Touch Screen Digitizer
AD7873
Rev. E
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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
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Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2006 Analog Devices, Inc. All rights reserved.
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, VCC of 2.2 V to 5.25 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
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 VCC and the analog input
range is from 0 V to VREF. The device includes a shutdown
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).
FUNCTIONAL BLOCK DIAGRAM
AD7873
+
V
CC
+V
CC
PENIRQ
SPORT
DIN DOUT DCLK BUSY
CS
PEN
INTERRUPT
CHARGE
REDISTRIBUTION
DAC
SAR + ADC
CONTROL LOGIC
6-TO-1
I/P
MUX
T/H
COMP
GND
TEMP
SENSOR
BATTERY
MONITOR
Y+
Y–
AUX
2.5V
REF
X+
X–
V
REF
V
BAT
BUF
02164-001
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 VCC at 3.6 V.
6. Package options include 4 mm × 4 mm LFCSP.
7. Analog input range from 0 V to VREF.
8. Versatile serial I/O ports.
AD7873
Rev. E | Page 2 of 28
TABLE OF CONTENTS
Features .............................................................................................. 1
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
Ter mi no log y ...................................................................................... 8
Typical Performance Characteristics ............................................. 9
Circuit Information........................................................................ 13
ADC Transfer Function............................................................. 13
Typical Connection Diagram ................................................... 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
REVISION HISTORY
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
AD7873
Rev. E | Page 3 of 28
SPECIFICATIONS
VCC = 2.7 V to 3.6 V, VREF = 2.5 V internal or external, fDCLK = 2 MHz; TA = −40°C to +85°C, unless otherwise noted.
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 VREF 0 to VREF 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
VREF Input Voltage Range 1/VCC 1/VCC V min/max
DC Leakage Current ±1 ± 1 μA max
VREF Input Impedance 1 1 GΩ typ CS = GND or +VCC; typically 260 Ω when the
on-board reference is enabled
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, VINH 2.4 2.4 V min
Input Low Voltage, VINL 0.4 0.4 V max
Input Current, IIN ±1 ±1 μA max Typically 10 nA, VIN = 0 V or +VCC
Input Capacitance, CIN5 10 10 pF max
AD7873
Rev. E | Page 4 of 28
Parameter AD7873A1AD7873B Unit Test Conditions/Comments
LOGIC OUTPUTS
Output High Voltage, VOH VCC – 0.2 VCC – 0.2 V min ISOURCE = 250 μA; VCC = 2.2 V to 5.25 V
Output Low Voltage, VOL 0.4 0.4 V max ISINK = 250 μA
PENIRQ Output Low Voltage, VOL 0.4 0.4 V max 100 kΩ pull-up; ISINK = 250 μA
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
ICC6 Digital I/Ps = 0 V or VCC
Normal Mode (fSAMPLE = 125 kSPS) 380 380 μA max Internal reference off, VCC = 3.6 V, 240 μA typ
670 670 μA typ Internal reference on, VCC = 3.6 V
Normal Mode (fSAMPLE = 12.5 kSPS) 170 170 μA typ Internal reference off, VCC = 2.7 V, fDCLK = 200 kHz
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 Dissipation6
Normal Mode (fSAMPLE = 125 kSPS) 1.368 1.368 mW max Internal reference off, VCC = 3.6 V
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.
AD7873
Rev. E | Page 5 of 28
TIMING SPECIFICATIONS
TA = TMIN to TMAX, unless otherwise noted; VCC = 2.7 V to 5.25 V, VREF = 2.5 V.
Table 2. Timing Specifications1
Parameter Limit at TMIN, TMAX Unit Description
fDCLK2 10 kHz min
2 MHz max
tACQ 1.5 μs min Acquisition time
t1 10 ns min CS falling edge to first DCLK rising edge
t2 60 ns max CS falling edge to busy three-state disabled
t33 60 ns max CS falling edge to DOUT three-state disabled
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
t93 200 ns max Data access time after DCLK falling edge
t10 0 ns min CS rising edge to DCLK ignored
t11 100 ns max CS rising edge to BUSY high impedance
t124 100 ns max CS rising edge to DOUT high impedance
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.
200µA I
OL
200µA I
OH
1.6V
TO OUTPUT
PIN C
L
50pF
02164-002
Figure 2. Load Circuit for Digital Output Timing Specifications
AD7873
Rev. E | Page 6 of 28
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
VREF to GND –0.3 V to VCC + 0.3 V
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
220°C (±5°C)
10 sec to 30 sec
6°C/sec max
Pb-free Parts Only
Peak Temperature 250°C
Time-to-Peak Temperature
Ramp-Up Rate
Ramp-Down Rate
20 sec to 40 sec
3°C/sec max
6°C/sec max
1 Transient currents of up to 100 mA do not cause SCR latch-up.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; 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 θJA θJC Unit
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
ESD CAUTION
AD7873
Rev. E | Page 7 of 28
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
PIN 1
INDICATOR
1AUX
2V
REF
3+V
CC
4PENIRQ
11 X+
12 Y+
10 +V
CC
9 DCLK
5
DOUT
6
BUSY
7
DIN
8
CS
15 GND
16 V
BAT
14 Y–
13 X–
AD7873
TOP VIEW
(Not to Scale)
02164-003
Figure 3. LFCSP Pin Configuration
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
X+
Y+
X–
VBAT
GND
Y–
+V
CC
CS
DIN
BUSY
+VCC
AUX VREF
PENIRQ
DOUT
DCLK
AD7873
TOP VIEW
(Not to Scale)
0
2164-004
Figure 4. QSOP/TSSOP Pin Configuration
Table 5. Pin Function Descriptions
Pin No.
LFCSP
QSOP/
TSSOP
Mnemonic
Function
3, 10 1, 10 +VCC Power Supply Input. The +VCC range for the AD7873 is from 2.2 V to 5.25 V. Both +VCC pins should be
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 VBAT Battery Monitor Input. ADC Input Channel 4.
1 8 AUX Auxiliary Input. ADC Input Channel 5.
2 9 VREF 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 +VCC. For specified performance, it is 2.5 V on
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 Pen Interrupt. CMOS logic open-drain output (requires 10 kΩ to 100 kΩ pull-up resistor externally).
5 12 DOUT
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
high impedance when CS is high.
6 13 BUSY BUSY Output. Logic output. This output is high impedance when CS is high.
7 14 DIN 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 Control Register section).
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.
AD7873
Rev. E | Page 8 of 28
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, VREF – 1 LSB) after the
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 Analog Input section for more details.
