Many people are familiar with RS232, RS485, CAN, and other commonly used industrial buses, all of which transmit digital signals. So, how do we transmit analog signals? Industrial applications commonly require the measurement of various non-electrical physical quantities, such as temperature, pressure, speed, and angle. These quantities need to be converted into analog electrical signals before being transmitted to control rooms or display devices hundreds of meters away. The most widely used method in industry is to transmit analog signals using a 4-20mA current. The reason for using current signals is that they are less susceptible to interference. While the amplitude of noise voltage in industrial environments can reach several volts, the power of the noise is very weak, so the noise current is usually less than nA, resulting in very small errors in 4-20mA transmission. The internal resistance of the current source tends to infinity, and the resistance of the wire connected in series in the loop does not affect accuracy. Therefore, it can transmit over hundreds of meters on ordinary twisted-pair cables. Due to the high internal resistance and constant current output of the current source, at the receiving end, we only need to place a 250-ohm resistor to ground to obtain a 0-5V voltage. The advantage of a low input impedance receiver is that nA-level input current noise only produces very weak voltage noise.
The upper limit is set at 20mA because of explosion-proof requirements: the spark energy caused by switching on and off a 20mA current is insufficient to ignite methane. The lower limit is not set to 0mA to detect open circuits: during normal operation, the current will not fall below 4mA; when the transmission line breaks due to a fault, the loop current drops to 0. 2mA is commonly used as the open circuit alarm value. Current-type transmitters convert physical quantities into 4-20mA current outputs, requiring an external power supply. Typically, a transmitter needs two power lines and two current output lines, totaling four wires, hence the name four-wire transmitter. Of course, the current output can share a single wire with the power supply (shared VCC or GND), saving one wire; therefore, four-wire transmitters are now generally referred to as three-wire transmitters. In fact, you may have noticed that the 4-20mA current itself can power the transmitter; the transmitter acts as a special load in the circuit. This type of transmitter only requires two external wires and is therefore called a two-wire transmitter. The lower limit of the industrial current loop standard is 4mA. Therefore, within the measurement range, transmitters are typically powered only at 24V, 4mA. (Therefore, high-efficiency DC/DC power supplies (TPS54331, TPS54160), low-power sensors and signal chain products, and low-power processors (such as MSP430) are crucial for two-wire 4-20mA transceivers under light load conditions.) This makes the design of two-wire sensors both possible and challenging.
Generally, a VI converter needs to be designed, with an input of 0-3.3V and an output of 4mA-20mA. An operational amplifier LM358 can be used, with a power supply of +12V.
