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AD7879 データシート(PDF) 14 Page - Analog Devices |
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AD7879 データシート(HTML) 14 Page - Analog Devices |
14 / 40 page AD7879/AD7889 Data Sheet Rev. D | Page 14 of 40 Figure 23 shows an equivalent circuit of the analog input structure of the AD7879/AD7889, including the touch screen switches, the main analog multiplexer, the ADC, and the dual 3 to 1 multiplexer that selects the reference source for the ADC. AUX/VBAT/GPIO 12-BIT SUCCESSIVE APPROXIMATION ADC WITH TRACK-AND-HOLD INPUT MUX TEMPERATURE SENSOR Y– Y+ X– X+ VCC REF– IN+ REF+ DUAL 3-TO-1 MUX X– Y– GND X+ Y+ VCC Figure 23. Analog Input Structure The AD7879/AD7889 can be set up to automatically convert either specific input channels or a sequence of channels. The results of the ADC conversions are stored in the result registers. When measuring the ancillary analog inputs (AUX, TEMP, or VBAT), the ADC uses a VCC reference and the measurement is referred to GND. MEASURING TOUCH SCREEN INPUTS When measuring the touch screen inputs, it is possible to use VCC as a reference or instead to use the touch screen excitation voltage as the reference and to perform a ratiometric, differential measurement. The differential method is the default method and is selected by clearing the SER/DFR bit (Bit 9 in Control Register 2) to 0. The single-ended method is selected by setting this bit to 1. Single-Ended Method Figure 24 shows the single-ended method for the Y position. For the X position, the excitation voltage is applied to X+ and X− and the voltage is measured at Y+. ADC REF+ INPUT (VIA MUX) X+ REF– TOUCH SCREEN Y+ Y– GND VREF VCC Figure 24. Single-Ended Conversion of Touch Screen Inputs The voltage seen at the input to the ADC in Figure 24 is YTOTAL Y CC IN R R V V (1) The advantage of the single-ended method is that the touch screen excitation voltage is switched off when the signal is acquired. Because a screen can draw over 1 mA, this is a significant consideration for a battery-powered system. The disadvantage of the single-ended method is that voltage drops across the switches can introduce errors. Touch screens can have a total end-to-end resistance ranging from 200 Ω to 900 Ω. By taking the lowest screen resistance of 200 Ω and a typical switch resistance of 14 Ω, the user can reduce the apparent excitation voltage to 200/228 × 100 = 87% of its actual value. In addition, the voltage drop across the low-side switch adds to the ADC input voltage. This introduces an offset into the input voltage; thus, it can never reach 0. Ratiometric Method The ratiometric method illustrated in Figure 25 shows the negative input of the ADC reference connected to Y− and the positive input connected to Y+. Thus, the screen excitation voltage provides the reference for the ADC. The input of the ADC is connected to X+ to determine the Y position. ADC REF+ INPUT (VIA MUX) REF– VCC X+ TOUCH SCREEN Y+ Y– GND Figure 25. Ratiometric Conversion of Touch Screen Inputs For greater accuracy, the ratiometric method has two significant advantages. One is that the reference to the ADC is provided from the actual voltage across the screen; therefore, any voltage dropped across the switches has no effect. The other advantage is that because the measurement is ratiometric, it does not matter if the voltage across the screen varies in the long term. However, it must not change after the signal has been acquired. The disadvantage of the ratiometric method is that the screen must be powered up at all times because it provides the reference voltage for the ADC. |
同様の部品番号 - AD7879_17 |
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同様の説明 - AD7879_17 |
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