Transmitting data over long distances is fraught with all kinds of potential problems. A ground loop can be a source of interference that can create a noise voltage between the grounds at both ends of the transmission that, if large enough, can cause data errors at the receiving end.

What is a ground loop?

A ground loop is a physical loop in a system’s grounding scheme that results from multiple ground paths between circuits. These ground paths can act as a large loop antenna that picks up noise from the environment, causing current flow in the ground system.

For example, “ground loops” are the most common in industrial production processes. In order to achieve monitoring and control, various automation instruments, control systems and actuators are used. The signal transmission between them includes both low-frequency DC signals and high-frequency pulse signals, ranging from weak to millivolt and microampere. There are small signals of tens of volts, even thousands of volts, and large signals of hundreds of amperes.

After the system is formed, it is often found that the signal transmission between the instrument and the equipment interferes with each other, resulting in system instability or even misoperation. In addition to the performance reasons of each instrument and equipment itself, such as anti-electromagnetic interference, there is another very important factor that causes a “ground loop” due to the potential difference between the signal reference point between the instrument and the equipment. “Causes distortion during signal transmission.

Therefore, to ensure the stable and reliable operation of the system, the “ground loop” problem must be solved in the process of system signal processing.

Designers often design circuits with a single-point ground to avoid loops, but some interfaces require a ground connection between transceivers . This ground connection must be interrupted while maintaining information flow from the transmitter to the receiver . In other words, galvanic isolation is required between the two devices.

Option One:

One of the possible ways to break ground loops is to use optocouplers. By having the ground connection of the optocoupler cable eliminated, noise currents are prevented from flowing between device #1 and device #2, allowing information to be transmitted in the form of light.

This approach has limitations as the performance and complexity of the interface increases. Optically isolated interfaces can become complex, expensive, and require a lot of board space. Optocouplers have considerable propagation delay and are only suitable for low-speed signals.

When using multiple optocouplers, the power dissipation of the LEDs and pull-up resistors can become quite high. Digital isolation techniques can be used to break ground loops without compromising interface performance, and the application circuit is simple and requires relatively few components. Digital isolators are non-optical isolators that utilize a CMOS interface IC to transfer information through capacitive or magnetic coupling.

Option II:

Connecting two AC-powered devices with a single USB cable can create a ground loop that disrupts bus communication. USB communication takes place over a pair of bidirectional differential lines. The master device controls the bus and communicates with the peripherals. The direction of data packets is determined by the USB protocol, not by control signals. The master device provides power and ground connections to the peripherals. This ground connection of the USB cable creates a ground loop with the safety grounds of the host and peripheral, which can cause the peripheral’s ground potential to shift relative to the host’s ground, making communication unreliable.

However, isolating USB ports to eliminate cable ground connections is difficult since there are no control signals to indicate whether data is being transferred downstream (peripheral) or upstream (host). Without access to the Serial Interface Engine (SIE) internal signals that control the bus, the only way to determine the direction of the data is through bus processing. The reason why the signal of SIE is not available is that SIE is often integrated into the processor.

third solution:

① All field devices are not grounded, 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 achieve in practical applications, because some equipment requires grounding to ensure measurement accuracy Or to ensure personal safety, some equipment may form a new ground point due to long-term corrosion and wear or climate influence.

② Make the potentials of the two grounding points the same, but because the resistance of the grounding point is affected by many factors such as geological conditions and climate change, this scheme cannot be completely realized in practice.

③ Use the signal isolation method in each process loop to disconnect the process loop without affecting the normal transmission of process signals, thereby completely solving the ground loop problem.

The most important signal isolation – signal isolator

The method introduced in method three ③ is the focus of this article: use an isolator (signal isolator) or “signal isolator”.

In the isolator, the principle of linear optocoupler isolation is adopted to convert the input signal to output. The input, output and working power are isolated from each other, and advanced digital technology is adopted, which is excellent in the 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 the isolators in practical applications are basically composed of the above four units, the types and quantities of input and output are different, forming a wide variety of models.

Isolators are widely used in major projects in oil fields, petrochemicals, manufacturing, electric power, metallurgy and other industries. 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 used reliably even in high-power frequency conversion control systems. At present, they have become an important part of industrial control systems.

The interior adopts many advanced technologies such as digital adjustment, no zero point and full-scale potentiometer, automatic dynamic zero point calibration, automatic temperature drift compensation, etc., and conforms to IEC61000-4-4: the application of this series of technologies makes the product more stable and reliable. Backed by science.

The above-mentioned technologies lead the international advanced level.