ANALOG DEVICES AD7877 Service Manual

A

3

A

2

A

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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 × 5 mm package
25-ball,WLCSP, 2.5 mm × 2.8 mm package

APPLICATIONS

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

of 2.7 V to 5.25 V

CC

level for serial interface

DRIVE

UX1/GPIO1

UX2/GPIO

UX3/GPIO

BAT1

BAT2

V

AOUT

ARNG

FUNCTIONAL BLOCK DIAGRAM

V

CC

7

12

X+

10

X–

13

Y+

11

Y–

6

5

4

3

BATTERY
MONITOR

2

BATTERY
MONITOR

TEMPERATURE

SENSOR

31

REF

2.5V
REF

30

8-BIT

DAC

29

CONTROL LOGIC AND SERIAL PORT

19

18

DIN26DCLK27DOUT28V

CS23DAV

9 TO 1

BUF

DAC

REGISTER

MUX

I/P

X–

Y– GND X+ Y+ V

12-BIT SUCCESSIVE

APPROXIMATION ADC

WITH TRACK-AND-HOLD

SEQUENCER

REGISTERS

REGISTERS

ALERT STATUS/

MASK REGISTER

REGISTERS

REGISTERS

DRIVE

Figure 1.

RESULTS

CONTROL

DUAL 3-1

MUX

REF–INREF+

COMPARATOR

LIMIT

GPIO

PEN INTERRUPT

AND WAKE-UP

ON TOUCH

AD7877

AD7877

REF

CLOCK

STOP

ACQ

ADC DATA

LOGIC

LIMIT

ALERT
LOGIC

TO

GPIO1-3

20

14

15

22

21

17

STOPACQ

AGND

DGND

ALERT

GPIO4

PENIRQ

03796-001

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 2.2 V), 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

e, it can be shut down to conserve power. An external

in us
reference can also be applied and varied from 1 V to +V
an analog input range of 0 V to V

. The device includes a

REF

, with

CC

shutdown mode that reduces its current consumption to less
than 1 μA.

Rev. B

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her
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.

To reduce the effects of noise from LCDs, the acquisition phase

the on-board ADC is controlled via the STOPACQ pin. User-

of
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 runs 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

P), and a 25-ball wafer level chip scale package (WLCSP).

(LFCS

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] ©2006 Analog Devices, Inc. All rights reserved.

AD7877

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TABLE OF CONTENTS

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

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

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

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

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

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

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

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

Timing Diagrams.......................................................................... 5

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

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

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

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

Terminology .................................................................................... 13

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

Touch Screen Principles ............................................................14

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

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

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

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

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

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

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

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

Writing Data ............................................................................... 28

Write Timing............................................................................... 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

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

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

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

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

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

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

REVISION HISTORY

6/06—Changed from Rev. A to Rev. B

Added Wafer Level Chip Scale Package...........................Universal

Changes to Table 3............................................................................ 6

Changes to Figure 21...................................................................... 11

Change to Figure 25 ....................................................................... 12

Changes to Figure 38 and Figure 39............................................. 23

Change to Figure 40 ....................................................................... 24

Changes to Data Available Output (

Updated Outline Dimensions....................................................... 42

Changes to Ordering Guide.......................................................... 42

DAV

) Section ....................24

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

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

GPIO Registers ........................................................................... 40

Outline Dimensions....................................................................... 42

Ordering Guide .......................................................................... 42

11/04—Changed from Rev. 0 to Rev. A

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

C

hanges 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

Rev. B | Page 2 of 44

AD7877

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SPECIFICATIONS

VCC = 2.7 V to 3.6 V, V

Table 1.