On Resistance
On resistance is a measure of the ohmic resistance between the
drain and source of the switch drivers.
AD7873
Rev. E | Page 9 of 28
TYPICAL PERFORMANCE CHARACTERISTICS
198
199
200
201
202
203
204
205
206
207
SUPPLY CURRENT (µA)
–40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
02164-005
Figure 5. Supply Current vs. Temperature
150
160
170
180
190
200
SUPPLY CURRENT (µA)
210
220
230
2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0
+V
CC
(V)
fSAMPLE
= 12.5kHz
V
REF
=+V
CC
0
2164-006
Figure 6. Supply Current vs. +VCC
–0.20
–0.15
–0.10
–0.05
0
0.05
DELTA FROM 25°C (LSB)
0.10
0.15
0.20
–40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
0
2164-007
Figure 7. Change in Gain vs. Temperature
134
135
136
137
138
139
140
141
SUPPLY CURRENT (nA)
–40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
02164-008
Figure 8. Power-Down Supply Current vs. Temperature
3.2 3.72.2 2.7 4.2 4.7 5.2
+V
CC
(V)
100
1000
SAMPLE RATE (kSPS)
V
REF
=+V
CC
02164-009
Figure 9. Maximum Sample Rate vs. +VCC
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
DELTA FROM 25°C (LSB)
–40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
02164-010
Figure 10. Change in Offset vs. Temperature
AD7873
Rev. E | Page 10 of 28
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
REFERENCE CURRENT (µA)
705525 4010 85 100 115 130
SAMPLE RATE (kHz)
02164-011
Figure 11. Reference Current vs. Sample Rate
Y+
X+
X–
Y–
4
5
6
7
8
9
10
R
ON
(Ω)
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
+V
CC
(V)
02164-012
Figure 12. Switch On Resistance vs. +VCC
(X+, Y+: +VCC to Pin; X−, Y−: Pin to GND)
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
ERROR (LSB)
15 35 55 75 95 115 135 155 175 195
SAMPLING RATE (kSPS)
INL: R = 2kΩ
INL: R = 500Ω
DNL: R = 2kΩ
DNL: R = 500Ω
02164-013
Figure 13. Linearity Error vs. Sampling Rate for Various RIN
2
4
6
5
3
8
7
REFERENCE CURRENT (µA)
10
9
12
11
14
13
020–40 –20 40 60 80
TEMPERATURE (°C)
0
2164-014
Figure 14. Reference Current vs. Temperature
Y+
X+
X–
Y–
3
4
5
6
7
8
9
R
ON
(Ω)
–40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
02164-015
Figure 15. Switch On Resistance vs. Temperature
(X+, Y+: +VCC to Pin; X−, Y−: Pin to GND)
2.4988
2.4990
2.4992
2.4994
2.4996
2.4998
2.5000
2.5002
2.5004
2.5006
INTERNAL
V
REF
(V)
TEMPERATURE (°C)
–40 –30 –20 –10 0 10 20 30 40 50 60 70 80
02164-016
Figure 16. Internal VREF vs. Temperature
AD7873
Rev. E | Page 11 of 28
2.484
2.486
2.488
2.490
2.492
2.494
2.496
2.498
2.500
2.502
2.504
V
REF
(V)
2.9 3.12.5 2.7 3.3 3.5 3.7
+V
CC
(V)
02164-017
Figure 17. Internal VREF vs. +VCC
–40 –30 –20 –10 0 10 20 30 40 50 60 70 80
450
500
550
600
650
700
TEMP DIODE VOLTAGE (mV)
750
800
850
TEMPERATURE (°C)
95.95mV
TEMP1
TEMP0
142.15mV
02164-018
Figure 18. Temp Diode Voltage vs. Temperature (2.7 V Supply)
720
721
722
723
724
725
726
727
728
729
730
TEMP1 DIODE VOLTAGE (mV)
V
SUPPLY
(V)
2.7 3.33.0 3.6
02164-019
Figure 19. TEMP1 Diode Voltage vs. VSUPPLY (25°C)
INTERNAL
V
REF
(V)
TURN-ON TIME (µs)
0
1
2
3
4
5
0 200 400 600 800 1000 1200 1400 1600 1800
NO CAP (7µs)
SETTLING TIME
1µF CAP (1800µs)
SETTLING TIME
02164-020
Figure 20. Internal VREF vs. Turn-on Time
600
601
602
603
604
605
606
607
608
609
610
TEMP0 DIODE VOLTAGE (mV)
2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6
V
SUPPLY
(V)
02164-021
Figure 21. TEMP0 Diode Voltage vs. VSUPPLY (25°C)
120
100
80
60
40
20
0
SNR (dB)
30.022.57.5 15.00 37.5 45.0 52.5 60.0
FREQUENCY (kHz)
f
SAMPLE
= 125kHz
f
IN
= 15kHz
SNR = 68.34dB
0
2164-022
Figure 22. Auxiliary Channel Dynamic Performance
(fSAMPLE =125 kHz, fINPUT = 15 kHz)
AD7873
Rev. E | Page 12 of 28
–120
–100
–80
–60
–40
–20
0
PSRR (dB)
0 102030405060708090100
V
CC
RIPPLE FREQUENCY (kHz)
V
CC
=3V
100mV p-p SINE WAVE ON +V
CC
V
REF
= 2.5V EXT REFERENCE
f
SAMPLE
= 125kHz,
f
IN
= 20kHz
02164-023
Figure 23. AC PSRR vs. Supply Ripple Frequency
Figure 23 shows the power supply rejection ratio vs. VDD supply
frequency for the AD7873. The power supply rejection ratio is
defined as the ratio of the power in the ADC output at full-scale
frequency, f, to the power of a 100 mV sine wave applied to the
ADC VCC supply of frequency fS
PSSR (dB) = 10log(Pf/Pfs)
where:
Pf is power at frequency, f, in ADC output.
Pfs is power at frequency, fs, coupled onto the ADC VCC supply.
Here a 100 mV p-p sine wave is coupled onto the VCC supply.
Decoupling capacitors of 10 µF and 0.1 µF were used on the supply.
AD7873
Rev. E | Page 13 of 28
CIRCUIT INFORMATION
The AD7873 is a fast, low power, 12-bit, single-supply analog-
to-digital converter (ADC). The AD7873 can be operated from
a 2.2 V to 5.25 V supply. When operated from either a 5 V
supply or a 3 V supply, the AD7873 is capable of throughput
rates of 125 kSPS when provided with a 2 MHz clock.
