FEATURES
FUNCTIONAL BLOCK DIAGRAM
VCC/REF
X– Y– X+ Y+
X+
X–
REF–
REF–
REF+
GND
TEMPERATURE
SENSOR
12-BIT
SAR ADC
AD7879W/
AD7879-1W
APPLICATIONS
SERIAL PORT
CONTROL
REGISTERS
SEQUENCER
AND TIMER
TO
RESULT
REGISTERS
DIN/ DOUT/ SCL
CS/
ADD0 ADD1 SDA
10408-001
Automotive applications
Personal digital assistants
Smart handheld devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones
RESULT
REGISTERS
PENIRQ/INT/DAV
Y–
FILTERING
Y+
AUX/VBAT/GPIO
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
6-TO-1 MUX
Data Sheet
Low Voltage Controller for Touch Screens
AD7879W
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
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 (DAV), as an interrupt to indicate when limits
are exceeded (INT), or as a pen-down interrupt when the
screen is touched (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 I2C® interface
(AD7879-1W).
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 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: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2011 Analog Devices, Inc. All rights reserved.
AD7879W
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Battery Input ............................................................................... 18
Applications ....................................................................................... 1
Limit Comparison ...................................................................... 18
Functional Block Diagram .............................................................. 1
GPIO ............................................................................................ 18
General Description ......................................................................... 1
Conversion Timing ........................................................................ 20
Revision History ............................................................................... 2
Register Map ................................................................................... 21
Specifications..................................................................................... 3
Detailed Register Descriptions ..................................................... 22
SPI Timing Specifications (AD7879W) .................................... 4
Control Registers ............................................................................ 26
I2C Timing Specifications (AD7879-1W) .................................. 5
Control Register 1 ...................................................................... 26
Absolute Maximum Ratings ............................................................ 6
Control Register 2 ...................................................................... 28
Thermal Resistance ...................................................................... 6
Control Register 3 ...................................................................... 29
ESD Caution .................................................................................. 6
Interrupts ..................................................................................... 30
Pin Configurations and Function Descriptions ........................... 7
Synchronizing the AD7879W to the Host CPU .................... 31
Typical Performance Characteristics ............................................. 9
Serial Interface ................................................................................ 32
Terminology .................................................................................... 12
SPI Interface ................................................................................ 32
Theory of Operation ...................................................................... 13
I2C-Compatible Interface .......................................................... 34
Touch Screen Principles ............................................................ 13
Grounding and Layout .................................................................. 37
Measuring Touch Screen Inputs ............................................... 14
Lead Frame Chip Scale Packages ............................................. 37
Touch-Pressure Measurement .................................................. 15
Outline Dimensions ....................................................................... 38
Temperature Measurement ....................................................... 15
Ordering Guide .......................................................................... 39
Median and Averaging Filters ....................................................... 17
Automotive Products ................................................................. 39
AUX/VBAT/GPIO Pin ................................................................... 18
Auxiliary Input ............................................................................ 18
REVISION HISTORY
12/11—Revision 0: Initial Version
Rev. 0 | Page 2 of 40
Data Sheet
AD7879W
SPECIFICATIONS
VCC = 1.6 V to 3.6 V, TA = −40°C to +85°C, unless otherwise noted.
Table 1.
Parameter
DC ACCURACY
Resolution
No Missing Codes
Integral Nonlinearity (INL)1
Differential Nonlinearity (DNL)1
Negative DNL
Positive DNL
Offset Error 1, 2
Gain Error1, 2
Noise 3
Power Supply Rejection3
Internal Clock Frequency
Internal Clock Accuracy
SWITCH DRIVERS
On Resistance1
Y+, X+
Y−, X−
ANALOG INPUTS
Input Voltage Range
DC Leakage Current
Input Capacitance
Accuracy
TEMPERATURE MEASUREMENT
Temperature Range
Resolution
Accuracy2
BATTERY MONITOR
Input Voltage Range
Input Impedance3
Accuracy
LOGIC INPUTS (DIN, SCL, CS, SDA, GPIO)
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance, CIN3
LOGIC OUTPUTS (DOUT, GPIO, SCL, SDA, INT)
Output High Voltage, VOH
Output Low Voltage, VOL
Floating-State Leakage Current
Floating-State Output Capacitance2
CONVERSION RATE3
Conversion Time
Throughput Rate
Min
Typ
12
11
12
±2
Max
Unit
±3
Bits
Bits
LSB
−0.99
2
±6
±4
70
60
2
1.8
2.2
6
5
Test Conditions/Comments
LSB size = 390 µV.
