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AD7873 Datasheet(PDF) 17 Page - Analog Devices |
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AD7873 Datasheet(HTML) 17 Page - Analog Devices |
17 / 29 page AD7873 Data Sheet Rev. F | Page 16 of 28 the AD7873 is approximately 70 Ω. In this mode of operation, therefore, some voltage is likely to be lost across the internal switches, and it is unlikely that the internal switch resistance will track the resistance of the touch screen over temperature and supply, providing an additional source of error. Figure 28. Single-Ended Reference Mode (SER/DFR = 1) The alternative to this situation is to set the SER/DFR bit low. Again, making a Y coordinate measurement is considered, but now the +REF and –REF nodes of the ADC are connected directly to the Y+ and Y– pins. This means the analog-to-digital conversion is ratiometric. The result of the conversion is always a percentage of the external resistance, independent of how it could change with respect to the on resistance of the internal switches. Figure 29 shows the configuration for a ratiometric Y coordinate measurement. Figure 29. Differential Reference Mode (SER/DFR = 0) The disadvantage of this mode of operation is that during both the acquisition phase and conversion process, the external touch screen must remain powered. This results in additional supply current for the duration of the conversion. MEASUREMENTS Temperature Measurement Two temperature measurement options are available on the AD7873, the single conversion method and the differential conversion method. Both methods are based on an on-chip diode measurement. In the single conversion method, a diode voltage is digitized and recorded at a fixed calibration temperature. Any subsequent polling of the diode provides an estimate of the ambient tempera- ture through extrapolation from the calibration temperature diode result. This assumes a diode temperature drift of approximately –2.1 mV/°C. This method provides a resolution of approximately 0.3°C and a predicted accuracy of ±3°C. The differential conversion method is a two-point measurement. The first measurement is performed with a fixed bias current into a diode, and the second measurement is performed with a fixed multiple of the bias current into the same diode. The voltage difference in the diode readings is proportional to absolute temperature and is given by the following formula: ( ) ( ) N q kT VBE ln / × = ∆ where: VBE represents the diode voltage. N is the bias current multiple. k is Boltzmann’s constant. q is the electron charge. This method provides more accurate absolute temperature measurement of ±2°C. However, the resolution is reduced to approximately 1.6°C. Assuming a current multiple of 105 (typical for the AD7873) taking Boltzmann’s constant, k = 1.38054 ×10–23 electrons volts/degrees Kelvin, the electron charge q = 1.602189 × 10–19, then T, the ambient temperature in degrees centigrade, can be calculated as follows: ( ) ( ) ( ) ( ) Κ 273 10 49 . 2 ) C ( ln / ln / 3 / ∆ × × = ° × × ∆ = × = ∆ BE BE BE V T N k q V T N q kT V where ∆VBE is calculated from the difference in readings from the first conversion and second conversion. Figure 30 shows a block diagram of the temperature measurement mode. Figure 30. Block Diagram of Temperature Measurement Circuit +VCC VREF GND Y+ Y– X+ IN+ IN+ IN– REF+ ADC CORE REF– +VCC GND Y+ Y– X+ IN+ IN+ IN– REF+ ADC CORE REF– I TEMP0 TEMP1 105 × I MUX ADC |
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