延长AD7745电容输入范围/ AD7746电容数字转换器

　　延长AD7745电容输入范围/ AD7746电容数字转换器

　　本文将讨论一个电路，延长了AD7745电容输入范围/ AD7746提供了一种方法。如何使用片上CAPDAC充分为了减少范围扩展的因素，因此，对电路进行了优化，以达到最佳的性能说明。AD7745拥有一个电容输入通道，AD7746则拥有两个通道。每个通道可以配置为单端或差分。

　　电路描述

　　the AD7745 /数字转换器AD7746电容-电容by using开关电容技术测量电路负载平衡to build up a。你是比例to the product of负载电压和电容C，Q = V×转换比率，the result represents the之间的输入电容，csens，and the internal参考电容，CREF，as the激励电压参考电压VREF excx and the内部有固定的已知值。

　　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 factorof F， so that the sensing capacitance connected to the inputcanbe 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.