LSB size = 390 µV.
LSB
LSB
LSB
LSB
µV rms
dB
MHz
MHz
Ω
Ω
0
VCC
V
µA
pF
%
+85
°C
°C
°C
Calibrated at 25°C.
V
kΩ
%
Uncalibrated accuracy.
±0.1
30
0.3
−40
0.3
±2
0.5
5
16
2
5
0.7 × VCC
0.3 × VCC
0.01
10
VCC − 0.2
V
V
µA
pF
±0.1
5
V
V
µA
pF
9.5
µs
105
kSPS
0.4
Rev. 0 | Page 3 of 40
VIN = 0 V or VCC.
Including 2 µs of acquisition time, MAV
filter off. 2 µs of additional time is required
if MAV filter is on.
AD7879W
Data Sheet
Parameter
POWER REQUIREMENTS
VCC
ICC
Converting Mode
Static
Min
Typ
Max
Unit
Test Conditions/Comments
1.6
2.6
3.6
V
480
406
650
µA
µA
0.5
6
µA
Specified performance.
Digital inputs = 0 V or VCC.
ADC on, PM = 10.
ADC and temperature sensor are off; the
reference and oscillator are on; PM = 01
or 11.
PM = 00.
Shutdown Mode
1
2
3
See the Terminology section.
Guaranteed by characterization; not production tested.
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 tR = tF = 5 ns (10% to 90% of VCC) and timed from a voltage level of 1.4 V.
Table 2.
Parameter 1
fSCL
t1
t2
t3
t4
t5
t6
t7
t8
Unit
MHz max
ns min
ns min
ns min
ns min
ns min
ns max
ns max
ns min
Description
CS falling edge to first SCL falling edge
SCL high pulse width
SCL low pulse width
DIN setup time
DIN hold time
DOUT access time after SCL falling edge
CS rising edge to DOUT high impedance
SCL rising edge to CS high
Guaranteed by design; not production tested.
CS
t1
t2
1
SCL
t8
t3
2
3
15
16
1
2
15
16
t4
t5
DIN
LSB
MSB
t6
DOUT
MSB
Figure 2. Detailed SPI Timing Diagram
Rev. 0 | Page 4 of 40
t7
LSB
10408-002
1
Limit
5
5
20
20
15
15
20
16
15
Data Sheet
AD7879W
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
fSCL
t1
t2
t3
t4
t5
t6
t7
t8
tR
tF
Unit
kHz max
μs min
μs min
μs min
ns min
ns min
μs min
μs min
μs min
ns max
ns max
Description
Start condition hold time, tHD; STA
Clock low period, tLOW
Clock high period, tHIGH
Data setup time, tSU; DAT
Data hold time, tHD; DAT
Stop condition setup time, tSU; STO
Start condition setup time, tSU; STA
Bus-free time between stop and start conditions, tBUF
Clock/data rise time
Clock/data fall time
Guaranteed by design; not production tested.
tR
t2
tF
t1
SCL
t3
t1
t5
t7
t6
t4
SDA
t8
STOP START
START
Figure 3. Detailed I2C Timing Diagram
Rev. 0 | Page 5 of 40
STOP
10408-003
1
Limit
400
0.6
1.3
0.6
100
300
0.6
0.6
1.3
300
300
AD7879W
Data Sheet
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
THERMAL RESISTANCE
Table 4.
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
1
Rating
−0.3 V to +3.6 V
−0.3 V to VCC + 0.3 V
−0.3 V to +5 V
−0.3 V to VCC + 0.3 V
−0.3 V to VCC + 0.3 V
10 mA
Table 5. Thermal Resistance
Package Type1
16-Lead TSSOP
16-Lead LFCSP
1
θJA
112.6
30.4
Unit
°C/W
°C/W
4-layer board.
