Design of Linear Photoelectric Isolation Electronic Circuit

　　Photoelectric isolation is an important measure for data acquisition and control system anti-interference. Due to the nonlinearity of photoelectric coupling devices , photoelectric isolation of analog quantities will cause greater signal distortion. In order to improve the linearity of the photoelectric isolation circuit, the output current of the photocoupler device is fed back to the input end by negative feedback. The static characteristic test of the photoelectric isolation circuit is carried out.

　　Deep negative feedback circuit design, using deep negative feedback can improve the linearity of the system. According to this principle, an analog signal optoelectronic isolation circuit is designed here. One LP521 optocoupler and two LM2904 integrated operational amplifiers are used in this circuit. LP521 has two optocouplers, one for signal conversion and one for negative feedback; operational amplifier 2904, 1A is used to form a negative feedback circuit. The operational amplifier 2904 constitutes a voltage follower to increase the circuit's load capacity. The analog signal optoelectronic isolation circuit is shown in Figure 3.

　　The circuit output voltage and input voltage are in a linear relationship: the light-emitting diodes of the two optocoupler channels of LP521 are connected in series, and the current If1 through the two diodes is If2. Because the two optocouplers are packaged in one device, the photoelectric characteristics are basically the same. Ideally, the collector output currents of the two optocouplers can be considered equal, that is, Ice1=Ice2.

　　According to the properties of the ideal op amp, the following formula can be obtained:

　　We=Ice2R1（1）

　　Vo=Ice1R2 （2）

　　So Vo = (R2/R1) Vi, the output voltage is proportional to the input voltage, and the proportionality coefficient is determined by R1 and R2.