“When using 4-20mA analog quantity for communication, the circuit design of both the transmitting end and the receiving end will be more complicated than digital communication, so why use it? This article will take you to understand 4-20mA communication in conjunction with design cases.
When using 4-20mA analog quantity for communication, the circuit design of both the transmitting end and the receiving end will be more complicated than digital communication, so why use it? This article will take you to understand 4-20mA communication based on design cases.
1. Why use 4-20mA communication?
In long-distance, complex industrial field applications, there are often large interference sources, such as magnetic field radiation interference, conduction interference, etc. If you use traditional digital communication, it is easy to be interfered, because the input impedance of the receiving end is infinite, and it is interfered by weak noise signals. Later, higher voltage noise will be generated, which is not conducive to data transmission and safe use of the interface. When using analog (4-20mA) for communication, because the coupled noise signal is relatively weak, usually nA level, it is not affected by this, and the current source drive has no line voltage drop problem.
2. What data meaning does 4-20mA represent?
Based on international standard documents, “Analog Signals for Process Control Systems (Part One): DC Current Signals (GB/T 3369.1-2008/IEC 60381-1:1982)” stipulates that 4-20mA signals are the preferred DC current communication signals , As shown in Table 1: DC current signal range. The document stipulates that when a 4-20mA communication signal is used, 4mA represents the 0-scale of the original data, 20mA represents the full-scale of the original data, and 0mA is used as a power failure or disconnection detection.
For example, the remote PT100 thermal resistance temperature monitoring system transmits remote field data back to the PLC through 4-20mA communication to realize the monitoring of field temperature changes. The measuring range of PT100 is -200-850℃, then 4mA represents -200℃, 20mA represents 850℃, the actual temperature calculation formula is as follows:
Among them: T is the current test temperature; I is the current acquisition current; generally, lower than 2mA is used to represent the thermal resistance temperature measurement module system is powered off or the communication line is disconnected.
3. General circuit design of 4-20mA communication system
The 4-20mA analog communication circuit is shown in Figure 2 4-20mA communication circuit architecture. The front end of the application field is composed of sensors. The non-standard sensor signal is converted into a standard 4-20mA communication signal through a transmitter, and then sent to the remote The control equipment is received by the receiver and uploaded to the PLC control terminal.
At present, there are many finished modules of sensor equipment or actuators on the market that have integrated 4-20mA communication function, and users only need to build the receiving module by themselves. The receiving module is shown in the following figure 2 4-20mA communication circuit architecture, including sampler, signal conditioning circuit, ADC (analog to digital conversion) and MCU (data transmission and processing). But if the 4-20mA communication carries a high-precision data with a relatively wide range of data (such as the remote PT100 thermal resistance temperature monitoring system mentioned in the previous chapter, the data range is -200-850℃, and the accuracy is 0.1%. ±1℃ takes the maximum value), if the accuracy of the receiving module is less than 0.1%, it will cause data transmission errors, and the sensor performance cannot be used. How to ensure that the sampling data of the receiving module is accurate?
4-20mA communication circuit architecture
First, let’s analyze the reasons that the receiving link may cause the accuracy error of the acquisition: a. The initial accuracy of the sampling resistor and when working in extreme environments (high and low temperature max), the temperature drift of the resistor causes the drift of the sampling voltage; b. Conditioning circuit , The circuit has many factors that limit the sampling accuracy, such as the offset voltage of the op amp, output noise, attenuation or gain network error, which causes the voltage acquisition error at the ADC terminal; c. ADC unit circuit errors, such as reference drift, reference noise, power supply noise, PCB layout, etc. These are all external factors that affect ADC conversion accuracy; d. Conversion accuracy errors caused by ADC itself, such as ADC offset error, gain error, noise-free low resolution, poor integral nonlinearity, etc., bring conversion Accuracy error.
In order to shorten the user’s development cycle, ZLG has introduced a high-precision analog quantity acquisition module with isolation function (TPS08U) to solve all the above problems at one time. In the design of this module, all the above factors are taken into consideration, using extremely low temperature drifting resistance, so-called zero-drift operational amplifier, 24bit resolution ADC, selecting the most cost-effective components under extremely excellent parameters, and optimizing the layout. Line layout, etc. realize 8-channel measurement with minimum volume. At the same time, each module has passed strict testing and calibration before leaving the factory to ensure that each module can meet the index requirements.
4. Introduction to the use of TPS08U module
The typical circuit of TPS08U is shown in the figure below. It only needs a simple peripheral circuit to realize 8-channel analog signal acquisition (4-20mA and 0-5V), with an accuracy of 0.1% (voltage is full-scale accuracy). The module power supply adopts 3.3V power supply, communication interface SPI, at the same time, the module integrates power supply and communication isolation circuit (isolation DC: 2500V), size length * width * height: 31.8mm*20.3mm*6.5mm. Detailed information can be obtained from local sales.