Signal Isolators Information
Last revised: December 05, 2024
Reviewed by: Scott Orlosky, consulting engineer
Signal isolators provide electrical (galvanic) isolation between the input and output circuits. They couple the signal to the output through a transformer or optical isolator. Signal isolators also break the direct electrical galvanic path between two or more loop points. They protect against dangerous measured-variable voltages and increase protection from surges and spikes.
Signal isolators are closely related to signal converters and signal interfaces. These devices are often used to share, split, boost, protect, step-down, linearize, and digitize process signals.
The most important role of a signal isolator is to break the galvanic path between circuits that are "grounded" to different potentials. A galvanic path is defined as a path in which there is a direct electrical connection between two or more electrical circuits that allow current to flow.
For example, if a differential pressure transmitter is sending a 4-20mA measurement to a receiver, such as a recorder, and the two ground points are different, an additional and unpredictable amount of current can be introduced into the loop, distorting the true measurement. This current path is known as a ground loop and is a very common cause of signal inaccuracies. Along with the current path having two grounds, the grounds are at different potentials, and there is a galvanic path between the grounds. In order to remove the ground loop, the galvanic path between the grounds must be removed.
Signal isolators are especially important since the other two causes for ground loops cannot always be removed safely. The ground may be necessary for safe operation of the electronic device.
Function
There are two prevalent methods for galvanic isolation.
Optical isolation uses light to transfer a signal between elements of a circuit. The isolator is usually in a small module mounted on a circuit board. The isolation circuit is composed of an LED and a photo-sensitive detector, such as a phototransistor. The insulating air gap between the LED and the photo transistor serves as the galvanic separation between the two circuits.
The advantages of optical isolation are the device is small, can sometimes provide higher levels of isolation than transformer isolation, and the device has better common-mode noise rejection. Disadvantages of optical isolation are that each element needs its own power supply and the signals on both sides must be quite small.
Transformer isolation is also referred to as electromagnetic isolation. It uses a transformer to electromagnetically couple the desired signal across an air gap or non-conductive isolation gap.
Transformers are very efficient at transforming AC signals. This is a disadvantage since many process control signals are DC and must be changed into an AC signal so they can pass across the transformer. Once passed, they have to be rectified and amplified back into the desired DC signal output.
Advantages of a transformer circuit include: the device does not need a power supply and it works well with AC signals such as audio. A disadvantage is that a transformer does not work with DC signals since at some frequencies the device will begin to act as a filter for AC signals.
Specifications
Signal isolators are often specified by what the isolation levels are from input to output. Two-way (input-to-output) isolation is used to describe a 2-wire transmitter since it is powered from either its input or output terminals.
Three-way isolation is defined as input-to-output, power-to-input, and power-to-output isolation. It is important to note if the isolator is powered by a DC supply which could cause a problem with common mode noise, or failing to switch power supply, creating unwanted output signal errors.
If the signal isolation is a 4-wire device it may require 24 V DC, 110 V AC, or 220 V AC to operate its circuits. For these devices, it is important to ensure the isolator has full three-way isolation.
Each type of signal isolator should provide signal isolation between the input, output, and power source. A typical signal isolator is capable of withstanding 1,000 V, or more common-mode signals at the input.
The level of isolation is also important when selecting a signal isolator. Level of isolation describes the level of voltage needed to create an arc from one side of the circuit to the other. Optical isolators have the advantage here as the electrical parts of the circuit can be designed to be further apart.
Environmental Factors
Due to the widespread application of signal isolation many devices are available with built-in safety features for use in various environments.
For example, intrinsically-safe, nonincendive, and explosion proof options are all available for hazardous areas within a plant. Hazardous conditions also include the amount of heat the electronics will be exposed to while installed in the plant.
Other devices may need protection from radio frequency interference (RFI) and electromagnetic interference (EMI) which can cause unpredictable and non-repeatable degradation in instrument performance.
Variable frequency drives (VFDs) are also known to cause noise that interferes with signal isolators and are common in plants. This interference can be avoided by installing a 4-wire isolator to filter the unwanted common mode noise.
Applications
Signal isolators are used in industrial, medical, and other environments in which electrical isolation is essential for safety. Signal isolators can also be used to amplify signals, enable instruments to be added to an overburdened loop, or to step down dangerous, high-voltage signals to safer levels.
Specific examples of applications include resistance input for use in RTD (Resistance Temperature Device), slidewire, strain, and potentiometer transmitters, and current/voltage isolators for use as alarm tripping and deviation alarm notification.
Signal Isolators FAQs
How do signal isolators help in preventing ground loops in industrial applications?
Signal isolators are effective in eliminating undesirable ground loop currents and induced electrical noise. Ground loops occur when there are multiple ground points at different potentials, causing an additional and unpredictable amount of current to flow, which distorts the true measurement. By isolating power sources and sensor signals, signal isolators remove the galvanic path between these grounds, thereby preventing ground loops from forming.
Ground loops can invalidate measured sensor signals, ruin measurement accuracy, and even physically destroy equipment. Signal isolators protect sensors and measurement equipment by breaking the current paths that cause ground loops. This ensures that the data acquisition systems maintain their accuracy and reliability, even when moved from controlled environments to more challenging industrial settings.