Parameter Min Typ Max Unit Test Conditions/Comments

DC ACCURACY

Resolution 12 Bits
No Missing Codes 11 12 Bits
Integral Nonlinearity
Differential Nonlinearity (DNL)1 Minimum LSB size = 610 μV

Negative DNL −0.99 LSB

Positive DNL +2 LSB
Offset Error1 ±2 ±6 LSB VCC = 2.7 V
Gain Error1 ±4 LSB External reference
Noise 70 μV rms
Power Supply Rejection 70 dB
Internal Clock Frequency 2 MHz

SWITCH DRIVERS

On Resistance

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 1.6 °C

Single Conversion Method3 0.3 °C
Accuracy

Differential Method2 ±4 °C 0°C to 70°C

Single Conversion Method3 ±2 °C Calibrated at 25°C

BATTERY MONITOR

Input Voltage Range 0.5 5 V @ V
Input Impedance 14 kΩ Sampling, 1 GΩ when the battery monitor is off
Accuracy 1 3.2 % External/internal reference, see Figure 26

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 − VCC

Slew Rate −0.4, +0.5 V/μs

Output Settling Time 12 15 μs 0 to 3/4 scale, R

1

= 2.5 V internal or external, f

REF

1

±2 LSB LSB size = 610 μV

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

DCLK

V

REF

CS

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

reference is enabled

= 2.5 V

REF

Rev. B | Page 3 of 44

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

= 10 kΩ, C

LOAD

REF

= 50 pF

LOAD

AD7877

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Parameter Min Typ Max Unit Test Conditions/Comments

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

INL

V

DRIVE

0.3
V

DRIVE

V

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

V

CC

ICC Digital I/Ps = 0 V or VCC

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 Terminology section.

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.

= 10 kΩ

LOAD

CC

= 250 μA, VCC/V

SOURCE

= 250 μA

SINK

high to DAV low

CS

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

RNG

Rev. B | Page 4 of 44

AD7877

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TIMING SPECIFICATIONS

TA = T

signals are specified with t

Table 2.

Parameter Limit at T

f

DCLK

20 MHz max

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 Figure 3 and defined as the time required for the output to cross 0.4 V or 2.0 V.

3

t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit shown in Figure 3. The measured number is then

extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t8, quoted in the timing characteristics is the true bus
relinquish time of the part and is independent of the bus loading.

to T

MIN

1

16 ns min

, unless otherwise 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

, T

MIN

Unit Description

MAX

10 kHz min

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

REF

falling edge to first DCLK rising edge

CS

20 ns min DCLK high pulse width

16 ns max

falling edge to DOUT, three-state disabled

CS

16 ns max DCLK falling edge to DOUT valid

16 ns max

rising edge to DOUT high impedance

CS

rising edge to DCLK ignored

CS

TIMING DIAGRAMS

CS

t

DCLK

DIN

DOUT

1

1 2 3 15 16

MSB LSB

t

6

MSB LSB

t

2

t

4

Figure 2. Detailed Timing Diagram

TO OUTPUT

PIN

Figure 3. Load Circuit for Digital Out

t

3

t

5

t

7

200μAI

C

L

50pF

200μAI

t

9

t

8

OL

1.6V

OH

03796-003

put Timing Specifications

03796-004

Rev. B | Page 5 of 44

AD7877

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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 (IEC 1000-4-2, Air Discharge)

Tablet Pins (X+, X−, Y+, Y−) 4 kV
Other Pins 2 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 sec) 300°C

1

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

Stresses above those listed under Absolute Maximum Ratings
ma

y 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.

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. B | Page 6 of 44

AD7877

A

A

A

E

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PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

PIN 1

REF

NC

V

32 31 30 29 28 27 26 25

1

NC

2

BAT2

3

BAT1

UX3/GPIO3

UX2/GPIO2

UX1/GPIO1

NC = NO CONNECT

4

5

6

7

V

CC

8

NC

9 10 11 12 13 14 15 16

NC

X–

Figure 4. LFCSP Pin Configuration

Table 4. Pin Function Descriptions

Pin No. Mnemonic Description

1, 8, 9, 16,

NC No Connect.

24, 25, 32
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

Power Supply Input. The V

CC

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

15 DGND

17
18

PENIRQ

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

CS

19 DIN

20 STOPACQ

21 GPIO4 Dedicated General-Purpose Logic Input/Output Pin.
22

23

Digital Active Low Output. Interrupt output that goes low if a GPIO data bit is set, or if the AUX1, TEMP1, BAT1,

ALERT

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

DAV

26 DCLK External Clock Input. Logic input. DCLK provides the serial clock for accessing data from the part.
27 DOUT

28 V

DRIVE

DRIV

AOUT

ARNG

V

DOUT

DCLK

NC

DCLK

DOUT

V

ARNG

V

REF

BAT1

V

CC

Y–

AOUT

BAT2

AUX3/
GPI03

AUX1/
GPI01

X–

03796-051

24

NC

23

DAV

22

DGND

NC

ALERT

21

GPIO4

20

STOPACQ

19

DIN

18

CS

17

PENIRQ

03796-002

range for the AD7877 is from 2.2 V to 5.25 V.

