AD7889 Analog Devices, AD7889 Datasheet - Page 13

• Manufacturer Part Number: AD7889
• Description: Low Voltage Controller for Touch Screens
• Manufacturer: Analog Devices

Specifications of AD7889

• Resolution (bits): 12bit
• # Chan: 6
• Sample Rate: 105kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni Vdd
• Adc Architecture: SAR
• Pkg Type: CSP

THEORY OF OPERATION
The AD7879/AD7889 are a complete 12-bit data acquisition
system for digitizing positional inputs from a 4-wire resistive
touch screen. To support this function, data acquisition on the
AD7879/AD7889 is highly programmable to ensure accurate
and noise-free results from the touch screen.
The core of the AD7879/AD7889 is a high speed, low power,
12-bit analog-to-digital converter (ADC) with an input multi-
plexer, on-chip track-and-hold, and on-chip clock. Conversion
results are stored in on-chip result registers. The results from
the auxiliary input or the battery input can be compared with
high and low limits stored in limit registers to generate an out-
of-limit interrupt ( INT ).
The AD7879/AD7889 also contain low resistance analog
switches to switch the X and Y excitation voltages to the touch
screen and to the on-chip temperature sensor. The high speed
SPI serial bus provides control of the devices, as well as com-
munication with the devices. The AD7879-1/AD7889-1 are
available with an I
Operating from a single supply from 1.6 V to 3.6 V, the AD7879/
AD7889 offer a throughput rate of 105 kHz. The device is avail-
able in a 1.6 mm × 2 mm, 12-ball wafer level chip scale package
(WLCSP) and in a 4 mm × 4 mm, 16-lead lead frame chip scale
package (LFCSP).
The AD7879/AD7889 have an on-chip sequencer that schedules
a sequence of preprogrammed conversions. The conversion
sequence starts automatically when the screen is touched or
at preset intervals, using the on-board timer.
To ensure that the AD7879/AD7889 work well with different
touch screens, the user can select the acquisition time. A
programmable delay ensures that the voltage on the touch
screen settles before a measurement is taken.
To help reduce noise in the system, the ADC takes up to 16
conversion results from each channel and writes the average of
the results to the register. To further improve the performance
of the AD7879/AD7889, the median filter can also be used if
there is noise present in the system.
TOUCH SCREEN PRINCIPLES
A 4-wire touch screen consists of two flexible, transparent,
resistive-coated layers that are normally separated by a small
air gap (see Figure 22). The X layer has conductive electrodes
running down the left and right edges, allowing the application
of an excitation voltage across the X layer from left to right.
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C interface.
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The Y layer has conductive electrodes running along the top
and bottom edges, allowing the application of an excitation
voltage down the Y layer from top to bottom.
Provided that the layers are of uniform resistivity, the voltage
at any point between the two electrodes is proportional to the
horizontal position for the X layer and the vertical position for
the Y layer.
When the screen is touched, the two layers make contact. If
only the X layer is excited, the voltage at the point of contact
and, therefore, the horizontal position, can be sensed at one of
the Y layer electrodes. Similarly, if only the Y layer is excited,
the voltage and, therefore, the vertical position, can be sensed
at one of the X layer electrodes. By switching alternately
between X and Y excitation and measuring the voltages, the
X and Y coordinates of the contact point can be determined.
In addition to measuring the X and Y coordinates, it is also
possible to estimate the touch pressure by measuring the
contact resistance between the X and Y layers. The AD7879/
AD7889 are designed to facilitate this measurement.
CONDUCTIVE ELECTRODE
CONDUCTIVE ELECTRODE
ON BOTTOM SIDE
ON TOP SIDE
Figure 22. Basic Construction of a Touch Screen
LCD SCREEN
Y+
X+
Y–
TRANSPARENT, RESISTIVE
AD7879/AD7889
COATING ON BOTTOM SIDE
TRANSPARENT, RESISTIVE
COATING ON TOP SIDE
PLASTIC FILM WITH
PLASTIC FILM WITH
X–