE ( Code)

10408-019

0

–20

–40

–60

–80

–100

–120

–140

–160

0

1603

3206

4809

6412

8015

9618

11221

12824

14427

16030

17633

19236

20839

22442

24045

25648

27251

28854

30457

32060

33663

35266

36869

FREQUENCY ( Hz )

INPUT TONE AMPLITUDE (dB)

SNR = 61.58dB
THD = 72.34dB

10408-020

250

200

150

100

50

0

NUMBER OF UNI TS

–4 –3 –2 –1 0

ERROR (%)

MEAN: –1.98893
SD: 0.475534

10408-021

Figure 19. Temperature Code vs. VCC for 25°C

Figure 21. Typical Uncalibrated Accuracy for the Battery Channel (25°C)

Figure 20. Typical FFT Plot for the Auxiliary Channels at 25 kHz Sampling

Rate and 1 kHz Input Frequency

Rev. 0 | Page 11 of 40

AD7879W Data Sheet

TERMINOLOGY

Differential Nonlinearity (DNL)

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

Integral Nonlinearity (INL)

INL is 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 at 1 LSB below
the first code transition and full scale at 1 LSB above the last
code transition.

Gain Error

Gain error is the deviation of the last code transition
(111 … 110 to 111 … 111) from the ideal (V
after the offset error has been calibrated out.

Offset Error

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

On Resistance

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

− 1 LSB)

REF

Rev. 0 | Page 12 of 40