CC

AD7877

TOP VIEW

(Not to Scale)

Y–

X+

Y+

AGND

Analog Ground. Ground reference point f

DRIVE

ALERT

STOP

ACQ

CS

DGND

DAV

GPIO4

AUX2/

DIN

GPI02

PENIRQ

AGND

Y+

X+

AD7877 WLCSP

TOP VIEW

Not to Scal e

Figure 5. WLCSP Pin Configuration

or all analog circuitry on the AD7877. All analog input signals and any

external reference signal should be referred to this voltage.
Digital Ground. Ground reference for all digital circuitry on the AD7877. Refer all digital input signals to this

ltage.

vo
Pen Interrupt. Digital active low output (has a 50 kΩ in

ternal pull-up resistor).

AD7877 and enabling the serial input/output register.
SPI® Serial Data Input. Data to be written to the AD7877 r

egisters are provided on this input and clocked into

the register on the rising edge of DCLK.
Stop Acquisition Pin. A signal applied to this pin can be monit

ored 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.

or BAT2 measurements are out of range.

registers. This output is high impedance when CS

Serial Data Output. Logic output. The conversion result fr

is high.

om 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
when CS

is high.

Logic Power Supply Input. The voltage supplied at this pin determines the operating voltage for the serial
interface of the AD7877.

Rev. B | Page 7 of 44

AD7877

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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 referenc
device. The reference output must be buffered before it is applied elsewhere in a system. To reduce system
noise effects, it is strongly recommended to place a capacitor of 100 nF between the V
Alternatively, an external reference can be applied to this input. The voltage range for the external reference is

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

e is available on this pin for use external to the

pin and GND.

REF

Rev. B | Page 8 of 44

AD7877

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TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C, VCC = 2.7 V, V

800

= 2.5 V, f

REF

SAMPLE

= 125 kHz, f

DCLK

= 16 × f

= 2 MHz, unless otherwise noted.

SAMPLE

200

ADC, REF, AND DAC

700

ADC AND REF

CURRENT (μA)

600

500

–50 –30 –10 0 30 50 70 90

TEMPERATURE (°C)

Figure 6. 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 7. Supply Current vs. V

ADC, REF, AND DAC

ADC AND REF

VCC (V)

CC

03796-030

03796-031

180

160

140

CURRENT (nA)

120

100

80

–50–30–101030507090

Figure 9. Full Power-Down I

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

Figure 10. Change in ADC Offset v

TEMPERATURE (°C)

vs. Temperature

DD

TEMPERATURE (°C)

s. Temperature

03796-032

03796-040

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)

03796-039

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

Figure 8. Change in ADC Gain vs. Temperature

Rev. B | Page 9 of 44

CODE

Figure 11. ACD INL Plot

03796-044

AD7877

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1.0

0.8

0.6

0.4

0.2

0

–0.2

DNL (LSB)

–0.4

–0.6

–0.8

–1.0

0 500 1000 1500 2000 2500 3000 3500 4000

CODE

Figure 12. ADC DNL Plot

03796-045

16

14

12

10

8

6

4

REFERENCE CURRENT (μA)

2

0

–50 9070503010–10–30

TEMPERATURE (°C)

Figure 15. External Reference Current vs. Temperature

03796-046

22

20

18

16

(Ω)

ON

14

R

12

10

8

2.73.13.53.94.34.75.15.5

X+ TO V

Y+ TO V

CC

VCC (V)

Figure 13. 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

CC

CC

X– TO GND

Y– TO GND

X– TO GND

Y– TO GND

CC

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 16. 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 –20 0 20 40 60 80

CC

TEMPERATURE ( °C)

