AD7879 datasheet(13/40 Pages) AD

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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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