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AD7877 Scheda tecnica(PDF) 15 Page - Analog Devices |
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AD7877 Scheda tecnica(HTML) 15 Page - Analog Devices |
15 / 46 page AD7877 Data Sheet CIRCUIT INFORMATION The AD7877 is a complete, 12-bit data acquisition system for digitizing positional inputs from a touch screen in PDAs and other devices. In addition, it can monitor two battery voltages, ambient temperature, and three auxiliary analog voltages, with high and low limit comparisons on three of the inputs, and has up to four general-purpose logic I/O pins. The core of the AD7877 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. The results of conversions are stored in 11 results registers, and the results from one auxiliary input and two battery inputs can be com- pared with high and low limits stored in limit registers to generate an out-of-limit ALERT. The AD7877 also contains low resistance analog switches to switch the X and Y excitation voltages to the touch screen, a STOPACQ pin to control the ADC acquisition period, 2.5 V reference, on-chip temperature sensor, and 8-bit DAC to control LCD contrast. The high speed SPI serial bus provides control of, and communication with, the device. Operating from a single supply from 2.2 V to 5 V, the AD7877 offers throughput rates of up to 125 kHz. The device is available in a 5 mm × 5 mm, 32-lead, lead frame chip scale package (LFCSP), and in a 2.5 mm × 2.8 mm, wafer level chip scale package (WLCSP), with a 5 × 5 ball grid array. The data acquisition system of the AD7877 has a number of advanced features: • Input channel sequenced automatically or selected by the host. • STOPACQ feature to reduce noise from LCD. • Averaging of from 1 to 16 conversions for noise reduction. • Programmable acquisition time. • Power management. • Programmable ADC power-up delay before first conversion. • Choice of internal or external reference. • Conversion at preprogrammed intervals. 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 27. 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 AD7877 is designed to facilitate this measurement. Figure 28 shows an equivalent circuit of the analog input structure of the AD7877, showing the touch screen switches, the main analog multiplexer, the ADC with analog and differential reference inputs, and the dual 3-to-1 multiplexer that selects the reference source for the ADC. Rev. E | Page 14 of 45 |
Codice articolo simile - AD7877_17 |
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Descrizione simile - AD7877_17 |
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