The Versatility of Signal Interface Instruments

Summary

Whether they are called “signal isolators,” “signal converters” or “signal interfaces,” these useful process instruments solve important ground loop and signal conversion challenges every day. Just as important, they are called on to do much more. They can be used to share, split, boost, protect, step down, linearize, and even digitize process signals. This article explains many of the important ways these signal interface instruments can be used, and what to look for when specifying them. 

Signal isolation 

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 4-20 mA (or other 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 (DP) transmitter sending a 4-20 mA measurement to a receiver such as a proportional-integral-derivative (PID) Controller. But when the voltages at the two ground points are different, a circulating, closed current (I) path is formed by the copper wires used for the 4-20 mA signal and ground (Figure 1). 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 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 because the number of grounds cannot always be controlled, and it is often impossible to just “lift” a ground. 

The ground may be required for the safe operation of an electronic device. It’s also possible that 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, one cannot reach into the earth and regulate the voltage at these permanent ground points. 

Breaking the galvanic path

The duty of an isolator is to break the galvanic path between circuits that are tied or “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. Breaking this galvanic path can be accomplished by electromagnetic, optic, capacitive, inductive and even acoustic methods. 

With most industrial measuring equipment, the two prevalent methods chosen for galvanic isolation are optical and transformer.

Optical 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 an 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 the circuits, thus 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. 

Transformer isolation. 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 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 must be rectified and amplified back into the desired dc signal output. 

Two-way versus three-way isolation 

Two common terms used within the process control industry with respect to isolation are two-way and three-way isolation. Isolation specifications often detail what the isolation levels are from input to output. This is often referred to as two-way (input-to-output) isolation and is the appropriate specification for a two-wire transmitter since it is powered from either its input or output terminals. 

However, many manufacturers fail to mention or outline the isolation details when their isolators are four-wire (line/mains-powered) and require 24 Vdc, 110 Vac or 220 Vac to operate its circuits. In these instances, ensure that an isolator has full three-way isolation. 

Three-way isolation is defined as input-to-output, power-to-input, and power-to-output isolation. If the isolator is powered by a dc supply, many manufacturers use common signal wires between the output and the power input. In these situations, there could be problems with common mode noise or a failing switching power supply that could create unwanted output signal errors. 

Signal conversion 

Signal converters are used to get legacy signal types, such as 10-50 mA, converted to a standard 4-20 mA or some other signal type that is compatible with a particular receiving device (Figure 3).

Fixed range or configurable signal converters? 

There are three approaches to performing signal conversion:

1. One is to use fixed-ranged signal converters designed and built specifically for the conversion need such as 0-10 V in and 4-20 mA out. The advantage is simplicity, as there is nothing to configure. Just mount and wire the device, and you’re up and running. The disadvantage is lack of flexibility. If the application changes, the fixed-range signal converter is not easily, or simply can’t be, modified to accommodate signal types other than what was originally specified. 

2. Another solution is to use a signal converter that has switches or jumpers to select or re-range the input and/or output. There’s a little more work to make the instrument suitable to the application, but a configurable signal interface is more flexible in addressing multiple applications or changing signal conversion needs. Fewer instruments need to be kept in stock. 

3. The third approach is to use a signal converter that is PC-configurable to provide similar application flexibility, plus some performance enhancements. Usually, the rangeability has more resolution, and there are no potentiometers, jumpers or switches that can be easily changed without authorization. 

Step down dangerous ac signals 

Normally, when one thinks of isolators, they think of solving a problem at the instrument control level layer, typically dealing with dc signals. However, very common applications use a signal converter to monitor, trend, or alarm on ac signals. With preventive maintenance budgets shrinking, companies are closely monitoring expensive and critical equipment purchases. Pumps, motors and fans are quick to fall into this category. 

Since much of this equipment is powered with ac voltage and high levels of current, a current transformer (CT) is installed. The role of a CT is two-fold. First, a CT is used to step down the current to a level that can easily be monitored. Second, safety is always a large concern. No one, especially a plant safety manager, wants technicians used to working with 24 Vdc grabbing hold of 5 amps ac. 

A signal converter with an ac current input is used in these situations to convert and isolate a “high level” ac signal to a lower level 4-20 mA dc signal. The secondary of the CT, which is almost always 0-5 amps, can be directly wired into the input of the signal converter. As an added measure of protection, some manufacturers offer an externally mounted CT option that makes use of a “mini-CT” to step the 0-5 amps ac down to 0-5 mA ac. This signal interface with the much lower ac signal is now very safe to wire and handle. 

Digital signal conversion 

An emerging method of converting signals ignores all the previous rules laid down by analog isolators and converters. This new “digital signal conversion” is becoming especially popular in locations where power is sparse, and wires are few. A common application deals with digitally converting or “mapping” HART digital signals to the popular MODBUS RTU serial communications protocol (Figure 4). 

Powering an isolator/converter 

A signal isolator/converter can be four-wire (line/mains powered) or two-wire (loop-powered). Selecting the correct type of isolator and power configuration depends on the application. 

Four-wire signal isolators. A four-wire isolator/converter is used when the instrument output must be voltage (i.e., 0-10 V), zero-based (i.e., 0-20 mA), or bipolar (i.e., -10 V to +10 V). A four-wire isolator usually sources its current output and typically has a drive capacity of around 1,000 to 1,200 ohms. Some isolators will drive up to 1,800 ohms. 

Two-wire signal isolators. A two-wire isolator/converter typically costs less to install than a four-wire unit because power wires don’t have to be run to the unit. Loop-powered instruments can be powered from the loop either on their output side or their input side. 

Isolators/converters that are output loop-powered are powered just like any other two-wire DP, pressure, or temperature transmitter. The output always has to be some form of 4-20 mA, but signal conversion (such as 1-5 V to 4-20 mA) and split ranging, like a 4-12 mA range, can still be performed. When powered with 24 V, these isolators typically drive into 600 ohms. 

A two-wire, input loop-powered isolator is a great solution when applied correctly. The beauty of this isolator is its overall simplicity, with integration into the loop nearly seamless. For example, refer to Figure 5 and imagine that the isolator was not originally implemented. Soon after startup, it is discovered that isolation is required for the process. The good news is that to install this type of isolator, just break the loop where convenient and insert the isolator. A simple solution, wiring changes and installation costs are minimal.