ANALOG DEVICES AD7873 Service Manual

V

V

Touch Screen Digitizer

FEATURES

4-wire touch screen interface
On-chip temperature sensor: −40°C to +85°C
On-chip 2.5 V reference
Direct battery measurement (0 V to 6 V)
Touch pressure measurement
Specified throughput rate of 125 kSPS
Single supply, V
Ratiometric conversion
High speed serial interface
Programmable 8-bit or 12-bit resolution
One auxiliary analog input
Shutdown mode: 1 μA maximum
16-lead QSOP, TSSOP, and LFCSP packages

APPLICATIONS

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

GENERAL DESCRIPTION

of 2.2 V to 5.25 V

CC

AUX

V

BAT

REF

AD7873

FUNCTIONAL BLOCK DIAGRAM

+

CC

X+

X–

Y+

Y–

BATTERY
MONITOR

2.5V
BUF

REF

TEMP

SENSOR

6-TO-1

I/P

MUX

REDISTRIBUTI ON

CONTROL LOGIC

T/H

CHARGE

DAC

SAR + ADC

PENIRQ

PEN

INTERRUPT

AD7873

COMP

GND

+V

CC

The AD7873 is a 12-bit successive approximation ADC with a
synchronous serial interface and low on resistance switches for
driving touch screens. The AD7873 operates from a single 2.2 V
to 5.25 V power supply and features throughput rates greater
than 125 kSPS.

The AD7873 features direct battery measurement, temperature
measurement, and touch pressure measurement. The AD7873
also has an on-board reference of 2.5 V that can be used for the
auxiliary input, battery monitor, and temperature measurement
modes. When not in use, the internal reference can be shut
down to conserve power. An external reference can also be
applied and can be varied from 1 V to V
range is from 0 V to V

. The device includes a shutdown

REF

and the analog input

CC

mode that reduces the current consumption to less than 1 µA.

The AD7873 features on-board switches. This, coupled with low
power and high speed operation, makes the device ideal for
battery-powered systems such as personal digital assistants with
resistive touch screens and other portable equipment. The part
is available in a 16-lead, 0.15" quarter size outline package
(QSOP), a 16-lead, thin shrink small outline package (TSSOP),
and a 16-lead, lead frame chip scale package (LFCSP).

SPORT

DIN DOUT DCLK BUSY

CS

Figure 1.

PRODUCT HIGHLIGHTS

1. Ratiometric conversion mode available, eliminating errors

due to on-board switch resistances.

2. On-board temperature sensor: −40°C to +85°C.

3. Battery monitor input.

4. Touch pressure measurement capability.

5. Low power consumption of 1.37 mW maximum with the

reference off, or 2.41 mW typical with the reference on, at
125 kSPS and V

6. Package options include 4 mm × 4 mm LFCSP.

7. Analog input range from 0 V to V

8. Versatile serial I/O ports.

at 3.6 V.

CC

REF

.

02164-001

Rev. E

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.

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.

AD7873

TABLE OF CONTENTS

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

Circuit Information........................................................................ 13

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

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

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

Product Highlights........................................................................... 1

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

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

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

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

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

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

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

Te r mi n ol o g y ...................................................................................... 8

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

REVISION HISTORY

ADC Transfer Function............................................................. 13

Typical C o n ne ction D i a g ram ................................................... 13

Analog Input............................................................................... 14

Measurements............................................................................. 16

Pen Interrupt Request ................................................................ 18

Control Register ......................................................................... 19

Power vs. Throughput Rate....................................................... 20

Serial Interface............................................................................ 21

Grounding and Layout .................................................................. 23

PCB Design Guidelines for Chip Scale Package .................... 23

Outline Dimensions ....................................................................... 24

Ordering Guide .......................................................................... 25

9/06—Rev. D to Rev. E

Changes to Figure 13 Caption....................................................... 10

Updated Outline Dimensions....................................................... 24

Changes to Ordering Guide.......................................................... 25

6/04—Rev. C to Rev. D

Updated Format..................................................................Universal

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

Additions to PD0 and PD1 Description...................................... 21

PBC Guidelines for Chip Scale Package Added......................... 23

Additions to Ordering Guide........................................................ 25

4/03—Rev. B to Rev. C

Changes to Formatting ......................................................Universal

Updated Outline Dimensions....................................................... 19

1/02—Rev. A to Rev. B
Addition of 16-Lead Lead Frame Chip Scale Package ..Universal

Edits to Features.................................................................................1

Edits to General Description ...........................................................1

Addition of LFCSP Pin Configuration...........................................4

Edit to Absolute Maximum Ratings................................................4

Addition to Ordering Guide............................................................4

Addition of CP-16 Outline Dimensions ..................................... 19

2/01—Rev. 0 to Rev A
Edits to Notes in the Ordering Guide

Rev. E | Page 2 of 28

AD7873

SPECIFICATIONS

VCC = 2.7 V to 3.6 V, V

Table 1.

