Low Voltage Controller for Touch Screens
AD7879/AD7889
Rev. C
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FEATURES
4-wire touch screen interface
1.6 V to 3.6 V operation
Median and averaging filter to reduce noise
Automatic conversion sequencer and timer
User-programmable conversion parameters
Auxiliary analog input/battery monitor (0.5 V to 5 V)
1 optional GPIO
Interrupt outputs (INT, PENIRQ)
Touch-pressure measurement
Wake-up on touch function
Shutdown mode: 6 μA maximum
12-ball, 1.6 mm × 2 mm WLCSP
16-lead, 4 mm × 4 mm LFCSP
APPLICATIONS
Personal digital assistants
Smart handheld devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones
FUNCTIONAL BLOCK DIAGRAM
REF+
X– Y– X+ Y+
FILTERING
RESULT
REGISTERS
CONTROL
REGISTERS
SEQUENCER
AND TI M E R
REF–
TEMPERATURE
SENSOR
TO
RESULT
REGISTERS
6-TO- 1 MUX
AD7879/
AD7879-1/
AD7889/
AD7889-1
CC
RE
X+
X–
Y+
Y–
GND
AUX/VBAT/GPIO
PENIRQ/INT/DAV
SERIAL PORT
DIN/
ADD1 DOUT/
SDA SCLCS/
ADD0
12-BIT
SAR ADC
REF–
07667-001
Figure 1.
GENERAL DESCRIPTION
The AD7879/AD7889 are 12-bit successive approximation
analog-to-digital converters (SAR ADCs) with a synchronous
serial interface and low on-resistance switches for driving 4-wire
resistive touch screens. The AD7879/AD7889 work with a very
low power supply—a single 1.6 V to 3.6 V supply—and feature
throughput rates of 105 kSPS. The devices include a shutdown
mode that reduces current consumption to less than 6 μA.
To reduce the effects of noise from LCDs and other sources, the
AD7879/AD7889 contain a preprocessing block. The prepro-
cessing function consists of a median filter and an averaging
filter. The combination of these two filters provides a more robust
solution, discarding the spurious noise in the signal and keeping
only the data of interest. The size of both filters is programmable.
Other user-programmable conversion controls include variable
acquisition time and first conversion delay; up to 16 averages
can be taken per conversion. The AD7879/AD7889 can run in
slave mode or standalone (master) mode, using an automatic
conversion sequencer and timer.
The AD7879/AD7889 have a programmable pin that can operate
as an auxiliary input to the ADC, as a battery monitor, or as a
GPIO. In addition, a programmable interrupt output can operate
in three modes: as a general-purpose interrupt to signal when
new data is available (DAV), as an interrupt to indicate when
limits are exceeded (INT), or as a pen-down interrupt when the
screen is touched (PENIRQ). The AD7879/AD7889 offer temp-
erature measurement and touch-pressure measurement.
The AD7879 is available in a 12-ball, 1.6 mm × 2 mm WLCSP
and in a 16-lead, 4 mm × 4 mm LFCSP. The AD7889 is available
in a backside-coated version of the WLCSP. Both parts support
an SPI interface (AD7879/AD7889) or an I2C interface
(AD7879-1/AD7889-1).