The AD7873 provides the user with on-chip track-and-hold,
multiplexer, ADC, reference, temperature sensor, and serial
interface, housed in a tiny 16-lead QSOP, TSSOP, or LFCSP
package, offering the user considerable space-saving advantages
over alternative solutions. The serial clock input (DCLK)
accesses data from the part and also provides the clock source
for the successive approximation ADC. The analog input range
is 0 V to VREF (where the externally applied VREF can be between
1 V and +VCC). The AD7873 has a 2.5 V reference on-board
with this reference voltage available for use externally if buffered.
The analog input to the ADC is provided via an on-chip
multiplexer. This analog input can be any one of the X, Y, and Z
panel coordinates, the battery voltage, or the chip temperature.
The multiplexer is configured with low resistance switches that
allow an unselected ADC input channel to provide power and
an accompanying pin to provide ground for an external device.
For some measurements, the on resistance of the switches could
present a source of error. However, with a differential input to
the converter and a differential reference architecture, this error
can be negated.
ADC TRANSFER FUNCTION
The output coding of the AD7873 is straight binary. The
designed code transitions occur at successive integer LSB values
(that is, 1 LSB, 2 LSBs, and so on). The LSB size is VREF/4096.
The ideal transfer characteristic for the AD7873 is shown in
Figure 24.
ADC CODE
ANALOG INPUT
1LSB
0V +V
REF
–1LSB
1LSB = V
REF
/4096
111...111
111...110
111...000
011...111
000...010
000...001
000...000
02164-024
Figure 24. Transfer Characteristic
TYPICAL CONNECTION DIAGRAM
Figure 25 shows a typical connection diagram for the AD7873
in a touch screen control application. The AD7873 features an
internal reference, but this can be overdriven with an external
low impedance source between 1 V and +VCC. The value of the
reference voltage sets the input range of the converter. The
conversion result is output MSB first, followed by the remaining
11 bits and three trailing zeros, depending on the number of
clocks used per conversion (see the Serial Interface section). For
applications where power consumption is a concern, the power
management option should be used to improve power perform-
ance. See Table 8 for available power management options.
16
15
14
13
12
11
9
8
1
2
3
4
7
6
5
10
TOUCH
SCREEN
1µF TO 10µF
(OPTIONAL)
2.2
V
TO 5
V
0.1µF
0.1µF
AUXILIARY
INPUT
AD7873
+VCC
X+
Y+
X–
Y–
GND
VBAT
AUX
DCLK
CS
DIN
BUSY
DOUT
PENIRQ
+VCC
VREF
SERIAL/CONVERSION CLOCK
CHIP SELECT
SERIAL DATA IN
CONVERTER STATUS
SERIAL DATA OUT
PEN INTERRUPT
50kΩ
TO BATTERY
VOLTAGE
REGULATOR
++
+
02164-025
Figure 25. Typical Application Circuit
AD7873
Rev. E | Page 14 of 28
ANALOG INPUT
Figure 26 shows an equivalent circuit of the analog input
structure of the AD7873 that contains a block diagram of the
input multiplexer, the differential input of the ADC, and the
differential reference.
Table 6 shows the multiplexer address corresponding to each
analog input, both for the SER/DFR bit in the control register
set high and low. The control bits are provided serially to the
device via the DIN pin. For more information on the control
register, see the Control Register section.
When the converter enters hold mode, the voltage difference
between the +IN and –IN inputs (see Figure 26) is captured on
the internal capacitor array. The input current on the analog
inputs depends on the conversion rate of the device. During the
sample period, the source must charge the internal sampling
capacitor (typically 37 pF). Once the capacitor is fully charged,
there is no further input current. The rate of charge transfer
from the analog source to the converter is a function of
conversion rate.
X+ Y+
REF
INT/
EXT
X– Y– GND
6-TO-1
MUX
3-TO-1
MUX
3-TO-1
MUX
IN+ IN+ IN– REF–
REF+
ADC CORE DATA O UT
V
CC
Y–
Y+
X+
X–
ON-CHIP SWITCHES
X+
Y+
Y–
V
BAT
AUX
TEMP
0
2164-026
Figure 26. Equivalent Analog Input Circuit
Table 6. Analog Input, Reference, and Touch Screen Control
A2 A1 A0 SER/ DFR Analog Input X Switches Y Switches +REF1 –REF1
0 0 0 1 TEMP0 Off Off VREF GND
0 0 1 1 X+ Off On VREF GND
0 1 0 1 VBAT Off Off VREF GND
0 1 1 1 X+ (Z1) X+ Off Y+ On VREF GND
X– On Y– Off
1 0 0 1 Y– (Z2) X+ Off Y+ On VREF GND
X– On Y– Off
1 0 1 1 Y+ On Off VREF GND
1 1 0 1 AUX Off Off VREF GND
1 1 1 1 TEMP1 Off Off VREF GND
0 0 0 0 Invalid Address. Test Mode: Switches out the TEMP0 diode to the PENIRQ pin.
0 0 1 0 X+ Off On Y+ Y–
0 1 0 0 Invalid Address
0 1 1 0 X+ (Z1) X+ Off Y+ On Y+ X–
X– On Y– Off
1 0 0 0 Y– (Z2) X+ Off Y+ On Y+ X–
X– On Y– Off
1 0 1 0 Y+ ON Off X+ X–
1 1 0 0 Outputs Identity Code, 1000 0000 0000.
1 1 1 0 Invalid address. Test mode: Switches out the TEMP1 diode to the PENIRQ pin.
1 Internal node, not directly accessible by the user.
AD7873
Rev. E | Page 15 of 28
Acquisition Time
The track-and-hold amplifier enters tracking mode on the
falling edge of the fifth DCLK after the start bit is detected (see
Figure 35). The time required for the track-and-hold amplifier
to acquire an input signal depends on how quickly the 37 pF
input capacitance is charged. With zero source impedance on
the analog input, three DCLK cycles are always sufficient to
acquire the signal to the 12-bit level. With a source impedance
(RIN) on the analog input, the actual acquisition time required is
calculated using the formula:
(
)
pF371004.8 ×+×= IN
ACQ Rt
where RIN is the source impedance of the input signal, and 100 Ω,
37 pF is the input RC. Depending on the frequency of DCLK
used, three DCLK cycles may or may not be sufficient to acquire
the analog input signal with various source impedance values.
