January/February 2010

Signal conditioners: The 'ins' and 'outs'

By Jay DeCastro

Whether you call them signal conditioners, isolators, converters, or interfaces, these useful process instruments solve important ground loop challenges every day.

The need for signal isolation began to flourish in the 1960s and continues today. Electronic transmitters were quickly replacing their pneumatic predecessors because of cost, installation, maintenance, and performance advantages. However, it was soon discovered that when 4-20mA (or other direct current, or DC) signal wires have paths to ground at both ends of the loop, problems are likely to occur.

The loop in question may be as simple as a differential pressure transmitter sending a 4-20mA measurement to a receiver, such as a recorder. But when the voltages (V) at the two ground points are different, a circulating, closed current (I) path is formed by the copper wires used for the 4-20mA signal and the ground. When this happens, an additional and unpredictable amount of current is introduced into the loop, which distorts the true measurement. This current path, known as a ground loop, is a very common source of signal inaccuracies.

A ground loop forms when three conditions are present:

  1. There are two grounds.
  2. The grounds are at different potentials.
  3. There is a galvanic path between the grounds.

To remove the ground loop, any one of these three conditions must be eliminated. The challenge is the first and second conditions are not plausible candidates for elimination. Why? Because you cannot always control the number of grounds, and it is often impossible to just "lift" a ground.

The ground may be required for the safe operation of an electronic device. It is also possible the ground exists because the instrument is in physical contact with the process which, in turn, is in physical contact with the ground. From a practical standpoint, you cannot reach into the earth and regulate the voltage at these permanent ground points.

 signal1

So what can be done? Use a signal isolator to "break" the galvanic path between the two grounds. When the conductive path between the differential voltages is broken, a current cannot form. The ground loop has been eliminated.

Breaking the galvanic path

The first and foremost duty of an isolator is to break the galvanic path between circuits that are tied or "grounded" to different potentials. A galvanic path is a path in which there is a direct electrical connection between two or more electrical circuits that allow current to flow. Breaking this galvanic path can be accomplished by any number of means. For most industrial measuring equipment, the two prevalent methods chosen for galvanic isolation are optical and transformer isolation.

Optical isolation uses light to transfer a signal between elements of a circuit. The opto-coupler or opto-isolator is usually self-contained in a small compact module that can be easily mounted on a circuit board.

An optical isolation circuit is comprised of two basic parts-a light source (usually a light emitting diode (LED), acting as the transmitter) and a photo-sensitive detector (usually a phototransistor, acting as the receiver). The output signal of the opto-coupler is proportional to the light intensity of the source. The insulating air gap between the LED and the phototransistor serves as the galvanic separation between circuits, providing the desired isolation between two circuits at different potentials.

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.

 signal2

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. The electromagnetic field intensity is proportional to the input signal applied to the transformer. Transformers are very efficient and fast at transferring alternating current (AC) signals. Since many process control signals are DC, they must be electrically "chopped" 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.

The ability to depend on accurate monitoring and control signals is literally priceless. Signal conditioners enhance measurement accuracy and protect signals from damaging conditions, thereby saving money.

ABOUT THE AUTHOR

Jay DeCastro is a project engineer at Moore Industries-International, Inc.