用PWM实现廉价隔离型标准电流输出D/A转换器

D/A Convertor Circuit Design for a Low Cost & Isolated Standard Output Current Using PWM Method

分析了将脉宽调制 (PWM)波形转换为模拟电压输出的 D/ A转换器基本理论 ,指出通过一个低通滤波器可以把经 PWM的数模信号解调出来 ,给出了滤波器设计的参数选择方法 ,分析了 D/ A转换器分辨率和 PWM参数的关系。在此基础上设计出简单的电压输出 D/ A转换器 ,该电路适合精度要求比较低的场合。在进一步分析该电路的基础上 ,设计了光电隔离、基准电压、I- V变换和功率放大等电路 ,得到了 0 m A～ 10 m A和 4 m A～ 2 0 m A模拟电流输出电路 ,该电路具有较高的精度和较大的负载驱动能力 ,可以应用于工业过程仪表的设计中。

This paper presents the basic theory of transforming PWM to analog voltage D/A convertor and points out that digital to analog signal can be demodulated from PWM signal by using a low pass filter. This paper also describes how to choose the parameters of the low pas filter and analyses the relationship between the resolution of D/A convertor and the parameters of PWM. A simple voltage output D/A convertor is designed basing on this theory,and the resultant circuit is suitable for the case of low resolution. By further analysing this D/A convertor circuit,a current output circuit of 0 mA～10 mA and 4 mA～20 mA is designed after adding optical isolation,reference voltage,I-V transformer and power amplifier. This circuit now has higher resolution and stronger load driving ability. It is applicable for use in industrial process instruments.