Signal isolators, converters and interfaces - Part 1

By using the right signal interface instruments, in the right ways, potential problems can be easily avoided well before they boil over.

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. Just as important, they are called on to do a whole lot more. They can be used to share, split, boost, protect, step down, linearise and even digitise process signals.

Signal isolation

The need for signal isolation began to flourish in the 1960s and continues today. 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 transmitter sending a 4-20 mA measurement to a receiver, such as a recorder. 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). 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: There are two grounds; The grounds are at different potentials; 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 that 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.

Figure 1: A ground loop forms when the voltages at two ground points in a loop are at different potentials.

So what can be done? Use a signal isolator to break the galvanic path between the two grounds (Figure 2). When the conductive path between the differential voltages is broken, a current cannot form. So even though there are two grounds and different voltages at each ground, there is no current flow. The ground loop has been eliminated.

The first and foremost duty of an isolator is to break the galvanic path between circuits that are tied or ‘grounded’ to different potentials. Breaking this galvanic path can be accomplished by any number of means including electromagnetic, optical, capacitive, inductive and even acoustic methods. Breaking the galvanic path The two most common methods chosen for galvanic isolation are optical and transformer isolation.

Figure 2: A signal isolator 'breaks' the galvanic path between two grounds.

Optical isolation

Optical isolation uses light to transfer a signal between elements of a circuit. The optocoupler or optoisolator is usually self-contained in a small compact module that can be easily mounted on a circuit board. 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 output signal of the phototransistor is proportional to the light intensity of the source. 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 have to be rectified and amplified back into the desired DC signal output.

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. 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.

Figure 3: Signal converters convert one signal type to another that is compatible with a particular receiving device.

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.

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 authorisation.

Signal conversion

Step down dangerous AC signals Normally when you think of isolators, you 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 preventative 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 at high current levels, a current transformer (CT) is installed.

Figure 4: An externally mounted mini-CT steps down a dangerous 0-5 A AC to a safe 0-5 mA signal.

The role of a CT is twofold - firstly to step down the current to a level that can easily be monitored, and secondly to reduce the current to a level that is safe to handle.

Digital signal conversion

Another method of converting signals ignores all the previous rules laid down by analog isolators and converters. Digital signal conversion is 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 5).

Figure 5: Digital signal conversion is becoming a popular strategy where power is sparse and wires are few.

Many PLCs and DCSs accept Modbus RTU, so this becomes a quick and efficient way to get HART data into a control system that doesn’t natively accept HART. HART devices and HART signals contain multiple pieces of data per instrument. Therefore, a HART-to-Modbus converter can be an effective tool when additional process variable and diagnostic data from field instruments is desired.

On the data acquisition side, you usually find an RTU or SCADA system that supports Modbus RTU. A HART-to-Modbus converter not only gathers all of the HART data from the HART transmitters and converts it to Modbus, but it also powers the HART bus using its 9-24 VDC power input. This allows any Modbus RTU-enabled RTU or SCADA system to monitor any HART variable from any of the up to 16 HART field devices on a multidrop HART network.

Powering an isolator/converter

A signal isolator/converter can be 4-wire (line/mains-powered) or 2-wire (loop-powered).

Selecting the correct type of isolator and power configuration depends on the application. 4-wire (line/mains-powered) isolators A 4-wire isolator/converter (Figure 6) is used when the instrument output has to be voltage (ie, 0-10 V) or zero-based (ie, 0-20 mA) or bipolar (ie, -10 to +10 V). A 4-wire isolator usually sources its current output, and typically has a drive capacity of around 1000 to 1200 Ω. Some isolators will drive up to 1800 Ω.

Figure 6: 4-wire (line/mains-powered) signal isolator/converter.

2-wire (loop-powered) isolators

A 2-wire isolator/converter typically costs less to install than a 4-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 2-wire DP, pressure or temperature transmitter, as shown in Figure 3. 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 Ω. A 2-wire input loop-powered isolator is a great solution when applied correctly (Figure 7). The beauty of this isolator is its overall simplicity, with integration into the loop nearly seamless.

Figure 7: 2-wire input loop-powered signal isolator/converter.

To install this type of isolator, you just break your loop where convenient and insert the isolator. A simple solution, and wiring changes and installation costs are minimal.

However, it’s also simple to misapply an input loop-powered isolator. Certain rules must be followed. An input loop-powered isolator is powered from the 4-wire transmitter in the field. The transmitter’s 4-20 mA output and its compliance voltage must power the isolator electronics and the isolator’s output. Because loop-power is limited, the isolator’s output load is held to 250 Ω. The receiver’s input impedance can be anywhere from 0-250 Ω, and it should be a fixed load. In addition, there can be no voltage on the output of the isolator.

To run the isolator electronics, the isolator consumes 5.5 V from the loop or, to put it another way, the isolator itself looks like a 275 Ω load on the transmitter. To calculate the total burden on the transmitter, you have to add the isolator load to the 275 Ω load. The total load could then be as high as 525 Ω plus wire resistance. That is not usually a challenge for a 4-wire transmitter, but it can be for a loop-powered transmitter limited to 600 Ω.

A caution about 2-way versus 3-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 2-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 4-wire (line/mains-powered) and require 24 VDC, 110 VAC or 220 VAC to operate their circuits. In these instances, you want to ensure that you have an isolator that 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 you could have problems with common mode noise, or a failing switching power supply that could create unwanted output signal errors.

In Part 2

We’ve covered some of the basics of signal isolation and conversion, and in Part 2 of this article we will look at some more advanced applications.