Touch Screen Digitizer
AD7873
FEATURES
FUNCTIONAL BLOCK DIAGRAM
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 max
16-lead QSOP, TSSOP, and LFCSP packages
VBAT
APPLICATIONS
VREF
+VCC
PENIRQ
PEN
INTERRUPT
TEMP
SENSOR
X+
X–
AD7873
Y+
T/H
Y–
6-TO-1
I/P
MUX
AUX
COMP
BATTERY
MONITOR
GND
Personal digital assistants
Smart hand-held devices
Touch screen monitors
Point-of-sale terminals
Pagers
2.5V
REF
CHARGE
REDISTRIBUTION
DAC
BUF
+VCC
SAR + ADC
CONTROL LOGIC
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.
DIN
CS
DOUT
DCLK
BUSY
Figure 1.
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, while 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).
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 max with the
reference off, or 2.41 mW typ 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.
Rev. D
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.326.8703
© 2004 Analog Devices, Inc. All rights reserved.
02164-D-001
SPORT
AD7873
TABLE OF CONTENTS
Specifications..................................................................................... 3
Analog Input ............................................................................... 14
Timing Specifications .................................................................. 5
Measurements............................................................................. 16
Absolute Maximum Ratings............................................................ 6
Pen Interrupt Request................................................................ 18
ESD Caution.................................................................................. 6
Control Register ......................................................................... 19
Pin Configurations and Function Descriptions ........................... 7
Power vs. Throughput Rate....................................................... 20
Terminology ...................................................................................... 8
Serial Interface ............................................................................ 21
Typical Performance Characteristics ............................................. 9
Grounding and Layout .................................................................. 23
Circuit Information ........................................................................ 13
PCB Design Guidelines for Chip Scale Package .................... 23
ADC Transfer Function............................................................. 13
Outline Dimensions ....................................................................... 24
Typical Connection Diagram ................................................... 13
Ordering Guide .......................................................................... 25
REVISION HISTORY
6/04—Changed from 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—Changed from Rev. B to Rev. C
Changes to Formatting ......................................................Universal
Updated Outline Dimensions ....................................................... 19
1/02—Changed from 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—Changed from Rev. 0 to Rev A
Edits to Notes in the Ordering Guide
Rev. D | Page 2 of 28
AD7873
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
DC ACCURACY
Resolution
No Missing Codes
Integral Nonlinearity2
Differential Nonlinearity2
Offset Error2
Gain Error2
Noise
Power Supply Rejection
SWITCH DRIVERS
On Resistance2
Y+, X+
Y–, X–
ANALOG INPUT
Input Voltage Ranges
DC Leakage Current
Input Capacitance
REFERENCE INPUT/OUTPUT
Internal Reference Voltage
Internal Reference Tempco
VREF Input Voltage Range
DC Leakage Current
VREF Input Impedance
TEMPERATURE MEASUREMENT
Temperature Range
Resolution
Differential Method3
Single Conversion Method4
Accuracy
Differential Method3
Single Conversion Method4
BATTERY MONITOR
Input Voltage Range
Input Impedance
Accuracy
LOGIC INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance, CIN5
AD7873A1
AD7873B
12
11
±2
±6
±4
70
70
12
12
±1
–0.9/+1.5
±6
±4
70
70
Bits
Bits min
LSB max
LSB max
LSB max
LSB max
µV rms typ
dB typ
5
6
5
6
Ω typ
Ω typ
0 to VREF
±0.1
37
0 to VREF
±0.1
37
Volts
µA typ
pF typ
2.45/2.55
± 15
1/VCC
±1
1
2.45/2.55
± 15
1/VCC
±1
1
V min/max
ppm/°C typ
V min/max
µA max
GΩ typ
–40/+85
–40/+85
°C min/max
1.6
0.3
1.6
0.3
°C typ
°C typ
±2
±2
±2
±2
°C typ
°C typ
0/6
10
± 2.5
±3
0/6
10
±2
±3
V min/max
kΩ typ
% max
% max
2.4
0.4
±1
10
2.4
0.4
±1
10
V min
V max
µA max
pF max
1
Unit
Footnotes on next page.