200µA
15 kV
10 kV
TO OUTPUT
PIN
4 kV
1 kV
0.2 kV
−40°C to +85°C
−65°C to +150°C
150°C
IOL
1.4V
CL
50pF
200µA
IOH
Figure 4. Circuit Used for Digital Timing
ESD CAUTION
577.2 mW
2.138 W
260°C (±0.5°C)
300°C
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; 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
10408-004
Parameter
VCC to GND
Analog Input Voltage to GND
AUX/VBAT to GND
Digital Input Voltage to GND
Digital Output Voltage to GND
Input Current to Any Pin Except Supplies1
ESD Rating (X+, Y+, X−, Y−)
Air Discharge Human Body Model
Contact Human Body Model
ESD Rating (All Other Pins)
Human Body Discharge
Field-Induced Charged Device Model
Machine Model
Operating Temperature Range
Storage Temperature Range
Junction Temperature
Power Dissipation
TSSOP (4-Layer Board)
LFCSP (4-Layer Board)
IR Reflow Peak Temperature
Lead Temperature (Soldering 10 sec)
Data Sheet
AD7879W
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
16 ADD0
VCC/REF 1
VCC/REF 1
16 CS
2
15 NC
NC
X+ 3
AD7879W
14 AUX/VBAT/GPIO
TOP VIEW
(Not to Scale)
13 PENIRQ/INT/DAV
15 NC
2
AD7879W
14 AUX/VBAT/GPIO
Y+ 4
TOP VIEW
(Not to Scale)
13 PENIRQ/INT/DAV
Y+ 4
X– 5
12 DOUT
X– 5
Y– 6
11 SCL
Y– 6
11 SCL
NC
10 NC
NC
7
10 NC
ADD1
8
9
7
DIN 8
9
GND
NC = NO CONNECT
10408-005
X+ 3
12 SDA
NC = NO CONNECT
Figure 5. AD7879W TSSOP Pin Configuration
GND
10408-006
NC
Figure 6. AD7879-1W TSSOP Pin Configuration
Table 6. Pin Function Descriptions, TSSOP
Pin No.
AD7879W AD7879-1W
1
1
2, 7, 10, 15 2, 7, 10, 15
3
3
4
4
5
5
6
6
8
N/A
N/A
8
Mnemonic
VCC/REF
NC
X+
Y+
X−
Y−
DIN
ADD1
9
9
GND
11
12
N/A
13
11
N/A
12
13
SCL
DOUT
SDA
PENIRQ/INT/
DAV
14
14
AUX/VBAT/GPIO
16
N/A
N/A
16
CS
ADD0
Description
Power Supply Input and ADC Reference.
No Connect.
Touch Screen Input Channel.
Touch Screen Input Channel.
Touch Screen Input Channel.
Touch Screen Input Channel.
SPI Serial Data Input to the AD7879W.
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).
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.
Serial Interface Clock Input.
SPI Serial Data Output for the AD7879W.
I2C Serial Data Input and Output for the AD7879-1W.
Interrupt Output. This pin is asserted when the screen is touched (PENIRQ), when a measurement exceeds the preprogrammed limits (INT), or when new data is available in the registers
(DAV). Active low, internal 50 kΩ pull-up resistor.
This pin can be programmed as an auxiliary input to the ADC (AUX), as a battery measurement input to the ADC (VBAT), or as a general-purpose digital input/output (GPIO).
Chip Select for the SPI Serial Interface on the AD7879W. Active low.
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
PIN 1
INDICATOR
NC 2
NC 3
14 ADD0
13 AUX/VBAT/GPIO
AD7879-1W
11 NC
TOP VIEW
(Not to Scale)
10 NC
NC 3
10 NC
9
X– 4
TOP VIEW
(Not to Scale)
9
SDA
NOTES
1. NC = NO CONNECT
2. 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.
10408-007
NOTES
1. NC = NO CONNECT
2. 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.
12 PENIRQ/INT/DAV
SCL 8
Y– 5
SCL 8
DOUT
GND 7
NC 2
ADD1 6
11 NC
GND 7
AD7879W
Y– 5
Y+ 1
DIN 6
X– 4
PIN 1
INDICATOR
12 PENIRQ/INT/DAV
Figure 7. AD7879W LFCSP Pin Configuration
10408-008
Y+ 1
16 X+
15 VCC/REF
13 AUX/VBAT/GPIO
Data Sheet
16 X+
15 VCC/REF
14 CS
AD7879W
Figure 8. AD7879-1W LFCSP Pin Configuration
Table 7. Pin Function Descriptions, LFCSP
Pin No.
