AD7879 datasheet(12/36 Pages) AD

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AD7879

Rev. 0 | Page 12 of 36

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 I2C interface.

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.

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 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.

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