Rev. D | Page 3 of 28
Test Conditions/Comments
+VCC = 2.7 V
External reference
CS = GND or +VCC; typically 260 Ω when onboard reference enabled
Sampling; 1 GΩ when battery monitor off
External reference
Internal reference
Typically 10 nA, VIN = 0 V or +VCC
AD7873
Parameter
LOGIC OUTPUTS
Output High Voltage, VOH
Output Low Voltage, VOL
PENIRQ Output Low Voltage, VOL
Floating-State Leakage Current
Floating-State Output Capacitance5
Output Coding
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition Time
Throughput Rate
POWER REQUIREMENTS
+VCC (Specified Performance)
ICC6
Normal Mode (fSAMPLE = 125 kSPS)
Normal Mode (fSAMPLE = 12.5 kSPS)
Normal Mode (Static)
Shutdown Mode (Static)
Power Dissipation6
Normal Mode (fSAMPLE = 125 kSPS)
Shutdown
AD7873A1
VCC – 0.2
0.4
0.4
±10
10
AD7873B
1
Unit
VCC – 0.2
V min
0.4
V max
0.4
V max
±10
µA max
10
pF max
Straight (Natural) Binary
Test Conditions/Comments
ISOURCE = 250 µA; VCC = 2.2 V to 5.25 V
ISINK = 250 µA
100 kΩ pull-up; ISINK = 250 µA
12
3
125
12
3
125
DCLK cycles max
DCLK cycles min
kSPS max
2.7/3.6
2.7/3.6
V min/max
380
380
µA max
670
170
670
170
µA typ
µA typ
150
580
1
150
580
1
µA typ
µA typ
µA max
Functional from 2.2 V to 5.25 V
Digital I/Ps = 0 V or VCC
Internal reference off. VCC = 3.6 V,
240 µA typ
Internal reference on. VCC = 3.6 V
Internal reference off. VCC = 2.7 V,
fDCLK = 200 kHz
Internal reference off. VCC = 3.6 V
Internal reference on. VCC = 3.6 V
200 nA typ
1.368
2.412
3.6
1.368
2.412
3.6
mW max
mW typ
µW max
VCC = 3.6 V. Internal reference disabled
VCC = 3.6 V. Internal reference enabled
VCC = 3.6 V
1
Temperature range as follows: A, B Versions: –40°C to +85°C.
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.
2
Rev. D | Page 4 of 28
AD7873
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
fDCLK2
tACQ
t1
t2
t33
t4
t5
t6
t7
t8
t9
t10
t11
t124
3
Limit at TMIN, TMAX
10
2
1.5
10
60
60
200
200
60
10
10
200
0
100
100
Unit
kHz min
MHz max
µs min
ns min
ns max
ns max
ns min
ns min
ns max
ns min
ns min
ns max
ns min
ns max
ns max
Description
Acquisition time
CS falling edge to first DCLK rising edge
CS falling edge to busy three-state disabled
CS falling edge to DOUT three-state disabled
DCLK high pulse width
DCLK low pulse width
DCLK falling edge to BUSY rising edge
Data setup time prior to DCLK rising edge
Data valid to DCLK hold time
Data access time after DCLK falling edge
CS rising edge to DCLK ignored
CS rising edge to BUSY high impedance
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.
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.
2
200µA
1.6V
CL
50pF
200µA
IOH
02164-D-002
TO
OUTPUT
PIN
IOL
Figure 2. Load Circuit for Digital Output Timing Specifications
Rev. D | Page 5 of 28
AD7873
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 3.
Parameter
+VCC to GND
Analog Input Voltage to GND
Digital Input Voltage to GND
Digital Output Voltage to GND
VREF to GND
Input Current to Any Pin Except Supplies1
Operating Temperature Range
Commercial (A, B Versions)
Storage Temperature Range
Junction Temperature
QSOP, TSSOP, LFCSP Packages, Power
Dissipation
θJA Thermal Impedance
θJC Thermal Impedance
IR Reflow Soldering
Peak Temperature
Time-to-Peak Temperature
Ramp-Down Rate
Pb-free Parts Only:
Peak Temperture
Time-to-Peak Temperature
Ramp-Up Rate
Ramp-Down Rate
Rating
–0.3 V to +7 V
–0.3 V to VCC + 0.3 V
–0.3 V to VCC + 0.3 V
–0.3 V to VCC + 0.3 V
–0.3 V to VCC + 0.3 V
±10 mA
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 listed in the operational sections
of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
–40°C to +85°C
–65°C to +150°C
150°C
450 mW
149.97°C/W (QSOP)
150.4°C/W (TSSOP)
135.7°C/W (LFCSP)
38.8°C/W (QSOP)
27.6°C/W (TSSOP)
220°C (±5°C)
10 sec to 30 sec
6°C/sec max
250°C
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.
