STM32|4-20mA output circuit

　　Equipment developed for industrial occasions usually has a 4-20mA output interface. In the past, single-chip systems without DAC modules required an external main chip DAC to realize analog quantity control, or used PWM to simulate DA, but it also brought temperature drift and long-term stability problems. In STM32-centered devices, the built-in DAC can be used to easily realize the 4-20mA output interface, which has the characteristics of high accuracy, good stability, small drift and easy programming.

　　In the STM32 MCU system, there is no external VREF pin below pin 100, but this makes the reference end of the DAC and VCC shared, resulting in a large bit error. To solve this problem, you can use the cheap TL431 to solve the power supply problem. The typical temperature drift of TL431 is 30ppm, so it is sufficient in general applications. Select two low-temperature drift resistors and adjust the output so that the output voltage of TL431 is between 3V-3.6V. Its parallel regulated current can reach 30mA, which just meets the power consumption requirements of the general STM32 core.

　　The power supply problem is solved by using TL431, and the remaining part is the 4-20mA conversion circuit, as shown below:

　　The figure above is a very accurate conversion circuit. OPA333 is a very excellent single-power rail-to-rail operational amplifier with an operating voltage of 2.7-5.5V and an offset voltage of only 10uV. The measured minimum output is 30uV and the maximum output can reach VCC-30uV. The circuit is composed of a voltage-controlled constant current source. The key lies in the excellent performance of the OPA333 chip, which enables the above circuit to achieve extremely high accuracy and stability. DACOUT comes from the DAC1 or DAC2 output of STM32. After digital noise filtering by C25, it enters the operation, performs 1:1 buffering, and then amplifies the current through Q2 to form a detection voltage on R7, and C17 performs de-jittering. The 4-20mA signal is output between AN_OUT+/AN_OUT-.

　　In the figure above, the current in the load forms a voltage drop on R7. After feedback from the op amp, Vdacout=Vr7=I*R7 is obtained, so: I=Vdacout/R7. When Vdacout changes between 400mV and 2000mV, a 4-20mA output can be obtained. Changing the size of R7 can change the required range of DACOUT. In the circuit, there will be a bias voltage of about 0.7V between the base and emitter of R2, so Vb[MAX]=2V+0.7V=2.7V, which is just within the output range of OPA333. In the circuit, R14 is used as the current limiting current at the output end, so that the maximum output current at the output end is Imax=Vcc/(R7+R14). If Vcc is 6V, then Imax=6V/200 O=30mA. If there is no R14, the maximum current may be 60mA. At this time, the power dissipation on R7 is 0.06*0.06*100=0.36W. If 0805 chip resistors are used, R7 will burn out, or the resistance value of R7 will change too much due to the temperature rise, causing a large deviation in the output. After adding R14, the maximum power dissipation on R7 is: 0.03*0.03*100=0.09W, which is within the normal range.

　　R14 and C17 in the circuit cannot be omitted. Possible slight interference or fluctuation of external load will cause the deep negative feedback circuit composed of OPA333 to oscillate and make the output current fluctuate. Adding C17 can suppress this fluctuation and make the output more stable. However, the value of C17 should not be too large.

　　When programming with STM32, please note that the internal DAC buffer should not be turned on, because the above circuit is already a buffer circuit with high input impedance. The output linearity will be lost due to the internal buffer circuit of STM32.