Touch Screen Settling
In some applications, external capacitors could be required
across the touch screen to filter noise associated with it, for
example, noise generated by the LCD panel or backlight
circuitry. The value of these capacitors causes a settling time
requirement when the panel is touched. The settling time
typically appears as a gain error. There are several methods for
minimizing or eliminating this issue. The problem can be that
the input signal, reference, or both, have not settled to their
final value before the sampling instant of the ADC. Additionally,
the reference voltage could still be changing during the conversion
cycle. One option is to stop or slow down the DCLK for the
required touch screen settling time. This allows the input and
reference to stabilize for the acquisition time, resolving the issue
for both single-ended and differential modes.
The other option is to operate the AD7873 in differential mode
only for the touch screen, and program the AD7873 to keep
the touch screen drivers on and not go into power-down
(PD0 = PD1 = 1). Several conversions could be required,
depending on the settling time required and the AD7873 data
rate. Once the required number of conversions have been made,
the AD7873 can then be placed in a power-down state on the
last measurement. The last method is to use the 15-DCLK cycle
mode, maintaining the touch screen drivers on until it is
commanded by the processor to stop.
Internal Reference
The AD7873 has an internal reference voltage of 2.5 V. The
internal reference is available on the VREF pin for external use in
the system; however, it must be buffered before it is applied
elsewhere. The on-chip reference can be turned on or off with
the power-down address, PD1 = 1 (see Table 8 and Figure 27).
Typically, the reference voltage is only used in single-ended
mode for battery monitoring, temperature measurement, and
for using the auxiliary input. Optimal touch screen performance
is achieved when using the differential mode. The power-up
time of the 2.5 V reference is typically 10 µs without a load;
however, a 0.1 µF capacitor on the VREF pin is recommended for
optimum performance because it affects the power-up time (see
Figure 20).
X+
Y+
V
REF
260Ω
SW1
3-TO-1
MUX
2.5V
REF
ADC
BUF
02164-027
Figure 27. On-Chip Reference Circuitry
Reference Input
The voltage difference between +REF and −REF (see Figure 26)
sets the analog input range. The AD7873 operates with a refer-
ence input in the range of 1 V to +VCC. Figure 27 shows the
on-chip reference circuitry on the AD7873. The internal
reference on the AD7873 can be overdriven with an external
reference; for best performance, however, the internal reference
should be disabled when an external reference is applied,
because SW1 in Figure 27 opens on the AD7873 when the
internal reference is disabled. The on-chip reference always is
available at the VREF pin as long as the reference is enabled. The
input impedance seen at the VREF pin is approximately
260 Ω when the internal reference is enabled. When it is
disabled, the input impedance seen at the VREF pin is in the
G region.
When making touch screen measurements, conversions can be
made in differential (ratiometric) mode or single-ended mode.
If the SER/DFR bit is set to 1 in the control register, then a
single-ended conversion is performed. Figure 28 shows the
configuration for a single-ended Y coordinate measurement.
The X+ input is connected to the analog-to-digital converter,
the Y+ and Y− drivers are turned on, and the voltage on X+ is
digitized. The conversion is performed with the ADC referenced
from GND to VREF. This VREF is either the on-chip reference or
the voltage applied at the VREF pin externally, and is determined
by the setting of the power management Bit PD0 and Bit PD1
(see Tabl e 7). The advantage of this mode is that the switches
that supply the external touch screen can be turned off once the
acquisition is complete, resulting in a power savings. However,
the on resistance of the Y drivers affects the input voltage that
can be acquired. The full touch screen resistance could be in the
order of 200 Ω to 900 Ω, depending on the manufacturer. Thus,
if the on resistance of the switches is approximately 6 Ω, true
full-scale and zero-scale voltages cannot be acquired, regardless
of where the pen/stylus is on the touch screen. Note that the
minimum touch screen resistance recommended for use with
AD7873
Rev. E | Page 16 of 28
the AD7873 is approximately 70 Ω. In this mode of operation,
therefore, some voltage is likely to be lost across the internal
switches, and it is unlikely that the internal switch resistance
will track the resistance of the touch screen over temperature
and supply, providing an additional source of error.
+
V
CC
V
REF
GND
Y+
Y–
X+ IN+
IN+
IN–
REF+
ADC CORE
REF–
0
2164-028
Figure 28. Single-Ended Reference Mode (SER/DFR = 1)
The alternative to this situation is to set the SER/DFR bit low.
Again, making a Y coordinate measurement is considered, but
now the +REF and –REF nodes of the ADC are connected
directly to the Y+ and Y– pins. This means the analog-to-digital
conversion is ratiometric. The result of the conversion is always
a percentage of the external resistance, independent of how it
could change with respect to the on resistance of the internal
switches. Figure 29 shows the configuration for a ratiometric
Y coordinate measurement.
+
V
CC
GND
Y+
Y–
X+ IN+
IN+
IN–
REF+
ADC CORE
REF–
02164-029
Figure 29. Differential Reference Mode (SER/DFR = 0)
The disadvantage of this mode of operation is that during both
the acquisition phase and conversion process, the external
touch screen must remain powered. This results in additional
supply current for the duration of the conversion.
MEASUREMENTS
Temperature Measurement
Two temperature measurement options are available on the
AD7873, the single conversion method and the differential
conversion method. Both methods are based on an on-chip
diode measurement.
In the single conversion method, a diode voltage is digitized
and recorded at a fixed calibration temperature. Any subsequent
polling of the diode provides an estimate of the ambient tempera-
ture through extrapolation from the calibration temperature
diode result. This assumes a diode temperature drift of
approximately –2.1 mV/°C. This method provides a resolution
of approximately 0.3°C and a predicted accuracy of ±3°C.
The differential conversion method is a two-point measurement.
The first measurement is performed with a fixed bias current
into a diode, and the second measurement is performed with a
fixed multiple of the bias current into the same diode. The
voltage difference in the diode readings is proportional to
absolute temperature and is given by the following formula:
(
)
(
)
NqkTVBE ln/
×
=
Δ
where:
VBE represents the diode voltage.
N is the bias current multiple.
k is Boltzmann’s constant.
q is the electron charge.
This method provides more accurate absolute temperature
measurement of ±2°C. However, the resolution is reduced to
approximately 1.6°C. Assuming a current multiple of 105
(typical for the AD7873) taking Boltzmann’s constant,
k = 1.38054 ×10–23 electrons volts/degrees Kelvin, the electron
charge q = 1.602189 × 10–19, then T, the ambient temperature in
degrees centigrade, can be calculated as follows:
(
)
(
)
()
()
Κ2731049.2)C(
ln/
ln/
3−Δ××=°
××Δ=
×
=
Δ
BE
BE
BE
VT
NkqVT
NqkTV
where ΔVBE is calculated from the difference in readings from
the first conversion and second conversion.