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. D | Page 6 of 28
AD7873
GND
Y–
X–
16 15 14 13
PENIRQ 4
AD7873
TOP VIEW
(Not to Scale)
6
7
DOUT
BUSY
DIN
5
12
Y+
10
X+
+VCC
9
DCLK
11
8
CS
VREF 2
+VCC 3
PIN 1
INDICATOR
16
DCLK
X+ 2
15
CS
Y+ 3
14
DIN
X– 4
AD7873
BUSY
TOP VIEW
Y– 5 (Not to Scale) 12 DOUT
02164-D-003
AUX 1
+VCC 1
13
GND 6
11
VBAT 7
10
+VCC
AUX 8
9
VREF
PENIRQ
02164-D-004
VBAT
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
Figure 4.QSOP/TSSOP Pin Configuration
Figure 3. LFCSP Pin Configuration
Table 4. Pin Function Descriptions
Mnemonic
+VCC
X+
Y+
X–
Y–
GND
VBAT
AUX
VREF
PENIRQ
DOUT
BUSY
DIN
CS
DCLK
Function
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.
X+ Position Input. ADC Input Channel 1.
Y+ Position Input. ADC Input Channel 2.
X– Position Input.
Y– Position Input. ADC Input Channel 3.
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.
Battery Monitor Input. ADC Input Channel 4.
Auxiliary Input. ADC Input Channel 5.
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.
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 high impedance when CS is high.
BUSY Output. Logic Output. This output is high impedance when CS is high.
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).
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.
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.
Rev. D | Page 7 of 28
AD7873
TERMINOLOGY
Integral Nonlinearity
This 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
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
This is the deviation of the first code transition (00 . . . 000) to
(00 . . . 001) from the ideal, that is, AGND + 1 LSB.
Gain Error
This 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
This is a measure of the ohmic resistance between the drain and
source of the switch drivers.
Rev. D | Page 8 of 28
AD7873
TYPICAL PERFORMANCE CHARACTERISTICS
207
141
206
140
SUPPLY CURRENT (nA)
SUPPLY CURRENT (µA)
205
204
203
202
201
139
138
137
136
200
–20
0
20
40
60
80
100
TEMPERATURE (°C)
134
–40
02164-D-005
198
–40
–20
0
20
40
60
80
100
TEMPERATURE (°C)
Figure 5. Supply Current vs. Temperature
02164-D-008
135
199
Figure 8. Power-Down Supply Current vs. Temperature
230
1000
fSAMPLE = 12.5kHz
220
SAMPLE RATE (kSPS)
SUPPLY CURRENT (µA)
VREF = +VCC
210
200
190
180
VREF = +VCC
170
2.6
3.0
3.4
3.8
4.2
4.6
5.0
+VCC (V)
100
2.2
02164-D-006
150
2.2
2.7
3.2
3.7
4.2
4.7
5.2
+VCC (V)
02164-D-009
160
Figure 9. Maximum Sample Rate vs. +VCC
Figure 6. Supply Current vs. +VCC
0.6
0.20
0.15
DELTA FROM 25°C (LSB)
0.05
0
–0.05
–0.10
0.2
0
–0.2
–0.4
–0.20
–40
–20
0
20
40
60
80
TEMPERATURE (°C)
100
–0.6
–40
–20
0
20
40
60
80
TEMPERATURE (°C)
Figure 10. Change in Offset vs. Temperature
