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AD7879 Scheda tecnica(PDF) 13 Page - Analog Devices |
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AD7879 Scheda tecnica(HTML) 13 Page - Analog Devices |
13 / 40 page 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. |
Codice articolo simile - AD7879_17 |
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Descrizione simile - AD7879_17 |
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