Figure 30 shows a block diagram of the temperature
measurement mode.
I
TEMP0 TEMP1
105 × I
MUX ADC
02164-030
Figure 30. Block Diagram of Temperature Measurement Circuit
AD7873
Rev. E | Page 17 of 28
Battery Measurement
The AD7873 can monitor a battery voltage from 0 V to 6 V.
Figure 31 shows a block diagram of a battery voltage monitored
through the VBAT pin. The voltage to the +VCC of the AD7873 is
maintained at the desired supply voltage via the dc-to-dc
regulator while the input to the regulator is monitored. This
voltage on VBAT is divided by 4 so that a 6 V battery voltage is
presented to the ADC as 1.5 V. To conserve power, the divider is
on only during the sampling of a voltage on VBAT. Table 6 shows
the control bit settings required to perform a battery
measurement.
+
B
A
TTERY
0V TO 6V
DC/DC
CONVERTER
+VCC
VBAT 0V TO 1.5V ADC CORE
7.5kΩ
2.5kΩ
0
2164-031
Figure 31. Block Diagram of Battery Measurement Circuit
Pressure Measurement
The pressure applied to the touch screen via a pen or finger
can also be measured with the AD7873 with some simple
calculations. The 8-bit resolution mode would be sufficient for
this measurement, but the following calculations are shown
with the 12-bit resolution mode. The contact resistance between
the X and Y plates is measured, providing a good indication of
the size of the depressed area and the applied pressure. The area
of the spot 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 first method requires the user to know the total resistance
of the X-plate tablet. Three touch screen conversions are
required, a measurement of the X-position, Z1-position, and
Z2-position (see Figure 32). The following equation calculates
the touch resistance:
(
)
(
)
(
)
[
]
1/4095/
−
×
×
=
12
POSITIONXPLATETOUCH ZZXRR
The second method requires that the resistance of both the
X-plate and Y-plate tablets are known. Again three touch screen
conversions are required, a measurement of the X-position,
Y-position, and Z1-position (see Figure 32).
The following equation also calculates the touch resistance:
(
)
(
)
(
)
[
]
{
}
()
[]
4095/
1/40964095//
POSITIONYPLATE
1
POSITION
1
XPLATETOUCH
YR
ZXZRR
×−
−
×
×
=
+
–
+
–+
–
X+
X–
X+
X–
Y+
Y–
Y–
TOUCH TOUCH
Z
2
-POSITION Z
1
-POSITION
TOUCH
X-POSITION
Y+ X+
X– Y–
Y+
ME
A
SU
R
E
X
-POSITION
MEASURE Z
1
-POSITION
MEASURE Z
2
-POSITION
0
2164-032
Figure 32. Pressure Measurement Block Diagram
AD7873
Rev. E | Page 18 of 28
PEN INTERRUPT REQUEST
The pen interrupt equivalent circuitry is outlined in Figure 33.
By connecting a pull-up resistor (10 kΩ to 100 kΩ) between +VCC
and this CMOS logic open-drain output, the PENIRQ output
remains high normally. If PENIRQ is enabled (see Table 8), when
the touch screen connected to the AD7873 is touched by a pen
or finger, the PENIRQ output goes low, initiating an interrupt to
a microprocessor. This can then instruct a control word to be
written to the AD7873 to initiate a conversion. This output can
also be enabled between conversions during power-down (see
Table 8), allowing power-up to be initiated only when the
screen is touched. The result of the first touch screen coordinate
conversion after power-up is valid, assuming any external
reference is settled to the 12-bit or 8-bit level as required.
Figure 34 assumes that the PENIRQ function was enabled in
the last write or that the part was just powered up so PENIRQ is
enabled by default. Once the screen is touched, the PENIRQ
output goes low a time tPEN later. This delay is approximately
5 µs, assuming a 10 nF touch screen capacitance, and varies
with the touch screen resistance actually used. Once the START
bit is detected, the pen interrupt function is disabled and the
PENIRQ cannot respond to screen touches. The PENIRQ
output remains low until the fourth falling edge of DCLK after
the START bit is clocked in, at which point it returns high as
soon as possible, irrespective of the touch screen capacitance.
This does not mean that the pen interrupt function is now
enabled again because the power-down bits have not yet been
loaded to the control register. Regardless of whether PENIRQ is
to be enabled again, the PENIRQ output normally always idles
high. Assuming the PENIRQ is enabled again as shown in
Figure 34, then once the conversion is complete, the PENIRQ
output again responds to a screen touch. The fact that PENIRQ
returns high almost immediately after the fourth falling edge of
DCLK means the user avoids any spurious interrupts on the
microprocessor or DSP, which can occur if the interrupt request
line on the micro/DSP were unmasked during or toward the
end of conversion and the PENIRQ pin was still low. Once the
next start bit is detected by the AD7843, the PENIRQ function
is again disabled.
If the control register write operation overlaps with the data
read, a start bit is always detected prior to the end of
conversion, meaning that even if the PENIRQ function is
enabled in the control register, it is disabled by the start bit
again before the end of the conversion is reached, so the
PENIRQ function effectively cannot be used in this mode.
However, as conversions are occurring continuously, the
PENIRQ function is not necessary and is therefore redundant.
Y+
Y– ON
X+
TOUCH
SCREEN
EXTERNAL
PULL-UP
PENIRQ
ENABLE
+V
CC
+
V
CC
PENIRQ
100kΩ
02164-033
Figure 33. PENIRQ Functional Block Diagram
SA2A1A0 1 0
81 1 13 16
MODE SER/
DFR
SC
R
EEN
TOUCHED
HERE NO RESPONSE TO TOUCH
t
PEN
PENIRQ
CS
DCLK
DIN (START)
PD1 = 1, PD0 = 0, PENIRQ
ENABLED AGAIN
INTERRUPT
PROCESSOR
02164-034
Figure 34. PENIRQ Timing Diagram
AD7873
Rev. E | Page 19 of 28
CONTROL REGISTER
The control word provided to the ADC via the DIN pin is
shown in Table 7. This provides the conversion start, channel
addressing, ADC conversion resolution, configuration, and
power-down of the AD7873. Table 7 provides detailed
information on the order and description of these control bits
within the control word.
Initiate START
The first bit, the S bit, must always be set to 1 to initiate the start
of the control word. The AD7873 ignores any inputs on the
DIN line until the start bit is detected.
