延長AD7745電容輸入范圍/ AD7746電容數字轉換器

　　本文將討論一個電路，延長了AD7745電容輸入范圍/ AD7746提供了一種方法。如何使用片上CAPDAC充分為了減少范圍擴展的因素，因此，對電路進行了優化，以達到最佳的性能說明。AD7745擁有一個電容輸入通道，AD7746則擁有兩個通道。每個通道可以配置為單端或差分。

　　電路描述

　　the AD7745 /數字轉換器AD7746電容-電容by using開關電容技術測量電路負載平衡to build up a。你是比例to the product of負載電壓和電容C，Q = V×轉換比率，the result represents the之間的輸入電容，csens，and the internal參考電容，CREF，as the激勵電壓參考電壓VREF excx and the內部有固定的已知值。

　　The range extension circuit has to ensure that the charge transfer within the sensing capacitance CSENS remains within the input range of the AD7745/AD7746. To achieve this， the excitation voltage needs to be decreased by a factor of F， so that the sensing capacitance connected to the input can be increased by a factor F. The AD7745/AD7746 has two independent excitation sources EXCA and EXCB. For the range extension， the excitation sources have to be set up in a way that EXCB is the inverse of EXCA. With resistor R1 and R2 connected as shown in Figure 1， the resulting range extension factor F is the ratio of the AD7746 excitation voltage between EXCA and EXCB （VEXC（A?B）） and the attenuated excitation signal （VEXCS） at the positive input of the AD8515 op amp. The range extension factor F can be calculated as follows：

　　By using both excitation sources， the mean voltage of the attenuated excitation voltage EXCS remains around VDD/2.

　　The AD8515 operational amplifier in the circuit functions as a low impedance source to ensure the sensing capacitance CSENS is fully charged when the AD7745/AD7746 starts sampling.