Parameter AD7873A1 AD7873B1 Unit Test Conditions/Comments

DC ACCURACY

Resolution 12 12 Bits
No Missing Codes 11 12 Bits min
Integral Nonlinearity2 ±2 ± 1 LSB max
Differential Nonlinearity2 –0.9/+1.5 LSB max
Offset Error2 ±6 ±6 LSB max +VCC = 2.7 V
Gain Error2 ±4 ±4 LSB max External reference
Noise 70 70 μV rms typ
Power Supply Rejection 70 70 dB typ

SWITCH DRIVERS

On Resistance2

Y+, X+ 5 5 Ω typ
Y–, X– 6 6 Ω typ

ANALOG INPUT

Input Voltage Ranges 0 to V
DC Leakage Current ±0.1 ±0.1 μA typ
Input Capacitance 37 37 pF typ

REFERENCE INPUT/OUTPUT

Internal Reference Voltage 2.45/2.55 2.45/2.55 V min/max
Internal Reference Tempco ±15 ±15 ppm/°C typ
V

Input Voltage Range 1/VCC 1/VCC V min/max

REF

DC Leakage Current ±1 ± 1 μA max
V

Input Impedance 1 1 GΩ typ

REF

TEMPERATURE MEASUREMENT

Temperature Range –40/+85 –40/+85 °C min/max
Resolution

Differential Method3 1.6 1.6 °C typ
Single Conversion Method4 0.3 0.3 °C typ

Accuracy

Differential Method3 ±2 ±2 °C typ
Single Conversion Method4 ±2 ±2 °C typ

BATTERY MONITOR

Input Voltage Range 0/6 0/6 V min/max
Input Impedance 10 10 kΩ typ Sampling; 1 GΩ when battery monitor is off

Accuracy ±2.5 ±2 % max External reference
±3 ±3 % max Internal reference
LOGIC INPUTS

Input High Voltage, V

Input Low Voltage, V

Input Current, IIN ±1 ±1 μA max Typically 10 nA, VIN = 0 V or +VCC

Input Capacitance, C

= 2.5 V internal or external, f

REF

0 to V

REF

2.4 2.4 V min

INH

0.4 0.4 V max

INL

5

10 10 pF max

IN

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

DCLK

V

REF

= GND or +VCC; typically 260 Ω when the

CS
on-board reference is enabled

Rev. E | Page 3 of 28

AD7873

Parameter AD7873A1AD7873B Unit Test Conditions/Comments

LOGIC OUTPUTS

Output High Voltage, VOH VCC – 0.2 VCC – 0.2 V min I
Output Low Voltage, V

OL

PENIRQ Output Low Voltage, VOL

0.4 0.4 V max I

0.4 0.4 V max 100 kΩ pull-up; I
Floating-State Leakage Current ±10 ±10 μA max
Floating-State Output Capacitance510 10 pF max
Output Coding Straight (Natural) Binary

CONVERSION RATE

Conversion Time 12 12 DCLK cycles max
Track-and-Hold Acquisition Time 3 3 DCLK cycles min
Throughput Rate 125 125 kSPS max

POWER REQUIREMENTS

+VCC (Specified Performance) 2.7/3.6 2.7/3.6 V min/max Functional from 2.2 V to 5.25 V

6

I

CC

Normal Mode (f

= 125 kSPS) 380 380 μA max Internal reference off, VCC = 3.6 V, 240 μA typ

SAMPLE

Digital I/Ps = 0 V or VCC

670 670 μA typ Internal reference on, VCC = 3.6 V
Normal Mode (f

= 12.5 kSPS) 170 170 μA typ Internal reference off, VCC = 2.7 V, f

SAMPLE

Normal Mode (Static) 150 150 μA typ Internal reference off, VCC = 3.6 V
580 580 μA typ Internal reference on, VCC = 3.6 V
Shutdown Mode (Static) 1 1 μA max 200 nA typ

Power Dissipation

Normal Mode (f

6

= 125 kSPS) 1.368 1.368 mW max Internal reference off, VCC = 3.6 V

SAMPLE

2.412 2.412 mW typ Internal reference on, VCC = 3.6 V
Shutdown 3.6 3.6 μW max VCC = 3.6 V

1

Temperature range as follows: A, B Versions: –40°C to +85°C.