Figure 7. Change in Gain vs. Temperature
Rev. D | Page 9 of 28
100
02164-D-010
–0.15
02164-D-007
DELTA FROM 25°C (LSB)
0.4
0.10
AD7873
14
7.5
13
12
REFERENCE CURRENT (µA)
REFERENCE CURRENT (µA)
6.5
5.5
4.5
3.5
2.5
11
10
9
8
7
6
5
4
1.5
40
55
70
85
100
115
130
SAMPLE RATE (kHz)
2
–40
02164-D-011
25
–20
0
20
40
60
80
TEMPERATURE (°C)
Figure 11. Reference Current vs. Sample Rate
02164-D-014
3
0.5
10
Figure 14. Reference Current vs. Temperature
9
10
9
8
Y+
Y+
X+
RON (Ω)
RON (Ω)
X+
7
8
7
X–
6
Y–
X–
6
5
Y–
2.5
3.0
3.5
4.0
4.5
5.0
5.5
+VCC (V)
3
–40
02164-D-012
4
2.0
0
20
40
60
80
100
TEMPERATURE (°C)
Figure 15. Switch On Resistance vs. Temperature
(X+, Y+: +VCC to Pin; X-, Y-: Pin to GND)
Figure 12. Switch On Resistance vs. +VCC
(X+, Y+: +VCC to Pin; X-, Y-: Pin to GND)
2.0
2.5006
1.8
2.5004
1.6
2.5002
INL: R = 2kΩ
INTERNAL VREF (V)
1.4
1.2
INL: R = 500Ω
1.0
0.8
DNL: R = 2kΩ
0.6
2.5000
2.4998
2.4996
2.4994
2.4992
0.4
DNL: R = 500Ω
2.4990
0
15
35
55
75
95
115
135
155
175
SAMPLING RATE (kSPS)
195
2.4988
–40 –30 –20 –10
0
10
20
30
40
50
60
TEMPERATURE (°C)
Figure 16. Internal VREF vs. Temperature
Figure 13. Maximum Sampling Rate vs. RIN
Rev. D | Page 10 of 28
70
80
02164-D-016
0.2
02164-D-013
ERROR (LSB)
–20
02164-D-015
4
5
AD7873
5
2.504
2.502
4
2.500
INTERNAL VREF (V)
VREF (V)
2.498
2.496
2.494
2.492
2.490
3
NO CAP (7µS)
SETTLING TIME
2
1µF CAP (1800µS)
SETTLING TIME
1
2.488
2.7
2.9
3.1
3.3
3.5
3.7
+VCC (V)
0
02164-D-017
2.484
2.5
0
200
600
800
1000
1200
1600
1800
Figure 20. Internal VREF vs. Turn-on Time
610
850
800
609
TEMP1
TEMP0 DIODE VOLTAGE (mV)
95.95mV
750
700
650
TEMP0
142.15mV
550
608
607
606
605
604
603
602
601
0
10
20
30
40
50
60
70
80
TEMPERATURE (°C)
600
2.7
02164-D-018
450
–40 –30 –20 –10
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
VSUPPLY (V)
Figure 18. Temp Diode Voltage vs. Temperature (2.7 V Supply)
02164-D-021
600
500
Figure 21. Temp0 Diode Voltage vs. VSUPPLY (25°C)
730
0
729
fSAMPLE = 125kHz
fIN = 15kHz
20
728
SNR = 68.34dB
727
40
SNR (dB)
726
725
724
60
80
723
722
100
720
2.7
3.0
3.3
VSUPPLY (V)
3.6
Figure 19. Temp 1 Diode Voltage vs. VSUPPLY (25°C)
120
0
7.5
15.0
22.5
30.0
37.5
45.0
52.5
60.0
FREQUENCY (kHz)
Figure 22. Auxiliary Channel Dynamic Performance
( fSAMPLE =125 kHz, fINPUT = 15 kHz)
Rev. D | Page 11 of 28
02164-D-022
721
02164-D-019
TEMP1 DIODE VOLTAGE (mV)
1400
TURN-ON TIME (µS)
Figure 17. Internal VREF vs. +VCC
TEMP DIODE VOLTAGE (mV)
400
02164-D-020
2.486
AD7873
0
Figure 23 shows the power supply rejection ratio versus 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:
VCC = 3V
100mV p-p SINEWAVE ON +VCC
VREF = 2.5V EXT REFERENCE
fSAMPLE = 125kHz, fIN = 20kHz
–20
PSSR (dB) = 10 log (Pf/Pfs)
–60
where:
–80
Pf is power at frequency, f, in ADC output.
