Critical Technologies in Front-End Signal Conditioning Systems

Signal conditioning is one of the most important -- and most overlooked -- components of a data acquisition system. With it, you can bring real-world signals into your digitizer. Many sensors require special signal conditioning technology, and no instrument has the capability to provide all types of signal conditioning to all sensors. For example, thermocouples produce very low voltage signals, which require amplification, filtering and linearization. Other sensors, such as strain gauges and accelerometers, require power in addition to amplification and filtering, while other signals may require isolation to protect the system from high voltages. No single instrument can provide the flexibility required to make all of these measurements. However, with front-end signal conditioning, you can combine the necessary technologies to bring these various types of signals into a single data acquisition system.

Not all front-end signal conditioning options are equal. Most choices are nonintelligent, parallel-in/parallel-out configurations that offer the bare minimum of functionality for a select few signals or sensor types. However, for computer-based measurement and automation, you want a system designed to take advantage of the latest PC-based data acquisition and instrumentation technologies. This system should have programmable input settings, the ability to be automatically detected by your computer, and tight integration with your software to handle scaling and channel management. Your system should offer all of the conditioning technologies you need, proof of its accuracy, and the capability to take advantage of the advances in high-speed digitizers.

This document will first give you a general overview of signal conditioning and then explain these critical technologies so you can make an informed decision about your next front-end signal conditioning system. Experienced users of signal conditioning systems should skip directly to the Critical Technologies section.

Signal Conditioning Background

Most signals require some form of preparation before they can be digitized. As previously mentioned, thermocouple signals are very small voltage levels that must be amplified before they can be digitized. Other sensors, such as RTDs, thermistors, strain gauges, and accelerometers, require electrical power to operate. Even pure voltage signals can require special technologies for blocking large common-mode signals or for safely measuring high voltages. All of these preparation technologies are forms of signal conditioning.

Because of the vast array of signal conditioning technologies, the role and need for each technology can quickly become confusing. Therefore, we’ve provided a list of common types of signal conditioning, their functionality, and examples of when you need them.

• Amplification -- When the voltage levels you are measuring are very small, amplification is used maximize the effectiveness of your digitizer. By amplifying the input signal, the conditioned signal uses more of the effective range of the analog-to-digital converter (ADC) and enhances the accuracy and resolution of the measurement. Typical sensors that require amplification are thermocouples and strain gauges.

• Attenuation -- Attenuation is the opposite of amplification. It is necessary when the voltages to be digitized are beyond the input range of the digitizer. This form of signal conditioning diminishes the amplitude of the input signal so that the conditioned signal is within the range of the ADC. Attenuation is necessary for measuring high voltages.

• Isolation -- Voltage signals well outside the range of the digitizer can damage the measurement system and harm the operator. For that reason, isolation is usually required in conjunction with attenuation to protect the system and the user from dangerous voltages or voltage spikes. Isolation may also be required when the sensor is on a different ground plane from the measurement sensor (such as a thermocouple mounted on an engine).

• Multiplexing -- Typically, the digitizer is the most expensive part of a data acquisition system. By multiplexing, you can sequentially route a number of signals into a single digitizer, thus achieving a cost-effective way to greatly expand the signal count of your system. Multiplexing is necessary for any high-channel-count application.

• Filtering -- Filtering is required to remove unwanted frequency components from a signal, primarily to prevent aliasing and reduce signal noise. Thermocouple measurements typically require a lowpass filter to remove power line noise from the signals. Vibration measurements normally require an antialiasing filter to remove signal components beyond the frequency range of the acquisition system.

• Excitation -- Many sensors, such as RTDs, strain gauges, and accelerometers, require some form of power to make a measurement. Excitation is the signal conditioning technology required to provide this power. This excitation can be a voltage or current source, depending on the sensor type.

• Linearization -- Some types of sensors produce voltage signals that are not linearly related to the physical quantity they are measuring. Linearization, the process of interpreting the signal from the sensor as a physical measurement, can be done either with signal conditioning or through software. Thermocouples are the classic example of a sensor that requires linearization.