Channel Addressing
The next three bits in the control register, A2, A1, and A0, select
the active input channel(s) of the input multiplexer (see Table 6
and Figure 26), touch screen drivers, and the reference inputs.
Mode
The MODE bit sets the resolution of the analog-to-digital
converter. With a 0 in this bit, the following conversion has
12 bits of resolution. With a 1 in this bit, the following
conversion has eight bits of resolution.
SER/DFR
The SER/DFR bit controls the reference mode, set to either
single-ended or differential when a 1 or a 0 is written to this bit,
respectively. The differential mode is also referred to as the
ratiometric conversion mode. This mode is optimum for
X-position, Y-position, and pressure-touch measurements. The
reference is derived from the voltage at the switch drivers,
which is almost the same as the voltage to the touch screen. In
this case, a separate reference voltage is not needed because the
reference voltage to the ADC is the voltage across the touch
screen. In single-ended mode, the reference voltage to the
converter is always the difference between the VREF and GND
pins. See Table 6 and Figure 26 through Figure 29 for further
information.
If X-position, Y-position, and pressure touch are measured in
single-ended mode, an external reference voltage or +VCC is
required for maximum dynamic range. The internal reference
can be used for these single-ended measurements; however, a
loss in dynamic range is incurred. If an external reference is
used, the AD7873 should also be powered from the external
reference. Because the supply current required by the device is
so low, a precision reference can be used as the supply source to
the AD7873. It might also be necessary to power the touch
screen from the reference, which can require 5 mA to 10 mA. A
REF19x voltage reference can source up to 30 mA, and, as such,
could supply both the ADC and the touch screen. Care must be
taken, however, to ensure that the input voltage applied to the
ADC does not exceed the reference voltage and therefore the
supply voltage. See the Absolute Maximum Ratings section.
Note that the differential mode can only be used for X-position,
Y-position, and pressure touch measurements. All other
measurements require single-ended mode.
PD0 and PD1
The power management options are selected by programming
the power management bits, PD0 and PD1, in the control
register. Table 8 summarizes the options available and the
internal reference voltage configurations. The internal reference
can be turned on or off independent of the analog-to-digital
converter, allowing power saving between conversions using the
power management options. On power-up, PD0 defaults to 0,
while PD1 defaults to 1.
MSB LSB
S A2 A1 A0 MODE SER/DFR PD1 PD0
Table 7. Control Register Bit Function Description
Bit No. Mnemonic Comment
7 S Start Bit. The control word starts with the first high bit on DIN. A new control word can start every 15th DCLK cycle
when in the 12-bit conversion mode or every 11th DCLK cycle when in 8-bit conversion mode.
6 to 4 A2 to A0 Channel Select Bits. These three address bits along with the SER/DFR bit control the setting of the multiplexer
input, switches, and reference inputs, as detailed in Table 6.
3 MODE 12-Bit/8-Bit Conversion Select Bit. This bit controls the resolution of the following conversion. With a 0 in this bit,
the conversion has 12-bit resolution or, with a 1 in this bit, 8-bit resolution.
2 SER/DFR Single-Ended/Differential Reference Select Bit. Together with Bit A2 to Bit A0, this bit controls the setting of the
multiplexer input, switches, and reference inputs as described in Table 6.
1, 0 PD1, PD0 Power Management Bits. These two bits decode the power-down mode of the AD7873 as shown in Table 8.
AD7873
Rev. E | Page 20 of 28
Table 8. Power Management Options
PD1 PD0 PENIRQ Description
0 0 Enabled This configuration results in immediate power-down of the on-chip reference as soon as PD1 is set to 0. The ADC
powers down only between conversions. When PD0 is set to 0, the conversion is performed first and the ADC
powers down upon completion of that conversion (or upon the rising edge of CS, if it occurs first). At the start of
the next conversion, the ADC instantly powers up to full power. This means if the device is being used in the
differential mode, or an external reference is used, there is no need for additional delays to ensure full operation
and the very first conversion is valid. The Y– switch is on while in power-down. When the device is performing
differential table conversions, the reference and reference buffer do not attempt to power up with Bit PD1 and
Bit PD0 programmed in this way.
0 1 Enabled This configuration results in switching the reference off immediately and the ADC on permanently. When the
device is performing differential tablet conversions, the reference and reference buffer do not attempt to power
up with Bit PD1 and Bit PD0 programmed in this way.
1 0 Enabled This configuration results in switching the reference on and powering the ADC down between conversions. The
ADC powers down only between conversions. When PD0 is set to 0, the conversion is performed first, and the
ADC powers down upon completion of the conversion (or upon the rising edge of CS if it occurs first). At the start
of the next conversion, the ADC instantly powers up to full power. There is no need for additional delays to ensure
full operation as the reference remains permanently powered up.
1 1 Disabled This configuration results in always keeping the device powered up. The reference and the ADC are on.
POWER VS. THROUGHPUT RATE
By using the power-down options on the AD7873 when not
converting, the average power consumption of the device
decreases at lower throughput rates. Figure 35 shows how, as the
throughput rate is reduced while maintaining the DCLK
frequency at 2 MHz, the device remains in its power-down state
longer and the average current consumption over time drops
accordingly.
SUPPLY CURRENT (µA)
1
100
10
1000
0 120
THROUGHPUT (kSPS)
40 60020 80100
f
DCLK
=16×f
SAMPLE
f
DCLK
=2MHz
V
CC
=2.7V
T
A
= –40°C TO +85°C
02164-035
Figure 35. Supply Current vs. Throughput (μA)
For example, if the AD7873 is operated in a 24-DCLK continuous
sampling mode, with a throughput rate of 10 kSPS and a DCLK
of 2 MHz, and the device is placed in the power-down mode
between conversions, (PD0, PD1 = 0, 0), that is, the ADC shuts
down between conversions but the reference remains powered
down permanently, then the current consumption is calculated
as follows. The current consumption during normal operation
with a 2 MHz DCLK is 210 µA (VCC = 2.7 V). Assuming an
external reference is used, the power-up time of the ADC is
instantaneous, so when the part is converting, it consumes
210 µA. In this mode of operation, the part powers up on the
fourth falling edge of DCLK after the start bit is recognized. It
goes back into power-down at the end of conversion on the
20th falling edge of DCLK, meaning that the part consumes
210 µA for 16 DCLK cycles only, 8 µs during each conversion
cycle. If the throughput rate is 10 kSPS, the cycle time is 100 µs
and the average power dissipated during each cycle is
(8/100) × (210 µA) = 16.8 µA.