Figure 14. Switch On Resistance vs. Temperature

(X+, Y

+: V

to Pin; X−, Y−: Pin to GND)

CC

03796-049

2.498

2.496

2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5.0

Figure 17. Internal V

Rev. B | Page 10 of 44

VCC (V)

REF

vs. V

03796-034

CC

AD7877

www.BDTIC.com/ADI

3145

3135

3125

3115

3105

3095

3085

3075

ADC CODE (Decimal)

3065

3055

3045

–50–30–101030507090

TEMPERATURE (°C)

Figure 18. ADC Code vs. Temperature (2.7 V Supply)

03796-041

6

(V)

REF

2.5

INTERNAL V

0

NO CAP

0.711μs SETTLING TIME

20 40 60 80 100 120–20 0

TURN-ON TIME (μs)

Figure 21. Internal V

100nF CAP

54.64μs SETTLING TIME

vs. Turn-On Time

REF

03796-047

1183

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 19. 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)

VCC (V)

CC

03796-042

03796-043

10

–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 22. Typical FFT Plot for the Auxiliary Channels of the AD7877

at 9

0 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 20. Temp0 vs. V

CC

Figure 23. DAC Source and Si

Rev. B | Page 11 of 44

nk Current Capability

AD7877

www.BDTIC.com/ADI

1

CH1 200mV CH2 100mV M2.00μs CH1 780mV

Figure 24. DAC O/P Settling Time (Zero Scale to Half Scale)

Δ

: 144mV

@: 1.296V

VDD = 3V
TEMPERATURE = 25°C

03796-037

–2 –1 0 1 2

ERROR (%)

Figure 26. Typical Accuracy for Battery Channel (25°C)

03796-050

600

500

400

DAC SINK CURRENT

300

200

DAC SINK CURRENT (µA)

100

0

0 25 50 75 100 125 150 175 200 225 250

NOTE: MAXIMUM DAC SINK CURRENT I S
SET ACCORDING TO THE EQUATION:
I

= VCC /(R

MAX

INPUT CODE (Decimal)

Figure 25. DAC Sink Current vs. Input Code with R

× 6)

RNG

= 1 kΩ

RNG

03796-038

Rev. B | Page 12 of 44

AD7877

www.BDTIC.com/ADI

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

Differential Nonlinearity

ference between the measured and the ideal 1 LSB

The dif
change between any two adjacent codes in the ADC.

Offset Error

The devia
(00…001) from the ideal (AGND + 1 LSB).

Gain Error

The devia
(111…111) from the ideal (V
has been adjusted out.

On Resistance

A me
source of the switch drivers.

tion of the first code transition (00…000) to

tion of the last code transition (111…110) to

− 1 LSB) after the offset error

REF

asure of the ohmic resistance between the drain and the

Rev. B | Page 13 of 44

AD7877

www.BDTIC.com/ADI

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

nalog-to-digital converter (ADC) with input multiplexer,

a
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 generate
an out-of-limit
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

ffers throughput rates of up to 125 kHz. The device is available

o
in a 5 mm × 5 mm, 32-lead, lead frame chip scale package
(LFCSP), and in a 2.5 mm × 2.8 mm, wafer level chip scale
package (WLCSP), with a 5 × 5 ball grid array.

The data acquisition system of the AD7877 has a number of

vanced features:

ad

nput channel sequenced automatically or selected by

• I

the host.

TOPACQ feature to reduce noise from LCD.

• S

veraging of from 1 to 16 conversions for noise reduction.

• A

• Pr

ogrammable acquisition time.

• Po

wer management.

• Pr

ogrammable ADC power-up delay before first

conversion.

hoice of internal or external reference.

• C

• C

onversion 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.

ALERT

. The AD7877 also contains low resistance

PLASTIC FILM WITH

TRANSPARENT, RESISTIVE

CONDUCTIVE ELECTRODE

ON BOTTOM SIDE

CONDUCTIVE ELECTRODE

ON TOP SIDE

Figure 27. 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–

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
a

ny 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
o

nly 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 found.

In addition to measuring the X and Y coordinates, it is also
p

ossible 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

tructure of the AD7877, showing the touch screen switches,

s
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.

03796-005

Rev. B | Page 14 of 44