Electronic components – isolators, how to achieve anti-interference between systems?

Transmission of data over long distances is fraught with a variety 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.

Transmission of data over long distances is fraught with a variety 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 grounding scheme that results from multiple ground paths between circuits. These ground paths can act as a large loop antenna, picking up noise from the environment and thus creating currents in the grounded system.

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

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

Therefore, in order 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 to avoid loops by grounding a single point, but some interfaces require a ground connection between transceivers. This ground connection must be interrupted while maintaining the flow of information 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 allowing the ground connection of the optocoupler cable to be eliminated, noise current is prevented from flowing between device #1 and device #2, allowing information to be transmitted as 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 fairly large propagation delays 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, interface performance is not compromised, and the application circuit is simple and requires relatively few components. Digital isolation is a non-optical isolator that utilizes a CMOS interface IC to transmit information through capacitive or magnetic coupling.

Option II:

Using a single USB cable to connect two AC powered devices can create a ground loop that interrupts bus communication. USB communication takes place on a pair of bidirectional differential lines. The master device controls the bus and communicates with peripherals. The direction of data packets is determined by the USB protocol, not by control signals. The master device provides power and ground connections for peripherals. This ground connection of the USB cable forms 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 potential, making communication unreliable.

However, since there is no control signal to indicate whether data is going downstream (peripheral) or upstream (host), isolating the USB port to eliminate the cable ground connection is difficult. 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 the bus processing. The SIE signal is not available because the 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 devices must be grounded 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 weather.

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 fully implemented in practice.

③Use the signal isolation method in each process loop to disconnect the process loop without affecting the normal transmission of the process signal, so as to completely solve the ground loop problem.

The top priority of signal isolation – signal isolator

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

The principle of linear optocoupler isolation is used in the isolator 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 has excellent performance in suppressing high and low frequency interference signals.

The isolator is generally composed of four parts: input signal processing unit, isolation unit, output signal processing unit, and power supply. Although the isolators in practical applications are basically composed of the above four units, the different types and numbers of inputs and outputs form a wide variety of models.

See below:

Electronic components – isolators, how to achieve anti-interference between systems?

Isolators are widely used in major projects in oilfield, petrochemical, manufacturing, electric power, metallurgy and other industries, and are often used with equipment and meters that require electrical isolation: such as unit combination meters and DCS, PLC and other systems.

Isolators can still be used reliably even in high-power variable frequency control systems. At present, they have become an important part of industrial control systems.

The internal use of digital adjustment, no zero and full scale potentiometers, automatic dynamic zero calibration, automatic temperature drift compensation and many other advanced technologies, and in line with IEC61000-4-4: the application of this series of technologies makes the product stable and reliable. Guaranteed by science.

The above technologies lead the international advanced level.

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Author: Yoyokuo