Pfs is power at frequency, fS, coupled onto the ADC VCC supply.
–100
–120
0
10
20
30
40
50
60
70
80
90
VCC RIPPLE FREQUENCY (kHz)
100
02164-D-023
PSRR (dB)
–40
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.
Figure 23. AC PSRR vs. Supply Ripple Frequency
Rev. D | Page 12 of 28
AD7873
CIRCUIT INFORMATION
ADC TRANSFER FUNCTION
The AD7873 is a fast, low-power, 12-bit, single-supply A/D
converter. 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 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.
The AD7873 provides the user with an on-chip track-and-hold,
multiplexer, A/D converter, reference, temperature sensor, and
serial interface housed in a tiny 16-lead QSOP, TSSOP, or
LFCSP package, which offers 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.
ADC CODE
111...111
111...110
111...000
1LSB = VREF/4096
011...111
0V
02164-D-024
000...010
000...001
000...000
+VREF–1LSB
1LSB
ANALOG INPUT
Figure 24. Transfer Characteristic
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, battery voltage, or 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.
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 zeroes, 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
performance. See Table 7 for available power management
options.
2.2V TO 5V
TOUCH
SCREEN
0.1µF
1
+VCC
2
X+
3
Y+
DIN 14
4
X–
BUSY 13
CONVERTER STATUS
5
Y–
DOUT 12
SERIAL DATA OUT
6
GND
PENIRQ 11
7
VBAT
+VCC 10
8
AUX
VREF 9
TO BATTERY
AUXILIARY
INPUT
DCLK 16
AD7873
SERIAL/CONVERSION CLOCK
CHIP SELECT
CS 15
SERIAL DATA IN
PEN INTERRUPT
50kΩ
0.1µF
VOLTAGE
REGULATOR
Figure 25. Typical Application Circuit
Rev. D | Page 13 of 28
02164-D-025
1µF TO 10µF
(OPTIONAL)
AD7873
ANALOG INPUT
Figure 26 shows an equivalent circuit of the analog input
structure of the AD7873, which contains a block diagram of the
input multiplexer, the differential input of the ADC, and the
differential reference.
VCC
X+
X–
Table 5 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.
3-TO-1
MUX
ON-CHIP SWITCHES
X+
Y+
IN+
Y–
6-TO-1
MUX
VBAT
REF+
IN+ ADC CORE
IN– REF–
DATA OUT
AUX
3-TO-1
MUX
TEMP
02164-D-026
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.
REF
INT/
X+ Y+ EXT
Y+
Y–
X– Y– GND
Figure 26. Equivalent Analog Input Circuit
Table 5. Analog Input, Reference, and Touch Screen Control
SER/ DFR
–REF
GND
GND
GND
GND
1
1
1
0
X Switches
Y Switches
+REF1
OFF
OFF
VREF
OFF
ON
VREF
OFF
OFF
VREF
X+ OFF
Y+ ON
VREF
X– ON
Y– OFF
Y– (Z2)
X+ OFF
Y+ ON
VREF
X– ON
Y– OFF
Y+
ON
OFF
VREF
AUX
OFF
OFF
VREF
TEMP1
OFF
OFF
VREF
Invalid Address. Test Mode: Switches out the Temp0 diode to the PENIRQ pin.
1
0
1
0
0
0
X+
Invalid Address
X+ (Z1)
Y–
0
0
0
0
1
1
1
0
1
0
0
0
A2
0
0
0
0
A1
0
0
1
1
A0
0
1
0
1
1
1
1
1
1
0
0
1
1
1
1
0
0
1
1
0
1
0
1
0
0
0
0
0
1
1
1
1
1
1
1
Analog Input
TEMP0
X+
VBAT
X+ (Z1)
OFF
ON
Y+
GND
GND
GND
GND
X+ OFF
Y+ ON
Y+
X–
X– ON
Y– OFF
Y– (Z2)
X+ OFF
Y+ ON
Y+
X–
X– ON
Y– OFF
Y+
ON
OFF
X+
X–
Outputs Identity Code, 1000 0000 0000.
Invalid address. Test mode: Switches out the Temp1 diode to the PENIRQ pin.
