ad7879-1acbz-rl Analog Devices, Inc., ad7879-1acbz-rl Datasheet - Page 12

• Manufacturer Part Number: ad7879-1acbz-rl
• Description: Low Voltage Controller For Touch Screens
• Manufacturer: Analog Devices, Inc.

AD7879
THEORY OF OPERATION
The AD7879 is 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 is
highly programmable so as to ensure accurate and noise free
results from the touch screen.
The core of the AD7879 is a high speed, low power, 12-bit
analog-to-digital converter (ADC) with input multiplexer,
on-chip track-and-hold, and on-chip clock. Conversion
results are stored in on-chip results 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 INT .
The AD7879 also contains low resistance analog switches to
switch the X and Y excitation voltages to the touch screen and
the on-chip temperature sensor. The high speed SPI serial bus
provides control of the devices, as well as communication with
the device. The AD7879-1 is available with an I
Operating from a single supply from 1.6 V to 3.6 V, the AD7879
offers 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.
The AD7879 has 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 works well with different touch
screens, the user can select the acquisition time. There is also a
programmable delay to ensure 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 perfor-
mance of the AD7879, 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. 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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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 found.
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
is 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
COATING ON BOTTOM SIDE
COATING ON TOP SIDE
TRANSPARENT, RESISTIVE
PLASTIC FILM WITH
PLASTIC FILM WITH
X–