2

See the Terminology section.

3

Difference between TEMP0 and TEMP1 measurement. No calibration necessary.

4

Temperature drift is –2.1 mV/°C.

5

Sample tested @ 25°C to ensure compliance.

6

See the Power vs. Throughput Rate section.

= 250 μA; V

SOURCE

= 250 μA

SINK

= 2.2 V to 5.25 V

CC

= 250 μA

SINK

= 200 kHz

DCLK

Rev. E | Page 4 of 28

AD7873

TIMING SPECIFICATIONS

TA = T

Table 2. Timing Specifications

Parameter Limit at T

f

DCLK

2 MHz max
t

ACQ

t1 10 ns min
t2 60 ns max

3

t

3

t4 200 ns min DCLK high pulse width
t5 200 ns min DCLK low pulse width
t6 60 ns max DCLK falling edge to BUSY rising edge
t7 10 ns min Data setup time prior to DCLK rising edge
t8 10 ns min Data valid to DCLK hold time

3

t

9

t10 0 ns min
t11 100 ns max
t

12

1

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 1.6 V.

2

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

3

Measured with the load circuit of Figure 2 and defined as the time required for the output to cross 0.4 V or 2.0 V.

4

t12 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated

back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t12, quoted in the timing characteristics is the true bus relinquish
time of the part and is independent of the bus loading.

to T

MIN

2

10 kHz min

, unless otherwise noted; VCC = 2.7 V to 5.25 V, V

MAX

1

, T

MIN

Unit Description

MAX

= 2.5 V.

REF

1.5 μs min Acquisition time
CS

falling edge to first DCLK rising edge

CS

falling edge to busy three-state disabled

CS

60 ns max

falling edge to DOUT three-state disabled

200 ns max Data access time after DCLK falling edge

CS

rising edge to DCLK ignored

CS

rising edge to BUSY high impedance

4

100 ns max

CS

rising edge to DOUT high impedance

TO OUTPUT

PIN

50pF

C

200µA I

L

200µA I

OL

1.6V

OH

02164-002

Figure 2. Load Circuit for Digital Output Timing Specifications

Rev. E | Page 5 of 28

AD7873

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 Supplies1 ±10 mA
Operating Temperature Range

Commercial (A, B Versions) –40°C to +85°C

Storage Temperature Range –65°C to +150°C
Junction Temperature 150°C
Power Dissipation 450 mW
IR Reflow Soldering

Peak Temperature

Time-to-Peak Temperature

Ramp-Down Rate
Pb-free Parts Only

Peak Temperature 250°C

Time-to-Peak Temperature

Ramp-Up Rate

Ramp-Down Rate

1

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

220°C (±5°C)
10 sec to 30 sec
6°C/sec max

20 sec to 40 sec
3°C/sec max
6°C/sec max

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.

THERMAL RESISTANCE

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

Table 4. Thermal Resistance

Package Type θ

16-Lead QSOP 149.97 38.8 °C/W
16-Lead TSSOP 150.4 27.6 °C/W
16-Lead LFCSP 135.7 °C/W

JA

θ

JC

Unit

ESD CAUTION

Rev. E | Page 6 of 28

AD7873

+V

PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

BAT

GND

Y–

V

X–

14

13

15

16

PIN 1

REF

CC

1AUX

2V

3+V

4PENIRQ

INDICATOR

AD7873

TOP VIEW

(Not to Scale)

5

6

BUSY

DOUT

12 Y+

11 X+

10 +V

CC

9 DCLK

8

7

CS

DIN

02164-003

Figure 3. LFCSP Pin Configuration

Table 5. Pin Function Descriptions

Pin No.