AD7873
Rev. E | Page 21 of 28
SERIAL INTERFACE
Figure 36 shows the typical operation of the serial interface of
the AD7873. The serial clock provides the conversion clock and
also controls the transfer of information to and from the AD7873.
One complete conversion can be achieved with 24 DCLK cycles.
The CS signal initiates the data transfer and conversion process.
The falling edge of CS takes the BUSY output and the serial bus
out of three-state. The first eight DCLK cycles are used to write
to the control register via the DIN pin. The control register is
updated in stages as each bit is clocked in. Once the converter
has enough information about the following conversion to set
the input multiplexer and switches appropriately, the converter
enters the acquisition mode and, if required, the internal switches
are turned on. During acquisition mode, the reference input
data is updated. After the three DCLK cycles of acquisition, the
control word is complete (the power management bits are now
updated) and the converter enters conversion mode. At this
point, track-and-hold goes into hold mode, the input signal is
sampled, and the BUSY output goes high (BUSY returns low on
the next falling edge of DCLK). The internal switches can also
turn off at this point if in single-ended mode, battery-monitor
mode, or temperature measurement mode.
The next 12 DCLK cycles are used to perform the conversion
and to clock out the conversion result. If the conversion is
ratiometric (SER/DFR low), the internal switches are on during
the conversion. A 13th DCLK cycle is needed to allow the
DSP/micro to clock in the LSB. Three more DCLK cycles clock
out the three trailing zeros and complete the 24 DCLK transfer.
The 24 DCLK cycles can be provided from a DSP or via three
bursts of eight clock cycles from a microcontroller.
DCLK
DIN
BUSY
DOUT
X/Y SWITCHES
1
(SER/DFR HIGH)
X/Y SWITCHES
1, 2
(SER/DFR LOW)
THREE-STATE
(START)
IDLE
OFF OFF
(MSB) (LSB)
ON
ON
OFF OFF
ACQUIRE CONVERSION IDLE
ZERO FILLED
THREE-STATE
THREE-STATE
THREE-STATE
tACQ
18 8
11 10 9 8 7 6 5 4 3 2 1 0
11
SA2 PD1PD0A1 A0
MODE SER/
DFR
NOTES
1
Y DRIVERS ARE ON WHEN X+ IS SELECTED INPUT CHANNEL (A2 TO A0 = 001); X DRIVERS ARE ON WHEN Y+ IS SELECTED INPUT CHANNEL (A2 TO A0 = 101).
WHEN PD1, PD0 = 00, 01 OR 10, Y– WILL TURN ON AT THE END OF THE CONVERSION.
2
DRIVERS WILL REMAIN ON IF POWER-DOWN MODE IS 11 (NO POWER-DOWN) UNTIL SELECTED INPUT CHANNEL, REFERENCE MODE,
OR POWER-DOWN MODE IS CHANGED, OR CS IS HIGH.
8
CS
0
2164-036
Figure 36. Conversion Timing, 24 DCLKS per Conversion Cycle, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port.
CS
DCLK
DIN
BUSY
DOUT DB11
PD0
DB10
t1t4
t5
t6t6t9
t10
t11
t7
t2
t3
t8
t12
02164-037
Figure 37. Detail Timing Diagram
AD7873
Rev. E | Page 22 of 28
16 Clocks per Cycle
The control bits for the next conversion can be overlapped with
the current conversion to allow for a conversion every 16 DCLK
cycles, as shown in Figure 38. This timing diagram also allows
the possibility of communication with other serial peripherals
between each byte (eight DCLKs) transfer between the
processor and the converter. However, the conversion must
complete within a short enough time frame to avoid capacitive
droop effects that could distort the conversion result. It should
also be noted that the AD7873 is fully powered while other
serial communications are taking place between byte transfers.
15 Clocks per Cycle
Figure 39 shows the fastest way to clock the AD7873. This
scheme does not work with most microcontrollers or DSPs
because they are not capable of generating a 15 clock cycle per
serial transfer. However, some DSPs allow the number of clocks
per cycle to be programmed. This method can also be used with
FPGAs (field programmable gate arrays) or ASICs (application
specific integrated circuits). As in the 16 clocks per cycle case,
the control bits for the next conversion are overlapped with the
current conversion to allow a conversion every 15 DCLK cycles
using 12 DCLKs to perform the conversion and 3 DCLKs to
acquire the analog input. This effectively increases the
throughput rate of the AD7873 beyond that used for the
specifications that are tested using 16 DCLKs per cycle, and
DCLK = 2 MHz.
8-Bit Conversion
The AD7873 can be set up to operate in an 8-bit mode rather
than a 12-bit mode by setting the MODE bit in the control
register to 1. This mode allows a faster throughput rate to be
achieved, assuming 8-bit resolution is sufficient. When using 8-bit
mode, a conversion is complete four clock cycles earlier than in
12-bit mode. This can be used with serial interfaces that provide
12 clock transfers, or two conversions can be completed with
three 8-clock transfers. The throughput rate increases by 25% as
a result of the shorter conversion cycle, but the conversion itself
can occur at a faster clock rate because the internal settling time
of the AD7873 is not as critical, because settling to eight bits is
all that is required. The clock rate can be as much as 50% faster.
The faster clock rate and fewer clock cycles combine to provide
double the conversion rate.
DCLK
DIN
BUSY
DOUT
CS
1
S S
11 10 9 8 7 6 5 4 3 2 1 0 11 10 9
111
888
CONTROL BITS CONTROL BITS
02164-038
Figure 38. Conversion Timing, 16 DCLKs per Cycle, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port.
CS
DCLK
DIN
BUSY
DOUT
SA2 PD1PD0A1 A0
MODE SER/
DFR MODE SER/
DFR
1
11109876543210 1110987654
15 1 15 1
SA2 SA2A1 PD1 PD0A0
02164-039
Figure 39. Conversion Timing, 15 DCLKs per Cycle, Maximum Throughput Rate
AD7873
Rev. E | Page 23 of 28
GROUNDING AND LAYOUT
For information on grounding and layout considerations for the
AD7873, refer to Application Note AN-577, Layout and
Grounding Recommendations for Touch Screen Digitizers.
PCB DESIGN GUIDELINES FOR
CHIP SCALE PACKAGE
The lands on the chip scale package (CP-32) are rectangular.
