What is a grounding loop?
A ground loop is a physical loop in a system grounding scheme, resulting from multiple grounding paths between circuits. These grounding paths can act as a large loop antenna, picking up noise from the environment and thus generating current in the grounding system.
In industrial production processes, "grounding loops" are the most common example. To achieve monitoring and control, various automated instruments, control systems, and actuators are needed. The signal transmission between them includes both low-frequency DC signals and high-frequency pulse signals, ranging from small signals as weak as millivolts or microamperes to large signals as large signals as tens of volts, or even thousands of volts or hundreds of amperes.
After a system is built, it is often found that signal transmission between instruments and equipment interferes with each other, causing system instability or even malfunction. In addition to the performance of each instrument and equipment itself, such as the influence of electromagnetic interference, another very important factor is that there is a potential difference between the signal reference points of the instruments and equipment, which forms a "grounding loop" and causes distortion during signal transmission.
Therefore, to ensure the stable and reliable operation of the system, the "grounding loop" problem must be solved during the system signal processing process.
When designing circuits, designers often use single-point grounding to avoid loops. However, some interfaces require a ground connection between the transceiver and the receiver. This ground connection must be interrupted while maintaining the information flow from the transmitter to the receiver. In other words, current isolation is required between the two devices.
Option 1:
One possible way to break a ground loop is to use an optocoupler. By eliminating the ground connection of the optocoupler cable, noise current is prevented from flowing between device #1 and device #2, allowing information to be transmitted in the form of light.
This approach has limitations as interface performance and complexity increase. Optical isolation interfaces can become complex, expensive, and require significant board space. Optical couplers have considerable propagation delays and are only suitable for low-speed signals.
When using multiple optocouplers, the power consumption of the LEDs and pull-up resistors can become quite high. Digital isolation techniques can be used to break ground loops without affecting interface performance, and the application circuitry is simpler with relatively fewer components required. Digital isolation is a non-optical isolator that uses a CMOS interface IC to transmit information via capacitive or magnetic coupling.
Option 2:
Connecting two AC-powered devices using a single USB cable can create a ground loop, interrupting bus communication. USB communication occurs over a pair of bidirectional differential lines. The host device controls the bus and communicates with the peripheral. The direction of data packets is determined by the USB protocol, not by control signals. The host device provides power and ground to the peripheral. This ground connection of the USB cable creates a ground loop between the host and the peripheral, potentially causing the peripheral's ground potential to shift relative to the host's ground potential, making communication unreliable.
However, isolating USB ports to eliminate cable grounding connections is difficult because there are no control signals to indicate whether data is being transmitted downstream (peripheral) or upstream (host). Without access to the internal signals of the Serial Interface Engine (SIE) controlling the bus, the only way to determine data direction is through bus processing. The SIE signals are unavailable because the SIE is often integrated into the processor.
Option 3:
① Do not ground all field equipment, so that all process loops have only one grounding point and cannot form a loop. This method seems simple, but it is often difficult to implement in practice. This is because some equipment requires grounding to ensure measurement accuracy or personal safety, and some equipment may form new grounding points due to long-term corrosion and wear or the influence of climate.
② Make the potentials of the two grounding points the same, but since the resistance of the grounding points is affected by many factors such as geological conditions and climate change, this scheme cannot be completely achieved in practice.
③ Use signal isolation methods in each process loop to disconnect the process loop without affecting the normal transmission of process signals, thereby completely solving the grounding loop problem.
Signal isolation is of paramount importance – signal isolators
The method introduced in Method 3③ is the focus of this article: using an isolator (also known as a "signal isolator").
Isolators employ linear optocoupler isolation to convert input signals to output. The input, output, and power supply are mutually isolated, and advanced digital technology is used, resulting in excellent suppression of high and low frequency interference signals. An isolator generally consists of four parts: an input signal processing unit, an isolation unit, an output signal processing unit, and a power supply. Although isolators in practical applications are basically composed of these four units, the variety of models varies depending on the type and number of inputs and outputs.
Isolators are widely used in major projects in industries such as oil fields, petrochemicals, manufacturing, power, and metallurgy. They are often used with equipment and instruments that require electrical isolation, such as unit combination instruments and DCS, PLC and other systems.
Isolators can still be reliably used even in high-power frequency conversion control systems, and have now become an important component of industrial control systems.
Internally, it employs numerous advanced technologies such as digital calibration, zero-point and full-scale potentiometers, automatic dynamic zero-point calibration, and automatic temperature drift compensation, and conforms to IEC61000-4-4: the application of these technologies scientifically guarantees the product's stability and reliability. All of these technologies are at the forefront of international advanced levels.