Internal node, not directly accessible by the user.
Rev. D | Page 14 of 28
1
AD7873
Acquisition Time
Internal Reference
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:
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 7 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, which affects the power-up time. (See
Figure 20.)
t ACQ = 8.4 × (RIN + 100 Ω ) × 37 pF
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.
X+
Y+
3-TO-1
MUX
ADC
260Ω
Touch Screen Settling
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, which
maintains the touch screen drivers on until it is commanded by
the processor to stop.
VREF
SW1
2.5V
REF
BUF
02164-D-027
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 may 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, which resolves the
issue for both single-ended and differential modes.
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 reference input in the range of 1 V to +VCC. Figure 27 shows the onchip 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 gigaOhm 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 Bits PD0 and PD1 (see
Table 6). The advantage of this mode is that the switches that
Rev. D | Page 15 of 28
AD7873
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
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.
+VCC
Y+
IN+
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 temperature 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 2-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:
VREF
X+
MEASUREMENTS
REF+
IN+ ADC CORE
IN– REF–
∆VBE = (κT / q ) × (ln N )
02164-D-028
Y–
GND
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.
where VBE represents the diode voltage, N is the bias current
multiple, k is Boltzmann’s constant and 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
(which is 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, would be calculated as follows:
∆VBE = (κT / q ) × (ln N )
T = (∆VBE × q )/(κ × ln N )
T °C = 2.49 × 10 3 × ∆VBE − 273Κ
+VCC
∆VBE is calculated from the difference in readings from the first
conversion and second conversion.
Y+
X+
IN+
Figure 30 shows a block diagram of the temperature
measurement mode.
REF+
IN+ ADC CORE
IN– REF–
TEMP0
TEMP1
GND
02164-D-029
Y–
I
105*I
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.
Rev. D | Page 16 of 28
ADC
02164-D-030
MUX
Figure 29. Differential Reference Mode (SER/DFR = 0)
Figure 30. Block Diagram of Temperature Measurement Circuit
AD7873
Battery Measurement
Pressure 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/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 5 shows the
control bit settings required to perform a battery 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, which provides 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.
+
DC/DC
CONVERTER
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 Z2position (see Figure 32). The following equation calculates the
touch resistance:
+VCC
VBAT
0V TO 1.5V
ADC CORE
7.5kΩ
RTOUCH = (RXPLATE ) × ( X POSITION / 4095) × [(Z 2 / Z1 ) − 1]
02164-D-031
2.5kΩ
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).
Figure 31. Block Diagram of Battery Measurement Circuit
The following equation also calculates the touch resistance:
RTOUCH = {(R XPLATE / Z 1 )× ( X POSITION / 4095)[(4096 / Z 1 ) − 1]}
− {RYPLATE × (YPOSITION / 4095)}
MEASURE X-POSITION
Y+
X+
TOUCH
+
–
X-POSITION
X–
MEASURE Z1-POSITION
Y+
X+
TOUCH
Y–
TOUCH
+
–
Z2-POSITION
X–
Y+
X+
+
–
Z1-POSITION
Y–
MEASURE Z2-POSITION
X–
Figure 32. Pressure Measurement Block Diagram
Rev. D | Page 17 of 28
Y–
02164-D-032
BATTERY
0V TO 6V
AD7873
PEN INTERRUPT REQUEST
user avoids any spurious interrupts on the microprocessor or
DSP, which could 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.
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
SCREEN
TOUCHED
HERE
tPEN
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.
+VCC
100kΩ
Y+
+VCC
PENIRQ
X+
TOUCH
SCREEN
PENIRQ
ENABLE
Y–
ON
Figure 33. PENIRQ Functional Block Diagram
PD1 = 1, PD0 = 0, PENIRQ
ENABLED AGAIN
NO RESPONSE TO TOUCH
PENIRQ
INTERRUPT
PROCESSOR
CS
DIN
S
8
A2
A1
A0
SER/
MODE
DFR
1
0
(START)
Figure 34. PENIRQ Timing Diagram
Rev. D | Page 18 of 28
1
13
16
02164-D-034
1
DCLK
EXTERNAL
PULL-UP
02164-D-033
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 7), 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, which 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 7) 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.
