Photoelectric isolation circuit design plan

New circuit principle

Figure 1 shows the principle circuit of an isolation amplifier. This isolation amplifier circuit is mainly composed of an optocoupler and an operational amplifier. The optocoupler is the ordinary optocoupler TLP521, and the operational amplifier is the general operational amplifier LF353. Through the combination of these two common devices. The resulting isolation amplifier performance is similar to that of a dedicated analog isolation amplifier.

Figure 1 shows an isolated amplifier circuit composed of an amplifier and an ordinary optocoupler. This isolation amplifier circuit consists of two parts: input and isolated output, and the two parts are powered by isolated power supplies (Vcc1, Vee1 and Vcc2, Vee2).

The input part consists of operational amplifier U1, resistors R1, R2, R3, R4, R5, capacitors C1, C2, the light-emitting diode parts of the optocouplers OPT1, OPT2, OPT3, and OPT4 OPT1_A, OPT2_A, OPT3_A, OPT4_A, and the photosensitizers of OPT1 and OPT3. The transistor part is composed of OPT1_B and OPT3_B and is powered by the positive power supply Vcc1 and the negative power supply Vee1.

The currents of OPT1_A, OPT2_A and OPT3_A, OPT4_A constitute the differential amplifier input. R1 and R2 are the input resistors of the op amp, and R3 and R4 provide bias and control current for the four optocoupler light-emitting diodes (LEDs). The operational amplifier U1 and the optocouplers OPT1 and OPT3 form an emitter follower, and the voltage on R5 is the input voltage of the operational amplifier.

The bandwidth of the op amp determines the bandwidth of the isolation amplifier. The bandwidth of existing integrated analog isolation amplifiers is below 100kHz, while the bandwidth of commonly used operational amplifiers is several to dozens of times this bandwidth.

Therefore, this design uses a general operational amplifier to meet the bandwidth requirements of the input part. Therefore, an ordinary operational amplifier (such as LF353) can be used as the input stage operational amplifier. R7 and C3 are used for filtering. The isolated output part of this circuit consists of phototransistors OPT2_B and OPT4_B of OPT2 and OPT4, potentiometer W1 and output resistor R6.

OPT2_B and OPT4_B are isolated outputs. Its circuit structure is similar to the phototransistor in the input part and is used to provide current to the output stage. Potentiometer W1 is used for zero adjustment. When there is a deviation in the current transfer ratio of the two parts of the optocoupler, the optocoupler LED current will be equal but the output stage current difference will be different, causing the zero point of the output voltage vo to drift. Therefore, adjusting the potentiometer W1 can eliminate the zero point drift caused by the characteristic deviation of the optocoupler device. R6 is the output load, which together with the potentiometer W1 determines the output voltage vo.