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AD7879 Scheda tecnica(PDF) 13 Page - Analog Devices

Il numero della parte AD7879
Spiegazioni elettronici  Low Voltage Controller for Touch Screens
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Produttore elettronici  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7879 Scheda tecnica(HTML) 13 Page - Analog Devices

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Data Sheet
AD7879/AD7889
Rev. D | Page 13 of 40
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 ADC with an input multiplexer, on-chip track-and-hold,
and an on-chip clock. Conversion results are stored in on-chip
result registers. Compare the results from the auxiliary input or
the battery input with high and low limits stored in the 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 communication
with the devices. The AD7879-1/AD7889-1 are available with an
I2C interface.
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
available 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 is able to select the acquisition time. A
programmable delay ensures that the voltage on the touch
screen settles before a measurement is taken.
To 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. If there is noise present in the system, use the
median filter to improve the performance of the AD7879/
AD7889.
TOUCH SCREEN PRINCIPLES
A 4-wire touch screen consists of two flexible, transparent,
resistive coated layers typically 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.
X+
X–
Y–
Y+
CONDUCTIVE ELECTRODE
ON BOTTOM SIDE
PLASTIC FILM WITH
TRANSPARENT, RESISTIVE
COATING ON BOTTOM SIDE
PLASTIC FILM WITH
TRANSPARENT, RESISTIVE
COATING ON TOP SIDE
LCD SCREEN
CONDUCTIVE ELECTRODE
ON TOP SIDE
Figure 22. Basic Construction of a Touch Screen
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
facilitate this measurement.


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