AD7873
CONTROL REGISTER
The control word provided to the ADC via the DIN pin is
shown in Table 6. This provides the conversion start, channel
addressing, ADC conversion resolution, configuration, and
power-down of the AD7873.
Table 6 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 5
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 8 bits of resolution.
converter is always the difference between the VREF and GND
pins. See Table 5 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 could 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.
SER/DFR
PD0 and PD1
The SER/DFR bit controls the reference mode, which can be
either single-ended or differential if 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
The power management options are selected by programming
the power management bits, PD0 and PD1, in the control
register. Table 7 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.
Table 6. Control Register Bit Function Description
MSB
S
LSB
A2
Bit No.
7
Mnemonic
S
6–4
A2–A0
3
MODE
2
SER/DFR
1, 0
PD1, PD0
A1
A0
MODE
SER/DFR
PD1
PD0
Comment
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.
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 5.
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.
Single-Ended/Differential Reference Select Bit. Along with Bits A2–A0, this bit controls the setting of the
multiplexer input, switches, and reference inputs as described in Table 5.
Power Management Bits. These two bits decode the power-down mode of the AD7873 as shown in Table 7.
Rev. D | Page 19 of 28
AD7873
Table 7. Power Management Options
PENIRQ
PD1
0
PD0
0
Enabled
0
1
Enabled
1
0
Enabled
1
1
Disabled
Description
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 Bits PD1 and
PD0 programmed in this way.
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 bits PD1 and PD0 programmed in this way.
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.
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.
1000
100
fDCLK = 2MHz
10
VCC = 2.7V
TA = –40°C TO +85°C
1
0
20
40
60
80
100
120
THROUGHPUT (kSPS)
02164-D-035
SUPPLY CURRENT (µA)
fDCLK = 16 × fSAMPLE
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, which means 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.
Figure 35. Supply Current vs. Throughput (µA)
Rev. D | Page 20 of 28
AD7873
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.
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 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
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 zeroes 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.
CS
tACQ
DCLK
DIN
1
S
8
A2
1
8
1
8
A0 MODE SER/
PD1 PD0
DFR
A1
(START)
DOUT
THREE-STATE
ACQUIRE
IDLE
CONVERSION
IDLE
THREE-STATE
11
10
9
8
7
6
5
4
3
2
1
(MSB)
X/Y SWITCHES1
(SER/DFR HIGH)
OFF
X/Y SWITCHES1,2
(SER/DFR LOW)
OFF
0
THREE-STATE
THREE-STATE
ZERO FILLED
(LSB)
OFF
ON
OFF
ON
NOTES
1Y DRIVERS ARE ON WHEN X+ IS SELECTED INPUT CHANNEL (A2–A0 = 001), X DRIVERS ARE ON WHEN Y+ IS SELECTED INPUT CHANNEL (A2–A0 = 101).
1WHEN PD1, PD0 = 00, 01 OR 10, Y– WILL TURN ON AT END OF CONVERSION.
2DRIVERS WILL REMAIN ON IF POWER-DOWN MODE IS 11 (NO POWER-DOWN) UNTIL SELECTED INPUT CHANNEL, REFERENCE MODE,
1OR POWER-DOWN MODE IS CHANGED, OR CS IS HIGH.
Figure 36. Conversion Timing, 24 DCLKS per Conversion Cycle, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port.
CS
t4
t1
t6
t9
t6
t10
t5
DCLK
t8
t7
PD0
t2
t11
BUSY
t12
t3
DOUT
DB11
DB10
Figure 37. Detail Timing Diagram
Rev. D | Page 21 of 28
02164-D-037
DIN
02164-D-036
BUSY
AD7873
16 Clocks per Cycle
8-Bit Conversion
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 DCLK) 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.
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 8bit mode, a conversion is complete four clock cycles earlier than
in 12-bit mode. This could be used with serial interfaces that
provide 12 clock transfers, or two conversions could be completed
with three eight-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.
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 could 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.
CS
1
DCLK
8
1
8
1
S
DIN
8
1
S
CONTROL BITS
CONTROL BITS
11
DOUT
10
9
8
7
6
5
4
3
2
1
0
11
10
02164-D-038
BUSY
9
Figure 38. Conversion Timing, 16 DCLKS per Cycle, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port.
