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.