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 ground at different potentials, and 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 there 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 24Vdc, 110Vac, or 220Vac 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 1000 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, slidewire, strain, and potentiometer transmitters, and current/voltage isolators for use as alarm tripping and deviation alarm notification.