0～20 mA电流输出回路设计及其误差校正

0～20 mA current output circuit design and its error calibration

在工业现场,0～20 mA模拟电流输出回路不受传输线路阻抗的影响、具有较强的抗干扰能力,因而得到广泛的应用。LM231是一款具有频率电压转换功能的芯片,可以实现模拟电流0～20 mA输出,电路简单并且精度高。电路包括输出可变频率信号的单片机、光耦隔离电路、频率电压转换电路LM231、放大电路和电压电流转换电路。由于温度、电子元器件误差等因素影响,实际输出往往与理想值存在一定偏差,这种偏差可以用最小二乘法来校正。即对电路采样一组数据、计算处理采样所得数据,取偏差的平方和为最小值,得到一个最佳的拟合曲线。经过这个校正过程后大多数误差基本消除,电路的输出精度得到了明显提高。

In an industrial environment, the output of a 0~20 mA analog circuit is unaffected by transmission line impedance, and remains robust against electromagnetic interferences, which lead to its widespread use. LM231 is a Frequency-to-Voltage converter chip which realizes an analog output of 0~20 mA, the circuit achieves the characteristics of simple hardware and high precision. The circuit consists of a MCU which outputs a variable frequency signal, an opto-coupler, a frequency-to-voltage converter LM231, an amplifier and a voltage-to-current converter. Due to thermal effects and errors in the electronic parts, the actual output of LM231 suffers deviations from the theoretical value. Such deviations can be calibrated by the least square method. By sampling a set of data from the circuit, minimizing the sum of squared deviations of sampled data, a best fit curve is obtained. It is shown that accuracy of the output is remarkably improved after this calibration process.