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AD7879 データシート(PDF) 15 Page - Analog Devices |
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AD7879 データシート(HTML) 15 Page - Analog Devices |
15 / 40 page Data Sheet AD7879/AD7889 Rev. D | Page 15 of 40 TOUCH PRESSURE MEASUREMENT The pressure applied to the touch screen by a pen or a finger can also be measured with the AD7879/AD7889 by using simple calculations. The contact resistance between the X and Y plates is measured, providing a good indication of the size of the depressed area and, therefore, the applied pressure. The area of the spot that is touched is proportional to the size of the object touching it. The size of this resistance (RTOUCH) can be calculated using two different methods. First Method for Calculating the Size of RTOUCH The first method for calculating RTOUCH requires the user to know the total resistance of the X-plate tablet (RX). Three touch screen conversions are required: the measurement of the X position, XPOSITION (Y+ input); the measurement of the X+ input with the excitation voltage applied to Y+ and X− (Z1 measurement); and the measurement of the Y− input with the excitation voltage applied to Y+ and X− (Z2 measurement). These three measure- ments are shown in Figure 26. The AD7879/AD7889 has two special ADC channel settings that configure the X and Y switches for the Z1 and Z2 measurements and store the results in the Z1 and Z2 result registers. The Z1 measurement is selected by setting the CHNL ADD[2:0] bits to 101 in Control Register 1 (Address 0x01); the result is stored in the X+ (Z1) result register (Address 0x0A). The Z2 measurement is selected by setting the CHNL ADD[2:0] bits to 100 in Control Register 1 (Address 0x01); the result is stored in the Y− (Z2) result register (Address 0x0B). The touch resistance (RTOUCH) can then be calculated using Equation 2: RTOUCH = (RXPLATE) × (XPOSITION/4096) × ((Z2/Z1) − 1) (2) Y– Y+ X– X+ TOUCH RESISTANCE MEASURE Z1 POSITION X– X+ Y– Y+ TOUCH RESISTANCE MEASURE X POSITION Y– Y+ X– X+ TOUCH RESISTANCE MEASURE Z2 POSITION Figure 26. Three Measurements Required for Touch Pressure Second Method for Calculating the Size of RTOUCH The second method for calculating RTOUCH requires the user to know the resistance of the X-plate and Y-plate tablets. Three touch screen conversions are required: a measurement of the X position (XPOSITION), the Y position (YPOSITION), and the Z1 position. Equation 3 also calculates the touch resistance (RTOUCH): RTOUCH = RXPLATE × (XPOSITION/4096) × ((4096/Z1) − 1) − RYPLATE × (1 − (YPOSITION/4096)) (3) TEMPERATURE MEASUREMENT A temperature measurement option called the single-conversion method is available on the AD7879/AD7889. The conversion method requires only a single measurement on ADC Channel 001. The results are stored in the temperature conversion result register (Address 0x0D). The AD7879/AD7889 do not provide an explicit output of the temperature reading; the system must perform some external calculations. This method is based on an on-chip diode measurement. The acquisition time is fixed at 16 ms for temperature measurement. Conversion Method The conversion method makes use of the fact that the tempera- ture coefficient of a silicon diode is approximately −2.1 mV/°C. However, this small change is superimposed on the diode forward voltage, which can have a wide tolerance. Therefore, it is necessary to calibrate by measuring the diode voltage at a known temperature to provide a baseline from which the change in forward voltage with temperature can be measured. This method provides a resolution of approximately 0.3°C and a predicted accuracy of ±2°C. The temperature limit comparison is performed on the result in the temperature conversion result register (Address 0x0D), which is the measurement of the diode forward voltage. The values programmed into the high and low limits should be referenced to the calibrated diode forward voltage to make accurate limit comparisons. |
同様の部品番号 - AD7879_17 |
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同様の説明 - AD7879_17 |
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