• Cold-Junction Compensation -- Another technology required for thermocouple measurements is cold-junction compensation (CJC). Any time a thermocouple is connected to a data acquisition system, the temperature of the connection must be known in order to calculate the true temperature the thermocouple is measuring. A built-in CJC sensor must be present at the location of the connections.

• Simultaneous Sampling -- When it is critical to measure two or more signals at the same instant in time, simultaneous sampling is required. Front-end signal conditioning can provide a much more cost-effective simultaneous sampling solution than purchasing a digitizer for each channel. Typical applications that might require simultaneous sampling include vibration measurements and phase-difference measurements.

Most sensors require a combination of the previously described signal conditioning technologies. Again, the thermocouple is the classic example because it requires amplification, linearization, cold-junction compensation, filtering, and sometimes isolation. Ideally, a good measurement platform should give you the ability to select the type of signal conditioning that is needed for your application. In some systems, front-end signal conditioning is an option, but in other systems, front-end signal conditioning is a necessity to make the required measurements. As a rule of thumb, your measurement system should include front-end signal conditioning if you are planning to use any of the following sensors:

• Thermocouples
• RTDs
• Thermistors
• Strain gauges
• Force/load/torque transducers
• LVDTs/RVDTs/resolvers
• Accelerometers
• Mixed low-voltage/high-voltage sources
• Current sources
• Resistance sources

Critical Technologies

When front-end signal conditioning is necessary for your data acquisition system, you should choose a system that takes advantage of the latest advances in computer-based measurement and automation. For your signal conditioning platform to fully exploit these advances, there are several critical technologies that it should possess. These critical technologies ensure that you get a high-performance signal conditioning platform that integrates tightly with the rest of your system, all at a reasonable total cost. The primary technologies we will examine in depth are integration, calibration, connectivity, switching, isolation, expandability, bandwidth, software, and ease of use. By understanding each of these technologies, you will be able make an informed decision on the purchase of a front-end signal conditioning system.

Integration
The ability of your front-end signal conditioning system to integrate easily with the rest of your system is technology that is a must. Your system should be modular, thus giving you the ability to choose the types of signal conditioning necessary for your system. It is also critical to have a system that accommodates mixed signal types. For example, you should be able to connect currents, high voltages, various sensors, analog outputs, digital I/O, and switching all into the same platform.

Calibration
One of the most critical technologies that a front-end signal conditioning system should possess is the ability to be easily and accurately calibrated. Most measurement devices are calibrated at the factory, but the accuracy immediately starts to drift with time and temperature changes. To make the most accurate measurements possible, it is necessary to periodically calibrate the entire data acquisition system. If your system has precision onboard voltage references, you can adjust your measurement system to compensate for temperature changes. In addition, you must have access to external calibration services to keep your system performing up to the manufacturer’s specifications year after year. It is very important to learn the calibration process for any front-end signal conditioning system under consideration because that is the only way to ensure that your investment contains the technology you need to make accurate and reliable measurements.

Connectivity
Because connecting your signals to your signal conditioning system can be a major issue, it is critical to select a platform that gives you the connectivity options you need. A good front-end signal conditioning system should give you a wide range of connectivity options, including thermocouple plugs, screw terminals, and BNC connectors.

Switching
In today’s demanding test environments, the ability to route signals easily throughout your measurement system is a technology that can lead to huge improvements in test times. As an example, consider a case where a unit under test (UUT) must be subjected to four separate measurements in the testing process. Without the proper technology, the UUT must be reconnected to each different measurement device for each test. With state-of-the art switching technology, you can not only route the UUT leads automatically to each measurement device in turn, but also test several UUTs at the same time. You thus achieve more efficient use of your test equipment, faster test times, and less user intervention. The selection of a front-end signal conditioning system that offers this technology can have a huge impact on the overall performance of your system.

Isolation
Another important technology to consider is isolation. When you are measuring signals that either are high-voltage signals or are subject to voltage spikes, it is critical that those signals are isolated from the rest of your system. Inadequate isolation compromises the safety of the operator, as well as the integrity of the entire data acquisition system. When determining the isolation requirements of your system, it is imperative to have reliable and accurate isolation specifications, including both a safe working voltage rating and an installation rating.

