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摘
本文设计了一种NA200PLC的RTD扩展模块，主要运用工业现场对温度传感器信号的采集和处理。本文分析了A/D芯片CS5513的原理，设计了RTD信号采集的硬件电路与软件实现流程。本模块利用CS5513这款20位高精度Δ-ΣADC进行多通道RTD信号的采集和处理。经过实践验证，该模块具有成本低、实用性强、精度高、可靠性高等优点。
关键词：RTD；Δ-Σ；总线；N200PLC；20位A/D转换器。
Abstract: A RTD expansion module of NAPLC is designed in this article, which is mainly used in temperature sensor signal acquisition and processing in the industrial field. This paper describes the principle of CS5513, ha***are circuits, and the main software flowcharts.The module uses the Δ-Σ ADC CS5513 of 24 bit high precision to collect and process the RTD signal in multi-channel. After practice verification, this module has some advantages, such as low-cost, high practicability,high precision high reliability and so on.
Key wo***: RTD；Δ-Σ；Bus；N200PLC；20 bit A/D converter。
1 引言
工业控制行业中需要对现场模拟信号进行采集和处理，一般用于工业控制中的基本模拟信号采集（AI）模块有三类：电压或电流型模拟量信号输入模块、热电阻（RTD）输入模块、热电偶（TC）输入模块。其中热电阻是工业控制现场用来测量中低温度区***常用的一种温度检测传感器。它的主要特点是有较高的测量精度和稳定的工作性能。
本文主要设计了一款基于Δ-Σ 型ADC芯片CS5513的四通道RTD信号采集模块，该模块是作为南大傲拓公司研制的小型PLC NA200CPU的扩展模块，主要是将经过信号调理的四路RTD信号通过ADC芯片CS5513转换成数字量，并通过隔离的总线上送给NA200CPU模块。
2 总体设计
图1为NA200PLC的RTD扩展模块NA200AIM201-0403的总体设计框图，该模块由输入信号检测电路、通道选通电路、恒流源电路、信号调理电路、转换电路、通讯电路、电源转换电路、CPU电路等组成。模块通过接线端子引入四路现场RTD信号，当检测到某通道有RTD信号输入时选通相应通道，利用自制高精度恒流源电路产生一个高精度的恒定电流，采集通道RTD电阻上的电压，通过信号调理电路对信号进行放大、滤波，经过CS5513进行模数转换后通过隔离的串行总线送入CPU单元，CPU电路负责将采集的RTD数据通过NA200的高速内部总线上传至NA200CPU模块，从而完成RTD信号的采集。
图1 总体设计框图
3 A/D芯片选用和原理
3.1 A/D芯片选用
对于模拟量输入采集模块，AD转换芯片的选用是关键，鉴于现在小型PLC市场竞争日益激烈的现状，不仅要满足精度要求、功耗、可靠性等基本因素，还要考虑多通道集成及低成本设计等环节，为客户带来高性价比的产品。本次设计采用的AD芯片CS5513是Cirrus Logic公司生产的20位串行输出Δ-Σ 模数转换芯片，它***大的优点就是具有20位采样精度且成本较低，其8脚SOIC封装形式可以***大限度的节省设计空间；同时片上自带振荡器，无需外加时钟源，使AD外部电路设计更加的简单。该产品包含一个4阶的Δ-Σ 调制器和一个滤波器，可有效地***线性噪声。可以使用5V的电源进行供电，也可以使用其他多种双电源的配置，是将工业控制中的双极性信号数字化的理想选择。
3.2 CS5513的工作原理
如图2所示，CS5513芯片内部自带片上的振荡器频率为64KHz，上电后振荡器和时钟门就开始工作，同时CS5513也开始循环进行模数转换工作更新输出寄存器。CS5513内部有一个数字滤波器可以提供***高为107Hz的转换速率，该数字滤波器对连续4个转换周期的转换值进行取平均值后输出，所以在使用该芯片时应注意以下两点：***是在初次上电之后或者从休眠状态返回之后，该滤波器需要4个转换周期才能得到***个有效的转换值；第二是在对多路模拟信号进行转换时，在进行通道切换后，必须经过4个转换周期才能得到有效的转换值。所以在上电或者是切换后执行采样程序时要对前四次的采样值要进行滤波。
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