AD7879W AD7879-1W
1
1
2, 3, 10, 11
2, 3, 10, 11
4
4
5
5
6
N/A
N/A
6
Mnemonic
Y+
NC
X−
Y−
DIN
ADD1
7
7
GND
8
9
N/A
12
8
N/A
9
12
SCL
DOUT
SDA
PENIRQ/INT/DAV
13
13
AUX/VBAT/GPIO
14
N/A
N/A
14
CS
ADD0
15
16
15
16
VCC/REF
X+
EP
Description
Touch Screen Input Channel.
No Connect.
Touch Screen Input Channel.
Touch Screen Input Channel.
SPI Serial Data Input to the AD7879W.
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).
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.
Serial Interface Clock Input.
SPI Serial Data Output for the AD7879W.
I2C Serial Data Input and Output for the AD7879-1W.
Interrupt Output. This pin is asserted when the screen is touched (PENIRQ), when a measurement exceeds the preprogrammed limits (INT), or when new data is available in the registers
(DAV). Active low, internal 50 kΩ pull-up resistor.
This pin can be programmed as an auxiliary input to the ADC (AUX), as a battery measurement input to the ADC (VBAT), or as a general-purpose digital input/output (GPIO).
Chip Select for the SPI Serial Interface on the AD7879W. Active low.
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).
Power Supply Input and ADC Reference.
Touch Screen Input Channel.
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
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, VCC = 2.6 V, fSCL = 2 MHz, unless otherwise noted.
1.0
470
0.8
465
0.6
GAIN ERROR VARIATION (LSB)
475
455
450
445
440
435
430
0.4
0.2
2.6V
0
3.6V
–0.2
–0.4
1.6V
–0.6
–0.8
–40
–25
–10
10
25
40
TEMPERATURE (°C)
55
70
85
–1.0
10408-009
425
Figure 9. Supply Current vs. Temperature
–40
–25
–10
10
25
40
TEMPERATURE (°C)
55
70
85
10408-012
CURRENT (µA)
460
Figure 12. Change in ADC Gain vs. Temperature
700
1.0
0.8
600
OFFSET VARIATION (LSB)
0.6
CURRENT (µA)
500
400
300
200
0.4
1.6V
0.2
2.6V
0
–0.2
3.6V
–0.4
–0.6
100
1.8
2.0
2.2
2.4
2.6 2.8
VCC (V)
3.0
3.2
3.4
3.6
–1.0
3.5
1.5
3.0
1.0
2.5
0.5
INL (LSB)
2.0
2.0
–0.5
1.0
–1.0
0.5
–1.5
0
–10
10
25
50
TEMPERATURE (°C)
–10
10
25
40
TEMPERATURE (°C)
55
70
85
0
1.5
75
100
–2.0
10408-011
CURRENT (µA)
4.0
–25
–25
Figure 13. Change in ADC Offset vs. Temperature
Figure 10. Supply Current vs. VCC
–40
–40
0
512
1024
1536
2048
CODE
2560
Figure 14. ADC INL
Figure 11. Full Power-Down IDD vs. Temperature
Rev. 0 | Page 9 of 40
3072
3584
4096
10408-014
1.6
10408-010
0
10408-013
–0.8
AD7879W
Data Sheet
1.0
6.0
0.8
5.5
0.6
0.4
RON (Ω)
0
–0.2
4.5
4.0
–0.4
–0.6
X+ TO VCC
Y+ TO VCC
X– TO GND
Y– TO GND
3.5
–0.8
1
501
1001
1501
2001 2501
CODE
3001
3501
4001
3.0
10408-015
–1.0
–40
Figure 15. ADC DNL
–25
–10
10
25
40
TEMPERATURE (°C)
55
70
85
10408-017
DNL (LSB)
5.0
0.2
Figure 17. Switch On Resistance vs. Temperature
(X+, Y+: Pin to VCC; X−, Y−: Pin to GND)
7
2370
2369
6
2368
ADC CODE (Decimal)
4
3
X+ TO VCC
Y+ TO VCC
X– TO GND
Y– TO GND
2
2367
2366
2365
2364
2363
2362
1
1.6
1.8
2.0
2.2
2.4
2.6 2.8
VCC (V)
3.0
3.2
3.4
3.6
2360
–40 –25 –15 –5
5 15 25 35 45
TEMPERATURE (°C)
55
65
75
85
Figure 18. ADC Code vs. Temperature (Fixed Analog Input)
Figure 16. Switch On Resistance vs. VCC
(X+, Y+: Pin to VCC; X−, Y−: Pin to GND)
Rev. 0 | Page 10 of 40
10408-018
2361
0
10408-016
RON (Ω)
5
Data Sheet
AD7879W
1400
MEAN: –1.98893
SD: 0.475534
250
1000
NUMBER OF UNITS
TEMPERATURE (Code)
1200
800
600
200
150
100
400
50
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
VCC (V)
10408-019
0
0
INPUT TONE AMPLITUDE (dB)
SNR = 61.58dB
THD = 72.34dB
–40
–60
–80
–100
–120
10408-020
0
1603
3206
4809
6412
8015
9618
11221
12824
14427
16030
17633
19236
20839
22442
24045
25648
27251
28854
30457
32060
33663
35266
36869
–140
FREQUENCY (Hz)
–2
ERROR (%)
0
–160
–3
–1
0
Figure 21. Typical Uncalibrated Accuracy for the Battery Channel (25°C)
Figure 19. Temperature Code vs. VCC for 25°C
–20
–4
10408-021
200
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 (VREF − 1 LSB)
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.
