1 September 2005
Problem solvers in signal conditioning
By Patrick McCurdy and Davis Mathews
As the world of process control becomes increasingly complex, control and measurement signals that make or break the process application become more important. A stray signal can cause system inefficiencies, product contamination, and system failure. Consider several design parameters to maintain the accuracy and integrity of the overall system. Isolation and conditioning of signals have become top priorities. Related trends in process automation include more distributed measurement and control and smaller and lower cost input/output (I/O) modules. In order to meet these requirements, isolation and conditioning are sometimes left out of the I/O function, resulting in future problems, such as ground loops and noise, and therefore requiring external signal conditioners.
Why use signal conditioners?
We need signal conditioners to ensure accuracy, transmission, and signal type are compatible with the rest of the control system. Typical tasks of a signal conditioner include conversion, isolation, amplification, filtering, and linearization of analog temperature sensor signals (thermocouples and RTDs).
Signal conversion: Signal conversion changes an analog signal from one electrical form to another, allowing equipment with dissimilar signals to communicate. If a new field transmitter outputs 4-20mA and the older controller accepts only a 0-10V input, you must convert the signal from the field. Those signal conditioners accepting several different input signal types and producing different signal outputs are universal signal converters.
Signal amplification: Low-level signals, like thermocouple signals, are often small. Signal conditioners boost the level and can linearize the incoming signal to a range the controller can easily read. Also, using an external signal conditioner located closer to the signal source improves the signal-to-noise ratio of the measurement by amplifying the signal level before environmental noise affects it. Amplification also improves resolution. (See figure below.)
Filtering: Electric interference, or noise, is an unwanted signal that can cause intolerable error in an electronic control or measurement system. We can break down interference or electrical noise into two categories: radio frequency interference (RFI) and electromagnetic interference (EMI). The effects of RFI and EMI can cause unpredictable errors in instrumentation performance and can even result in equipment failure. A professional signal conditioner should typically include a low-pass filter to prevent noise from causing erroneous errors to the measurement system.
Signal isolation from ground loops: Most analog control loops are grounded at more than one point, whether by intention or accident. Grounds can exist at the instrument and at other points in the loop, such as the receiver, or the power supply. A single ground poses no problem, but multiple grounds cause ground loops. In a ground loop, you tie each ground to a different earth potential. This allows current to flow between the grounds, interfering with the actual signal.
All isolation devices, regardless of their design or technology, perform the same function: they interrupt common mode currents while preserving information flow. Isolation also permits separating (and accurately measuring) small normal mode signals that ride on large common mode carrier (unwanted) signals. It's this function that breaks the ground loop.
In a perfect world, an isolation circuit would allow information to flow across an isolation barrier with no signal degradation, no power consumption, no size or cost penalty, and infinite protection against common mode voltage differential (otherwise called ground loops) and transients. This type of isolator doesn't exist, but does serve as a benchmark to evaluate isolation technologies.
Isolation can be analog or digital. Both analog and digital isolation can use components, such as transformers and opto couplers, for the separation of unwanted signals.
Analog isolation using transformers
A common analog isolation technique uses transformers in the circuit design based on amplitude modulation. This design uses a chopper circuit to chop the analog waveform into positive and negative segments and inverts every other cycle at high frequencies. The resulting signal then passes across a transformer, which is then reconstructed, demodulated, and filtered. Although this method is popular with low-cost isolators, it does introduce errors into the control system. This method is common with isolated PLC cards or controller cards.
Digital isolation circuits
Design engineers are turning to digital technologies in order to improve accuracy, improve system functionality, and reduce cost. Advances in digital isolation technology provide better accuracy and higher resolution at lower costs. The digital circuit converts the analog signal to a digital format, which then passes through an isolation barrier (transformer or opto) and then to a micro-processor or digital signal processor that evaluates a large number of measurement criteria and passes the accurate signal form through. Most universal signal conditioner isolators-PC-programmable and dual inline package (DIP) switch selectable-use this technology. The two common technologies for digital isolation circuits are based on optos and magnetic (otherwise known as DC-to-DC converters).
Optical isolation uses a light emitting diode (LED) and a photodiode to convey information across an isolation barrier. Optocouplers, which see use in almost every branch of the electronic industry, are an effective low-cost method of isolating digital signals. However certain limitations exist in pure opto isolation circuits. These limitations are due to the time associated with the charging and discharging of the photodiode. When the energy from the LED strikes the photodiode, it immediately starts converting photons to electrons. When the LED turns off, the photodiode stops converting photons to electrons, but the existing charge needs to dissipate. There is a propagation delay associated with this process, which can distort input pulse widths. This affects the throughput rate of an optocoupler, which limits its use to slower applications.
Even at slower speeds, optos can experience problems. The photodiode, a semi-conductor, is very sensitive to electro-static discharge (ESD) and common mode transients, which can produce unwanted distortions to the output. Despite the limitations, optos offer high isolation levels and low cost in a wide variety of low-speed applications.
Magnetic isolation (DC/DC isolation) designs are based on transformers within the circuit. A magnetic isolator consists of a transformer with a driver at the primary and a receiver at the secondary. The driver encodes the input signal into an AC waveform that then couples from the primary to the secondary. The receiver then decodes the waveform and reconstructs the signal at the output.
Constructing transformer-based digital isolators involves using high-speed CMOS devices with slow propagation delays, which can thus see use at high speeds and low speeds, maintaining high accuracy. The disadvantage of this type design is it requires a hybrid construction and enough space on the board (or vented housing designs) for heat dissipation. A few have been introduced to the market recently that are cost effective and require less PCB real estate. These devices, however, cost more than optos but provide high integrity signal conversion in a compact design.
Industrial signal isolators
Whether you select isolation technology depends on cost, size, and performance. Optical isolators provide the lowest cost but are more sensitive to electrostatic discharge (ESD) and common mode transients. Magnetic isolation provides high accuracy, robustness, and efficiency but costs more. Manufacturers of electronic components and signal isolators use different types of isolation techniques depending on the form, function, and cost of the device. In some cases, they use a combination of technologies.
Both analog and digital isolation (transformers and opto coupler-based) see use in some designs, and many signal conditioning modules will contain one or both components. In general, transformers are a little more electrically rugged and can handle larger transient voltage spikes than optocouplers. Transformers can also run at fast speeds, perhaps up to 1 GHz, while optos are limited to <10 MHz. Transformers can pass energy or power from one area to another. The optos will isolate a voltage but do not really pass energy to another area. (The amount of energy in the photons is enough so the receiver recognizes it, but that's all.) Transformers then can see use on the power side to provide energy to the module electronics; it would be more difficult to use an opto in that case. Optos generally use less board space and are great for passing digital data because they do not depend on a changing voltage in order to maintain a digital on or off signal. <p />The industry trend for high performance signal isolators and conditioners is to provide highly functional, accurate, universal microprocessor-based products in a compact package at a moderate price per point. New transformer component technology and signal conditioner designs allow tiny packaging, full function, and high-isolation values (1.5kV) without heat dissipation problems. With power dissipation < 400mW per channel, these signal conditioners can mount adjacent to each other providing maximum functionality in even the smallest distributed measurement and control cabinet applications.
Whether designed on the front end or retrofitted later, signal conditioners are versatile and cost effective problem solvers for improving measurement and control system reliability, compatibility, and flexibility.
Behind the byline
Patrick McCurdy is a product marketing manager for signal conditioners at Phoenix Contact in Harrisburg, Penn. His e-mail is firstname.lastname@example.org. Davis Mathews is a regional business unit manager for Interface Products at Phoenix Contact. His e-mail is email@example.com.