智能儀表中的精確功率測量。第2部分：元件的指定

　　為滿足先進的計費和能源管理服務的需求，MCU驅動今天的智能電表和分戶計量系統采用高精度的模擬前端（AFE）電路和復雜的數據處理軟件。這兩部分系列的第一部分概述了現代儀表中使用的基本功能需求和體系結構方法。這一部分將深入討論指定組件的一些設計問題，這些組件確保智能儀表將為其目標應用程序提供必要的精確性和性能。

　　前端的基礎知識

　　智能電表的能量測量前端的基本要素是電流傳感器、模數轉換器（ADC），以及專用MCU或儀表主機處理器用來解釋原始數據的算法（圖1）。

　　Resistive shunts have traditionally been the most commonly-used current sensing technique for residential and other low-to-medium-power applications. Shunt-based sensors use a low-value resistor （typically 100 μ？ to 500 m？） to produce a small but reliable voltage drop from which the current is calculated. Although they are simple， inexpensive， and relatively reliable， Ohm’s Law dictates that shunt-based current sensors have a limited upper current range due to self-heating and power loss. Advances in the accuracy and dynamic range of ADCs have made it possible to use lower shunt resistances， but they remain impractical for larger commercial and industrial applications. Unlike the inductively based sensors that will be discussed next， resistive shunts require separate isolator elements if they are used in poly-phase systems. Nevertheless， shunt-based sensors’ lower cost and relative immunity against tampering techniques that use a DC magnetic field， still make them popular for residential applications.