Rev. 0 | Page 12 of 40
Data Sheet
AD7879W
THEORY OF OPERATION
CONDUCTIVE ELECTRODE
ON BOTTOM SIDE
Y+
The core of the AD7879W is a high speed, low power, 12-bit
analog-to-digital converter (ADC) with an input multiplexer,
on-chip track-and-hold, and on-chip clock. Conversion results
are stored in on-chip result registers. The results from the
auxiliary input or the battery input can be compared with high
and low limits stored in limit registers to generate an out-oflimit interrupt (INT).
The AD7879W also contains low resistance analog switches to
switch the X and Y excitation voltages to the touch screen and
to the on-chip temperature sensor. The high speed SPI serial
bus provides control of the devices, as well as communication
with the devices. The AD7879-1W is available with an I2C
interface.
Operating from a single supply from 1.6 V to 3.6 V, the AD7879W
offers a throughput rate of 105 kHz. The device is available in a
4.4 mm × 5.0 mm, 16-lead thin shrink small outline package
(TSSOP) and in a 4 mm × 4 mm, 16-lead lead frame chip scale
package (LFCSP).
The AD7879W has an on-chip sequencer that schedules a
sequence of preprogrammed conversions. The conversion
sequence starts automatically when the screen is touched or
at preset intervals, using the on-board timer.
To ensure that the AD7879W works well with different touch
screens, the user can select the acquisition time. A programmable delay ensures that the voltage on the touch screen settles
before a measurement is taken.
To help reduce noise in the system, the ADC takes up to 16
conversion results from each channel and writes the average of
the results to the register. To further improve the performance
of the AD7879W, the median filter can also be used if there is
noise present in the system.
PLASTIC FILM WITH
TRANSPARENT, RESISTIVE
COATING ON BOTTOM SIDE
X–
Y–
X+
CONDUCTIVE ELECTRODE
ON TOP SIDE
PLASTIC FILM WITH
TRANSPARENT, RESISTIVE
COATING ON TOP SIDE
LCD SCREEN
Figure 22. Basic Construction of a Touch Screen
The Y layer has conductive electrodes running along the top
and bottom edges, allowing the application of an excitation
voltage down the Y 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 layer electrodes. By switching alternately
between X and Y excitation and measuring the voltages, the
X and Y coordinates of the contact point can be determined.
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 AD7879W is
designed to facilitate this measurement.
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 (see Figure 22). 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.
Rev. 0 | Page 13 of 40
10408-022
The AD7879W is a complete 12-bit data acquisition system for
digitizing positional inputs from a 4-wire resistive touch screen.
To support this function, data acquisition on the AD7879W is
highly programmable to ensure accurate and noise-free results
from the touch screen.
AD7879W
Data Sheet
Figure 23 shows an equivalent circuit of the analog input structure
of the AD7879W, showing the touch screen switches, the main
analog multiplexer, the ADC, and the dual 3-to-1 multiplexer
that selects the reference source for the ADC.
The voltage seen at the input to the ADC in Figure 24 is
VIN = VCC ×
RY −
(1)
RYTOTAL
The advantage of the single-ended method is that the touch
screen excitation voltage is switched off when the signal is
acquired. Because a screen can draw over 1 mA, this is a
significant consideration for a battery-powered system.