LFCSP

QSOP/
TSSOP

3, 10 1, 10 +VCC

Mnemonic

Function

Power Supply Input. The +V

range for the AD7873 is from 2.2 V to 5.25 V. Both +VCC pins should be

CC

connected directly together.
11 2 X+ X+ Position Input. ADC Input Channel 1.
12 3 Y+ Y+ Position Input. ADC Input Channel 2.
13 4 X– X– Position Input.
14 5 Y– Y– Position Input. ADC Input Channel 3.
15 6 GND

Analog Ground. Ground reference point for all circuitry on the AD7873. All analog input signals

and any external reference signals should be referred to this GND voltage.
16 7 V

BAT

Battery Monitor Input. ADC Input Channel 4.
1 8 AUX Auxiliary Input. ADC Input Channel 5.
2 9 V

REF

Reference Output for the AD7873. Alternatively, an external reference can be applied to this input.

The voltage range for the external reference is 1.0 V to +V

the AD7873. 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 0.1 μF capacitor is

recommended between this pin and GND to reduce system noise effects.
4 11

PENIRQ

5 12 DOUT

Pen Interrupt. CMOS logic open-drain output (requires 10 kΩ to 100 kΩ pull-up resistor externally).

Data Out. Logic output. The conversion result from the AD7873 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

CS is high.
6 13 BUSY
7 14 DIN

high impedance when
BUSY Output. Logic output. This output is high impedance when
Data In. Logic Input. Data to be written to the AD7873 control register is provided on this input and

is clocked into the register on the rising edge of DCLK (see the

8 15

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

conversions on the AD7873 and enabling the serial input/output register.

9 16 DCLK

External Clock Input. Logic input. DCLK provides the serial clock for accessing data from the part.
This clock input is also used as the clock source for the AD7873 conversion process.

1

CC

2

X+

3

Y+

AD7873

TOP VIEW

4

X–

(Not to Scale)

5

Y–

6

GND

7

V

BAT

8

AUX V

16

15

14

13

12

11

10

9

DCLK

CS

DIN

BUSY

DOUT

PENIRQ

+V

CC

REF

Figure 4. QSOP/TSSOP Pin Configuration

. For specified performance, it is 2.5 V on

CC

CS is high.

Control Register section).

2164-004

Rev. E | Page 7 of 28

AD7873

TERMINOLOGY

Integral Nonlinearity

Integral nonlinearity 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, a point 1 LSB
below the first code transition, and full scale, a point 1 LSB
above the last code transition.

Differential Nonlinearity

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

Offset Error

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

Gain Error

Gain error is the deviation of the last code transition (111…110)
to (111…111) from the ideal (that is, V
offset error is adjusted out.

Track-and-Hold Acquisition Time

The track-and-hold amplifier enters the acquisition phase on
the fifth falling edge of DCLK after the start bit has been
detected. Three DCLK cycles are allowed for the track-and-hold
acquisition time. The input signal is fully acquired to the 12-bit
level within this time even with the maximum specified DCLK
frequency. See the

On Resistance

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

Analog Input section for more details.

– 1 LSB) after the

REF

Rev. E | Page 8 of 28

AD7873

TYPICAL PERFORMANCE CHARACTERISTICS

207

206

205

204

203

202

201

SUPPLY CURRENT ( µA)

200

199

198

–40 –20 0 20 40 60 80 100

TEMPERATURE (°C)

Figure 5. Supply Current vs. Temperature

230

f

220

210

SAMPLE

V

REF

= 12.5kHz

=+V

CC

02164-005

141

140

139

138

137

136

SUPPLY CURRENT (nA)

135

134

–40 –20 0 20 40 60 80 100

TEMPERATURE (°C)

Figure 8. Power-Down Supply Current vs. Temperature

1000

02164-008

200

190

180

SUPPLY CURRENT (µA)

170

160

150

2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0

(V)

+V

CC

Figure 6. Supply Current vs. +V

CC

0.20

0.15

0.10

0.05

0

–0.05

DELTA FROM 25°C (LSB)

–0.10

–0.15

V

=+V

REF

CC

SAMPLE RATE (kSPS)

100

2164-006

Figure 9. Maximum Sample Rate vs. +V

3.2 3.72.2 2.7 4.2 4.7 5.2

(V)

+V

CC

CC

02164-009

0.6

0.4

0.2

0

–0.2

DELTA FRO M 25°C (LSB)

–0.4

–0.20

–40 –20 0 20 40 60 80 100

TEMPERATURE (°C)

Figure 7. Change in Gain vs. Temperature

2164-007

Rev. E | Page 9 of 28

–0.6

–40 –20 0 20 40 60 80 100

TEMPERATURE (°C)

Figure 10. Change in Offset vs. Temperature

02164-010