Application of HK module in two-wire and HART isolation transmission

1. Ground loop problems and ground potential differences in signal transmission

      There are many reasons why ground loops can occur in instrumentation circuits. Ground loops occur when the transmitting and receiving devices are connected to different ground points or different power supplies, which is common over long cable runs. They can also occur when the metal housing of a device makes physical contact with the ground. Ground loops create an unnoticed electrical loop, creating a source of error.

      Since it is impossible to determine the accuracy of the signal through simple observation at the receiving end of the signal, the ground loop current generated by different ground potentials will introduce undetectable errors into the signal loop.

      In two-wire signal transmission without signal isolation, there are signal errors caused by these ground loops.

      In some cases, lightning, surge and other signals may be loaded onto the signal line along with the long line, damaging the modulation or detection devices on the port.

      In another case, the ground potentials referenced by the ports of two devices are different, which may cause the port voltage to exceed the standard and damage the device port.

2. Non-isolated two-wire signal transmission method

Figure 1

      In the circuit of Figure 1, the sensor signal is converted into a digital signal by the ADC, and then sent to the current loop DAC and HART modulator through the MCU to form a 4-20mA signal output. The current loop DAC draws power from the 4-20mA bus output by the PLC, and outputs 3.3V and 2.5V through the built-in voltage regulator to provide a small amount of power to the MCU and HART modulator.

      In the transmission mode of Figure 1, there is a potential difference between the ground potential of the two-wire instrument and the remote PLC, which introduces common-mode interference. This interference will cause transmission errors of the 4-20mA signal and affect the normal communication of the HART bus. If the common-mode interference is too large (exceeding the allowable common-mode voltage range of the internal components), it may even cause damage to the components inside the two-wire instrument.

3. Isolated two-wire signal transmission method

Figure 2

      Based on the above hidden dangers caused by different ground potentials, the MCU can be isolated from the current loop DAC and HART modulator by digital isolation. The current loop DAC and HART modulator are floating, so that high impedance is formed between the two ground potentials of the two-wire instrument and the PLC system, reducing the loop current between the ground wires and forming electrical isolation between the two devices, thereby reducing the transmission error of the 4-20mA signal and eliminating the hidden danger of common-mode interference.

      Because the two-wire instrument has no additional power supply port, its power supply is obtained through the 4-20mA loop. After adopting this isolation method, the 2.5V power supply of the HART modulator is still obtained from the built-in voltage regulator on the current loop DAC, and the 3.3V power supply of the MCU system becomes a problem. A better solution is to connect a voltage regulator diode in series in the 4-20mA circuit as shown in Figure 2, and then use the HK module of Guangzhou Jinshengyang Technology Co., Ltd. to isolate the output 3.3V to power the MCU system.

      In this way, using Jinshengyang's HK module and digital isolation circuit, the electrical isolation between the two-wire instrument and the PLC system can be achieved, avoiding system measurement errors and equipment damage caused by ground potential, and further improving the reliability of the system. The optional Jinshengyang HK module models are as follows:

 
      In circuit design, the low power consumption design of the MCU system should be fully considered. Try to choose a low power MCU, and try to reduce the operating frequency of the MCU while meeting the sampling rate. This can greatly reduce the power consumption of the MCU. Generally, the MCU in a two-wire instrument only needs 3.3V 1mA power.

      In general, the higher the voltage regulator value of the diode connected to the input end of the HK module, the greater the output power that the HK can provide. In order to maximize the transmission distance of the two-wire signal, under the premise of meeting the power consumption required by the MCU, try to choose a voltage regulator diode with a lower voltage regulator value as the input voltage of the HK module. For input voltages not listed, you can order it from Guangzhou Jinshengyang Technology Co., Ltd.

IV. Conclusion

      The HK module produced by Guangzhou Mornsun Technology Co., Ltd. can realize the isolated transmission of two-wire signals, eliminate the errors caused by ground potential differences in system testing and the possible impact of common-mode signals on port devices, and provide a new and better choice for signal transmission in industrial control sites.

      Of course, for non-isolated applications, if the MCU requires a slightly higher operating frequency and the current loop DAC chip cannot provide sufficient energy, you can also consider using a power supply circuit similar to Figure 2 to provide additional energy. This article will not go into details.

      The HK online power supply module of Guangzhou Mornsun Technology Co., Ltd. has made further innovations based on the patented circuit in 2012, improving the power conversion efficiency of the HK module. It is believed that it can meet the higher power requirements of two-wire instruments.