VCC
X+
X–
Y+
Y–
X– Y– GND X+ Y+ VCC
INPUT
MUX
DUAL 3-TO-1 MUX
AUX/VBAT/GPIO
IN+
REF+
12-BIT SUCCESSIVE
APPROXIMATION ADC
WITH TRACK-AND-HOLD
10408-023
REF–
TEMPERATURE
SENSOR
Figure 23. Analog Input Structure
The AD7879W can be set up to automatically convert either
specific input channels or a sequence of channels. The results of
the ADC conversions are stored in the result registers.
When measuring the ancillary analog inputs (AUX, TEMP, or
VBAT), the ADC uses a VCC reference and the measurement is
referred to GND.
The disadvantage of the single-ended method is that voltage
drops across the switches can introduce errors. Touch screens
can have a total end-to-end resistance ranging from 200 Ω to
900 Ω. By taking the lowest screen resistance of 200 Ω and a
typical switch resistance of 14 Ω, the user can reduce the apparent
excitation voltage to 200/228 × 100 = 87% of its actual value. In
addition, the voltage drop across the low-side switch adds to the
ADC input voltage. This introduces an offset into the input
voltage; thus, it can never reach 0.
Ratiometric Method
The ratiometric method illustrated in Figure 25 shows the
negative input of the ADC reference connected to Y− and the
positive input connected to Y+. Thus, the screen excitation
voltage provides the reference for the ADC. The input of the
ADC is connected to X+ to determine the Y position.
VCC
MEASURING TOUCH SCREEN INPUTS
Y+
When measuring the touch screen inputs, it is possible to use
VCC as a reference or instead to use the touch screen excitation
voltage as the reference and to perform a ratiometric, differential
measurement. The differential method is the default method
and is selected by clearing the SER/DFR bit (Bit 9 in Control
Register 2) to 0. The single-ended method is selected by setting
this bit to 1.
X+
TOUCH
SCREEN
Y+
X+
VREF
INPUT
(VIA MUX)
REF+
REF–
10408-025
For greater accuracy, the ratiometric method has two significant
advantages. One is that the reference to the ADC is provided
from the actual voltage across the screen; therefore, any voltage
dropped across the switches has no effect. The other advantage
is that because the measurement is ratiometric, it does not
matter if the voltage across the screen varies in the long term.
However, it must not change after the signal has been acquired.
10408-024
Y–
GND
Figure 25. Ratiometric Conversion of Touch Screen Inputs
The disadvantage of the ratiometric method is that the screen
must be powered up at all times because it provides the reference
voltage for the ADC.
ADC
TOUCH
SCREEN
REF–
Y–
GND
VCC
REF+
ADC
Single-Ended Method
Figure 24 illustrates the single-ended method for the Y position.
For the X position, the excitation voltage is applied to X+ and
X− and the voltage is measured at Y+.
INPUT
(VIA MUX)
Figure 24. Single-Ended Conversion of Touch Screen Inputs
Rev. 0 | Page 14 of 40
Data Sheet
AD7879W
TOUCH-PRESSURE MEASUREMENT
Second Method
The pressure applied to the touch screen by a pen or finger can
also be measured with the AD7879W using some simple
calculations. The contact resistance between the X and Y plates
is measured, providing a good indication of the size of the
depressed area and, therefore, the applied pressure. The area of
the spot that is touched is proportional to the size of the object
touching it. The size of this resistance (RTOUCH) can be calculated
using two different methods.
The second method requires the user to know the resistance of
the X-plate and Y-plate tablets. Three touch screen conversions
are required: a measurement of the X position (XPOSITION), the
Y position (YPOSITION), and the Z1 position.
The following equation also calculates the touch resistance
(RTOUCH):
RTOUCH = RXPLATE × (XPOSITION/4096) × [(4096/Z1) − 1] −
RYPLATE × [1 − (YPOSITION/4096)]
First Method
The first method requires the user to know the total resistance
of the X-plate tablet (RX). Three touch screen conversions are
required: measurement of the X position, XPOSITION (Y+ input);
measurement of the X+ input with the excitation voltage applied
to Y+ and X− (Z1 measurement); and measurement of the Y−
input with the excitation voltage applied to Y+ and X− (Z2
measurement). These three measurements are illustrated in
Figure 26.
