![]() Transformers are very efficient and fast at transferring AC signals. The electromagnetic field intensity is proportional to the input signal applied to the transformer. Transformer isolation, often referred to as electromagnetic isolation, uses a transformer to electromagnetically couple the desired signal across an air gap or non-conductive isolation gap. Optical isolation has better common-mode noise rejection, is usually seen in digital circuits, is not frequency sensitive, is smaller and can sometimes provide higher levels of isolation than transformer isolation. The output signal of the phototransistor is proportional to the light intensity of the source. The insulating air gap between an LED and a phototransistor serves as the galvanic separation between the circuits, thus providing the desired isolation between two circuits at different potentials. The optocoupler or optoisolator is usually self-contained in a small compact module that can be easily mounted on a circuit board. Optical isolation uses light to transfer a signal between elements of a circuit. The two most common methods chosen for galvanic isolation are optical and transformer isolation.įigure 2: A signal isolator 'breaks' the galvanic path between two grounds. Breaking this galvanic path can be accomplished by any number of means including electromagnetic, optical, capacitive, inductive and even acoustic methods. The first and foremost duty of an isolator is to break the galvanic path between circuits that are tied or ‘grounded’ to different potentials. So even though there are two grounds and different voltages at each ground, there is no current flow. ![]() ![]() When the conductive path between the differential voltages is broken, a current cannot form. So what can be done? Use a signal isolator to break the galvanic path between the two grounds (Figure 2). Why? Because you cannot always control the number of grounds, and it is often impossible to just ‘lift’ a ground.įigure 1: A ground loop forms when the voltages at two ground points in a loop are at different potentials. The challenge is that the first and second conditions are not plausible candidates for elimination. To remove the ground loop, any one of these three conditions must be eliminated. There is a galvanic path between the grounds.The grounds are at different potentials.This current path, known as a ground loop, is a very common source of signal inaccuracies.Ī ground loop forms when three conditions are present: When this happens, an additional and unpredictable amount of current is introduced into the loop, which distorts the true measurement. But when the voltages at the two ground points are different, a circulating, closed current path is formed by the copper wires used for the 4-20 mA signal and the ground (Figure 1). The loop in question may be as simple as a differential pressure transmitter sending a 4-20 mA measurement to a receiver, such as a recorder. However, it was soon discovered that when 4-20 mA (or other DC) signal wires have paths to ground at both ends of the loop, problems are likely to occur. The need for signal isolation began to flourish in the 1960s and continues today. They can be used to share, split, boost, protect, step down, linearise and even digitise process signals. ![]() Just as important, they are called on to do a whole lot more. Whether you call them signal isolators, signal converters or signal interfaces, these useful process instruments solve important ground loop and signal conversion challenges every day. By using the right signal interface instruments, in the right ways, potential problems can be easily avoided well before they boil over.
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