In addition to protecting measurement signals, signal isolators also enhance safety by preventing high current or voltage from causing physical harm to users. This is particularly important in industrial environments where electrical isolation is essential for safe operation. By preventing ground loops, signal isolators help avoid costly process delays or shutdowns due to measurement and control system failures.
Signal isolators also help in mitigating radio frequency interference (RFI) and electromagnetic interference (EMI), which can cause unpredictable and non-repeatable degradation in instrument performance. This is particularly relevant in industrial plants where variable frequency drives (VFDs) are common sources of noise. Installing a 4-wire isolator can filter out unwanted common mode noise, further ensuring the integrity of the measurement signals.
What are the environmental factors that affect the performance of signal isolators?
Signal isolators are designed to operate within a specific temperature range. Extreme temperatures, whether too high or too low, can affect their performance and longevity. For instance, high temperatures can cause thermal drift, leading to inaccuracies in signal transmission.
The ability of a signal isolator to maintain its performance over a range of temperatures is crucial. Poor thermal stability can result in signal degradation.
High humidity levels can lead to condensation inside the isolator, which can cause short circuits or corrosion of internal components. This can degrade the performance or even lead to failure.
Signal isolators used in environments with high humidity should have adequate sealing and protective coatings to prevent moisture ingress.
Industrial environments often involve significant vibration and mechanical shocks. Signal isolators must be robust enough to withstand these conditions without affecting their performance.
Proper mounting and durable housing can help mitigate the effects of vibration and shock.
Signal isolators must be designed to handle EMI and RFI, which are common in industrial settings with variable frequency drives (VFDs) and other sources of electrical noise. Effective shielding and filtering mechanisms are essential to maintain signal integrity.
By providing galvanic isolation, signal isolators prevent the propagation of interference through the system, thereby protecting the measurement signals.
In certain industrial applications, signal isolators may be exposed to corrosive chemicals. Materials used in the construction of the isolators should be resistant to such chemicals to prevent degradation.
Applying protective coatings can help in shielding the isolators from chemical exposure.
Signal isolators should have an appropriate IP rating to ensure they are protected against dust and particulate matter, which can interfere with their operation.
How do manufacturers test for thermal drift in signal isolators?
Manufacturers test for thermal drift in signal isolators through a series of controlled procedures designed to evaluate the performance of the isolators under varying temperature conditions.
Manufacturers calibrate the signal isolators over a specified range of temperatures. This involves:
Static Thermal Chamber: The isolator is placed in a static thermal chamber where the temperature is controlled and varied systematically.
Multiple Temperature Points: Calibration is performed at multiple temperature points, such as 1° C, 27° C, and 55° C. The calibration parameters, such as gain and offset, are recorded at these points.
Interpolation and Verification: A curve is fit over the range to provide interpolative values for all intervening temperatures. The calibration is then checked at additional points within the range to verify that the input and output calibration remains within specified limits, such as 0.2% Full Scale Range (FSR).
Thermal Hysteresis Testing: This testing characterizes the repeatability of the isolator over numerous thermal cycles:
Temperature Cycling: The isolator is subjected to cycles where the temperature is increased to a high value (e.g., 150° C) and then returned to room temperature, followed by a decrease to a low value (e.g., -40° C) and then returned to room temperature again.
Output Measurement: The output is measured at each temperature point to ensure that it remains consistent every time the temperature returns to a given value.
Long-Term Stability Testing: This involves testing the isolator over an extended period to ensure that its performance remains stable:
Sustained High Temperature: The isolator is tested at a high temperature (e.g., 150°C) for an extended duration (e.g., 300 hours).
Offset Drift Measurement: The offset drift is measured to determine how stable the output remains over time and sustained temperature.
Real-Time Sensitivity Compensation: Some advanced isolators incorporate real-time sensitivity compensation to maintain performance under operational conditions:
Zero-Drift Architecture: This architecture helps in minimizing sensitivity drift over temperature changes, ensuring consistent performance without the need for frequent recalibration.
What are the benefits of using zero-drift architecture in signal isolators?
Minimized Sensitivity Drift
Zero-drift architecture involves the use of an op-amp to stabilize the zero state of the isolator. This helps in minimizing sensitivity drift over temperature changes. This ensures that the performance of the signal isolator remains consistent even when there are fluctuations in temperature. The sensitivity drift over temperature is often less than 0.45% over 150 °C.
Reduced Calibration Needs
The zero-drift architecture eliminates the need for frequent device calibration. This reduces the maintenance requirements over time, making the system more reliable and cost-effective. The sensors provide inherent galvanic isolation and eliminate the need for device calibration.
Enhanced Long-Term Stability
Zero-drift architecture contributes to the long-term stability of the signal isolator. This means that the isolator can maintain its performance characteristics over extended periods, even under operational conditions such as temperature changes and equipment aging. This is particularly beneficial in industrial applications where long-term reliability is critical.
Improved Accuracy
Zero-drift architecture ensures high accuracy in signal transmission. This is essential for applications that require precise measurements and control.
Inherent Galvanic Isolation
The zero-drift architecture also provides inherent galvanic isolation, which is capable of delivering high levels of isolation per industry standards.
Signal Isolators Media Gallery
References
Electronics360—TI introduces zero-drift hall-effect current sensors
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