The AD7879W has two special ADC channel settings that
configure the X and Y switches for the Z1 and Z2 measurements and store the results in the Z1 and Z2 result registers. The
Z1 measurement is selected by setting the CHNL ADD[2:0] bits
to 101 in Control Register 1 (Address 0x01); the result is stored
in the X+ (Z1) result register (Address 0x0A). The Z2 measurement
is selected by setting the CHNL ADD[2:0] bits to 100 in Control
Register 1 (Address 0x01); the result is stored in the Y− (Z2)
result register (Address 0x0B).
The touch resistance (RTOUCH) can then be calculated using the
following equation:
RTOUCH = (RXPLATE) × (XPOSITION/4096) × [(Z2/Z1) − 1]
MEASURE
X POSITION
X+
Y+
Y–
Y+
X+
TEMPERATURE MEASUREMENT
A temperature measurement option called the single-conversion
method is available on the AD7879W. The conversion method
requires only a single measurement on ADC Channel 001. The
results are stored in the temperature conversion result register
(Address 0x0D). The AD7879W does not provide an explicit
output of the temperature reading; the system must perform
some external calculations. This method is based on an on-chip
diode measurement.
The acquisition time is fixed at 16 ms for temperature
measurement.
Conversion Method
The conversion method makes use of the fact that the temperature coefficient of a silicon diode is approximately −2.1 mV/°C.
However, this small change is superimposed on the diode forward
voltage, which can have a wide tolerance. Therefore, it is necessary
to calibrate by measuring the diode voltage at a known temperature
to provide a baseline from which the change in forward voltage
with temperature can be measured. This method provides a
resolution of approximately 0.3°C and a predicted accuracy
of ±2°C.
The temperature limit comparison is performed on the result
in the temperature conversion result register (Address 0x0D),
which is the measurement of the diode forward voltage. The
values programmed into the high and low limits should be
referenced to the calibrated diode forward voltage to make
accurate limit comparisons.
TOUCH
RESISTANCE
X–
(2)
MEASURE
Z1 POSITION
TOUCH
RESISTANCE
Y–
X–
Y+
X+
TOUCH
RESISTANCE
X–
MEASURE
Z2 POSITION
10408-026
Y–
(3)
Figure 26. Three Measurements Required for Touch Pressure
Rev. 0 | Page 15 of 40
AD7879W
Data Sheet
Temperature Calculations
Example
If an explicit temperature reading in degrees Celsius is required,
calculate for the single-measurement method as follows:
Using VCC = 2.5 V as reference,
1.
Calculate the scale factor of the ADC in degrees per LSB.
Degrees per LSB = ADC LSB size/−2.1 mV =
(VCC/4096)/−2.1 mV
2.
3.
4.
Degrees per LSB = (2.5/4096)/−2.1 × 10−3 = −0.291
The ADC output is 983 decimal at 25°C, equivalent to a diode
forward voltage of 0.6 V.
The ADC output at TAMB is 880.
Save the ADC output, DCAL, at the calibration temperature,
TCAL.
Take the ADC reading, DAMB, at the temperature to be
measured, TAMB.
Calculate the difference in degrees between TCAL and TAMB by
∆T = (DAMB − DCAL) × degrees per LSB
5.
Add ∆T to TCAL.
Rev. 0 | Page 16 of 40
∆T = (880 − 983) × −0.291 = 30°C
TAMB = 25 + 30 = 55°C
Data Sheet
AD7879W
MEDIAN AND AVERAGING FILTERS
As explained in the Touch Screen Principles section, touch
screens are composed of two resistive layers, normally placed
over an LCD screen. Because these layers are in close proximity
to the LCD screen, noise can be coupled from the screen onto
these resistive layers, causing errors in the touch screen
positional measurements.
When both filter values are 00, only one measurement is
transferred to the register map.
The AD7879W contains a filtering block to process the data
and discard the spurious noise before sending the information
to the host. The purpose of this block is not only the
suppression of noise; the on-chip filtering also greatly reduces
the host processing loading.
Table 10. Median Averaging Filters (MAVF) Settings
The processing function consists of two filters that are applied
to the converted results: the median filter and the averaging filter.
The median filter suppresses the isolated out-of-range noise and
sets the number of measurements to be taken. These measurements
are arranged in a temporary array, where the first value is the
smallest measurement and the last value is the largest measurement. Bit 6 and Bit 5 in Control Register 2 (MED1, MED0) set
the window of the median filter and, therefore, the number of
measurements taken.
M>A
M
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