The printed circuit board pad for these should be 0.1 mm
longer than the package land length and 0.05 mm wider than
the package land width. The land should be centered on the
pad. This ensures that the solder joint size is maximized.
The bottom of the chip scale package has a central thermal pad.
The thermal pad on the printed circuit board should be at least
as large as this exposed pad. On the printed circuit board, there
should be a clearance of at least 0.25 mm between the thermal
pad and the inner edges of the pad pattern. This ensures that
shorting is avoided.
Thermal vias can be used on the printed circuit board thermal
pad to improve thermal performance of the package. If vias are
used, they should be incorporated in the thermal pad at 1.2 mm
pitch grid. The via diameter should be between 0.3 mm and
0.33 mm and the via barrel should be plated with 1 oz. copper
to plug the via.
The user should connect the printed circuit board thermal pad
to GND.
AD7873
Rev. E | Page 24 of 28
OUTLINE DIMENSIONS
COMPLIANT TO JEDEC STANDARDS MO-137-AB
16 9
8
1
PIN 1
SEATING
PLANE
0.010
0.004 0.012
0.008
0.025
BSC 0.010
0.006
0.050
0.016
8°
0°
COPLANARITY
0.004
0.065
0.049
0.069
0.053
0.197
0.193
0.189
0.158
0.154
0.150 0.244
0.236
0.228
Figure 40. 16-Lead Shrink Small Outline Package [QSOP]
(RQ-16)
Dimensions shown in inches
16 9
81
PIN 1
SEATING
PLANE
8°
0°
4.50
4.40
4.30
6.40
BSC
5.10
5.00
4.90
0.65
BSC
0.15
0.05
1.20
MAX
0.20
0.09 0.75
0.60
0.45
0.30
0.19
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 41. 16-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-16)
Dimensions shown in millimeters
COMPLIANT TO JEDEC STANDARDS MO-220-VGGC
2.25
2.10 SQ
1.95
16
5
13
8
9
12 1
4
1.95 BSC
PIN 1
INDICATOR TOP
VIEW
4.00
BSC SQ
3.75
BSC SQ
COPLANARITY
0.08
EXPOSED
PAD
(BOTTOM VIEW)
12° MAX
1.00
0.85
0.80 SEATING
PLANE
0.35
0.30
0.25
0.80 MAX
0.65 TYP
0.05 MAX
0.02 NOM
0.20 REF
0.65 BSC
0.60 MAX
0.60 MAX
PIN 1
INDICATOR
0.25 MIN
010606-0
0.75
0.60
0.50
Figure 42. 16-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
4 mm × 4 mm Body, Very Thin Quad
(CP-16-4)
Dimensions shown in millimeters
AD7873
Rev. E | Page 25 of 28
ORDERING GUIDE
Model Temperature Range Package Description Linearity Error (LSB)1 Package Option2
AD7873ARQ –40°C to +85°C 16-Lead QSOP ±2 RQ-16
AD7873ARQ-REEL –40°C to +85°C 16-Lead QSOP ±2 RQ-16
AD7873ARQ-REEL7 –40°C to +85°C 16-Lead QSOP ±2 RQ-16
AD7873ARQZ3–40°C to +85°C 16-Lead QSOP ±2 RQ-16
AD7873ARQZ-REEL3–40°C to +85°C 16-Lead QSOP ±2 RQ-16
AD7873ARQZ-REEL73–40°C to +85°C 16-Lead QSOP ±2 RQ-16
AD7873BRQ –40°C to +85°C 16-Lead QSOP ±1 RQ-16
AD7873BRQ-REEL –40°C to +85°C 16-Lead QSOP ±1 RQ-16
AD7873BRQ-REEL7 –40°C to +85°C 16-Lead QSOP ±1 RQ-16
AD7873BRQZ 3–40°C to +85°C 16-Lead QSOP ±1 RQ-16
AD7873BRQZ-REEL3–40°C to +85°C 16-Lead QSOP ±1 RQ-16
AD7873BRQZ-REEL73–40°C to +85°C 16-Lead QSOP ±1 RQ-16
AD7873ARU –40°C to +85°C 16-Lead TSSOP ±2 RU-16
AD7873ARU-REEL –40°C to +85°C 16-Lead TSSOP ±2 RU-16
AD7873ARU-REEL7 –40°C to +85°C 16-Lead TSSOP ±2 RU-16
AD7873ARUZ3–40°C to +85°C 16-Lead TSSOP ±2 RU-16
AD7873ARUZ-REEL3–40°C to +85°C 16-Lead TSSOP ±2 RU-16
AD7873ARUZ-REEL73–40°C to +85°C 16-Lead TSSOP ±2 RU-16
AD7873ACP –40°C to +85°C 16-Lead LFCSP_VQ ±2 CP-16-4
AD7873ACP-REEL –40°C to +85°C 16-Lead LFCSP_VQ ±2 CP-16-4
AD7873ACP-REEL7 –40°C to +85°C 16-Lead LFCSP_VQ ±2 CP-16-4
AD7873ACPZ3–40°C to +85°C 16-Lead LFCSP_VQ ±2 CP-16-4
AD7873ACPZ-REEL3–40°C to +85°C 16-Lead LFCSP_VQ ±2 CP-16-4
AD7873ACPZ-REEL73–40°C to +85°C 16-Lead LFCSP_VQ ±2 CP-16-4
AD7873BCP –40°C to +85°C 16-Lead LFCSP_VQ ±1 CP-16-4
AD7873BCP-REEL –40°C to +85°C 16-Lead LFCSP_VQ ±1 CP-16-4
AD7873BCP-REEL7 –40°C to +85°C 16-Lead LFCSP_VQ ±1 CP-16-4
AD7873BCPZ3–40°C to +85°C 16-Lead LFCSP_VQ ±1 CP-16-4
AD7873BCPZ-REEL73–40°C to +85°C 16-Lead LFCSP_VQ ±1 CP-16-4
EVAL-AD7873CB4 Evaluation Board
EVAL-CONTROL BRD25 Controller Board
1 Linearity error here refers to integral linearity error.
2 RQ = QSOP = 0.15” quarter size outline package; RU = TSSOP, CP = LFCSP.
3 Z = Pb-free part.
4 This can be used as a standalone evaluation board or in conjunction with the evaluation board controller for evaluation/demonstration purposes.
5 This evaluation board controller is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in CB designators.
AD7873
Rev. E | Page 26 of 28
NOTES
AD7873
Rev. E | Page 27 of 28
NOTES
AD7873
Rev. E | Page 28 of 28
T
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C02164-0-9/06(E)
TTT