CS
DIN
S
15
A2
A1
A0 MODE SER/ PD1 PD0
DFR
1
15
S
A2
A1
5
4
3
SER/
A0 MODE DFR PD1 PD0
1
S
A2
5
4
BUSY
DOUT
11
10
9
8
7
6
2
1
0
Figure 39. Conversion Timing, 15 DCLKS per Cycle, Maximum Throughput Rate
Rev. D | Page 22 of 28
11
10
9
8
7
6
02164-D-039
1
DCLK
AD7873
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 will ensure 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 may 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 1.2mm
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.
Rev. D | Page 23 of 28
AD7873
OUTLINE DIMENSIONS
5.10
5.00
4.90
0.193
BSC
16
9
16
4.50
4.40
4.30
0.154
BSC
1
0.236
BSC
8
9
6.40
BSC
1
8
PIN 1
PIN 1
0.069
0.053
0.065
0.049
0.010
0.025
0.004
BSC
COPLANARITY
0.004
0.012
0.008
1.20
MAX
0.15
0.05
SEATING
PLANE
8°
0°
0.010
0.006
0.20
0.09
0.30
0.19
0.65
BSC
0.050
0.016
COPLANARITY
0.10
SEATING
PLANE
8°
0°
COMPLIANT TO JEDEC STANDARDS MO-153AB
COMPLIANT TO JEDEC STANDARDS MO-137AB
Figure 41. 16-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-16)
Dimensions shown in millimeters
Figure 40. 16-Lead Shrink Small Outline Package [QSOP]
(RQ-16)
Dimensions shown in inches
4.0
BSC SQ
0.60 MAX
0.65 BSC
PIN 1
INDICATOR
TOP
VIEW
13
12
3.75
BSC SQ
16
1
BOTTOM
VIEW
0.75
0.60
0.50
12° MAX
PIN 1
INDICATOR
0.60 MAX
4
9
8
5
0.25 MIN
1.95 BSC
0.80 MAX
0.65 TYP
0.05 MAX
0.02 NOM
1.00
0.85
0.80
SEATING
PLANE
0.35
0.28
0.25
0.20 REF
COPLANARITY
0.08
COMPLIANT TO JEDEC STANDARDS MO-220-VGGC
Figure 42. 16-Lead Lead Frame Chip Scale Package [LFCSP]
(CP-16)
Dimensions shown in millimeters
Rev. D | Page 24 of 28
2.25
2.10 SQ
1.95
0.75
0.60
0.45
AD7873
ORDERING GUIDE
Model
AD7873ARQ
AD7873ARQ-REEL
AD7873ARQ-REEL7
AD7873BRQ
AD7873BRQ-REEL
AD7873BRQ-REEL7
AD7873ARU
AD7873ARU-REEL
AD7873ARU-REEL7
AD7873ARUZ-REEL3
AD7873ARUZ-REEL73
AD7873ACP
AD7873ACP-REEL
AD7873ACP-REEL7
AD7873ACPZ3
AD7873ACPZ-REEL3
AD7873ACPZ-REEL73
AD7873BCP
AD7873BCP-REEL
AD7873BCP-REEL7
EVAL-AD7873CB4
EVAL-CONTROL BRD25
Temperature Range
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
Linearity Error (LSB)1
±2
±2
±2
±1
±1
±1
±2
±2
±2
±2
±2
±2
±2
±2
±2
±2
±2
±1
±1
±1
1
Package Option2
RQ-16
RQ-16
RQ-16
RQ-16
RQ-16
RQ-16
RU-16
RU-16
RU-16
RU-16
RU-16
CP-16
CP-16
CP-16
CP-16
CP-16
CP-16
CP-16
CP-16
CP-16
Evaluation Board
Controller Board
Linearity Error here refers to integral linearity error.
RQ = QSOP = 0.15” Quarter Size Outline Package; RU = TSSOP.
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.
2
3
Rev. D | Page 25 of 28
AD7873
NOTES
Rev. D | Page 26 of 28
AD7873
NOTES
Rev. D | Page 27 of 28
AD7873
NOTES
© 2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C02164–0–6/04(D)
Rev. D | Page 28 of 28