Expandability
Any front-end signal conditioning system should be easily expandable. Adding more channels or different types of signals to your system must not require a massive overhaul of your data acquisition system. With the right technology, expanding your system should be as simple as plugging in another module.

Bandwidth
In addition to being expandable, a system should also have the bandwidth to handle the data throughput from a high-channel-count system. The bandwidth should also be high enough to accommodate future growth in channel count. System bandwidth is typically expressed in samples/second (Hz). To determine the minimum necessary bandwidth of the system, you should multiply your total number of expected channels times the maximum sampling rate you will need on an individual channel. For a high-channel-count system, the required bandwidth for a modest acquisition rate can quickly reach several hundred kHz. Bandwidth is an often overlooked, but extremely important technology to consider when selecting a front-end signal conditioning system.

Software
A large portion of the total cost of a test and measurement system is application development. To keep application development costs to a minimum, you must use software tools that maximize your productivity. Your front-end signal conditioning system should be designed to integrate tightly with these software tools. Only with the capability to fully control your front-end signal conditioning system from your software application can you take full advantage of the latest technologies in computer-based measurement and automation.

Configuration/Installation
Finally, any signal conditioning system you consider should be easy to use. No one can afford to lose time due to overly complex installation or configuration issues. An ideal front-end signal conditioning system will poll the hardware, report which equipment you have, and provide you a software interface for setting up all signal conditioning settings. You should be able to configure channels through software, and have the capability to set up channel names and scaling to engineering units.

National Instruments Signal Conditioning Solutions

National Instruments produces a front-end signal conditioning system known as SCXI (Signal Conditioning eXtensions for Instrumentation). SCXI is a rugged, modular platform for high-performance signal conditioning. An SCXI system consists of multichannel signal conditioning modules installed in one or more rugged chassis. SCXI offers a wide range of signal conditioning modules -- analog input, analog output, digital I/O, counter/timer, switching, and multiplexing. To the analog input modules, you can connect an extensive variety of sensors, such as thermocouples, RTDs, thermistors, strain gauges, accelerometers, and LVDTs. The modules condition the input signals and multiplex them onto the backplane bus of the chassis, where they are typically routed into a data acquisition device to be digitized.The SCXI architecture has many advantages as a front-end signal conditioning solution. SCXI incorporates all of the critical technologies necessary to build a high-performance, fully integrated, and easy-to-use front-end signal conditioning system, all at an affordable cost per channel. It is modular and easily expandable, making it ideal for medium to high channel count applications. Because it is modular, the SCXI architecture makes it very easy to measure mixed signal types in one system. SCXI is also a high bandwidth system, capable of multiplexing up to 333 kS/s into a single data acquisition device. It integrates seamlessly with National Instruments data acquisition devices and timesaving application development software such as LabVIEW and Measurement Studio. SCXI modules are shipped with NI-DAQ driver software, which offers easy set up, including autodetection of modules and programmatic modification of settings. Many SCXI modules now come with both calibration certificates (proving the module is within manufacturer’s specifications) and onboard calibration sources (so you can easily calibrate it from software). Finally, with the wide range of options in SCXI modules, you can specify a system that includes signal routing (with matrix switching modules), isolation, filtering, and more.

The SCXI architecture, first introduced in 1992, has been continually improved through software enhancements and additional modules. It is now a mature product with the benefit of years of continuous technology enhancements. The SCXI front-end signal conditioning system is used by thousands of customers in a wide variety of applications.

Conclusions

A front-end signal conditioning system should be looked at as a platform that defines the measurement capabilities of your data acquisition system. It is critical to invest in a platform that is both proven by time and constantly improving by incorporating the latest technologies. The most important technologies to be considered in any front-end data acquisition system are calibration, isolation, bandwidth, expandability, measurement coverage, signal routing, and, of course, ease of use. The National Instruments SCXI system is a proven platform that integrates these critical technologies into a full-featured data acquisition system that can meet a broad range of measurement requirements.

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