Data Acquisition Fundamentals

This document discusses the elements of a PC-based data acquisition system and considerations for users who are selecting components for such a system.

Introduction

Today, most scientists and engineers use personal computers (PCs) with PCI, PXI, CompactPCI, PCMCIA, USB, FireWire, parallel, or serial ports for data acquisition in laboratory research, test and measurement, and industrial automation. Many applications use plug-in devices to acquire data and transfer it directly to computer memory. Others use data acquisition (DAQ) hardware remote from the PC that is coupled via Ethernet, parallel, or serial port. Obtaining proper results from a PC-based DAQ system depends on each of the following system elements (see Figure 1):

• The PC
• DAQ Hardware
• Software
• Sensors
• Signal Conditioning

This document gives an overview of each element and explains the most important criteria of each element. The document also defines much of the terminology common to each element of a PC-based DAQ system.

The computer you use for your DAQ system can drastically affect the maximum speeds at which you can continuously acquire data. As computers continuously improve, your DAQ system can take advantage of the computer’s enhanced capabilities, including improved real-time processing, the ability to usdse complex video graphics, and higher streaming-to-disk throughput. Today's technology boasts Pentium IV and PowerPC class processors coupled with the high-performance bus architectures. The PCI bus and USB port are standard equipment on most of today's desktop computers and yield up to 132 Mbytes/s theoretical data transfer capabilities. External and portable PC buses such as PCMCIA, USB, and FireWire offer a flexible alternative to desktop PC-based DAQ systems while achieving up to 40 Mbytes/s transfer rates. For remote or distributed DAQ applications, you can place measurement nodes near sensors and signal sources and use standard networking technology, such as Ethernet, serial, or wireless. When choosing a DAQ device and bus architecture, keep in mind the data transfer methods supported by your chosen device and bus and the transfer rates.

The data transfer capabilities of your computer can significantly affect the performance of your DAQ system. Twenty years ago, PCs were capable of transferring at rates around 5 MHz, whereas today’s computers can transfer significantly faster. As PC speed continuously increases, DAQ system speed increases as a result.

Today’s PCs are capable of programmed I/O and interrupt data transfers. Direct memory access (DMA) transfers increase the system throughput by using dedicated hardware to transfer data directly into system memory. Using this method, the processor is not burdened with moving data and is therefore free to engage in more complex processing tasks. With National Instruments driver software, NI-DAQ 7, which serves as the interface between the hardware and the computer, the DMA routines to transfer waveform data across the PC bus were optimized, thus providing the ability to transfer data as fast as possible. To reap the benefits of DMA or interrupt transfers, your DAQ device must be capable of these transfer types. For example, while PCI and FireWire devices offer both DMA and interrupt-based transfers, PCMCIA and USB devices use interrupt-based transfers. Depending on how much processing is needed during data transfer, the rate at which the data is transferred from the DAQ device to PC memory may be affected by the data transfer mechanism.

The limiting factor for real-time storage of large amounts of data often is the hard drive. Hard drive access time and hard drive fragmentation can significantly reduce the maximum rate at which data can be acquired and streamed to disk. For systems that must acquire high-frequency signals, select a high-speed hard drive for your PC and ensure that there is enough contiguous (unfragmented) free disk space to hold the data. In addition, dedicate a hard drive to the acquisition and run the operating system (OS) on a separate disk when streaming data to disk.

In the past, applications requiring real-time processing of high-frequency signals needed a high-speed, 32-bit processor with its accompanying coprocessor or a dedicated plug-in processor such as a digital signal processing (DSP) board. However, with today’s processors, you can perform the same real-time analysis without a specialized DSP because they are capable of rates around 2.5 GHz.

Determine which operating system and computer platform will yield the greatest long-term return on investment while still meeting your short-term goals. Factors that influence your choice may include the experience and needs of both your developers and end users, other uses for the PC (now and in the future), cost constraints, the availability of different computers with respect to your implementation time frame, and software support on that particular operating system. Traditional platforms include Mac OS, which is known for its simple graphical user interface, and Windows 2000 or XP which include native plug and play and power management. Furthermore, real-time operating systems provide reliability and robustness that may appeal to your particular application.

DAQ Hardware

Depending on your application, there are several different classes of PC-based data acquisition devices that you can use.

• Analog Input/Output
• Digital Input/Output
• Counter/Timers
• Multifunction – a combination of analog, digital and counter operations

Analog Inputs

Basic Considerations of Analog Inputs – The analog input specifications give you information on both the capabilities and the accuracy of the DAQ product. Basic specifications, which are available on most DAQ products, tell you the number of channels, the sampling rate, the resolution, and the input range.

• Number of Channels – The number of analog channel inputs is specified for both single-ended and differential inputs for devices with both input types. Single-ended inputs are all referenced to a common ground reference. These inputs are typically used when the input signals are high level (greater than 1 V), the leads from the signal source to the analog input hardware are short (less than 15 ft), and all input signals share a common ground reference. If the signals do not meet these criteria, you should use differential inputs. With differential inputs, each input has its own ground reference; noise errors are reduced because the common-mode noise picked up by the leads is canceled out.

• Sampling Rate – This parameter determines how often conversions can take place. A faster sampling rate acquires more data in a given time and can therefore often form a better representation of the original signal. Data can be sampled simultaneously with multiple converters, or it can be multiplexed, where the analog-to-digital converter (ADC) samples one channel, switches to the next channel, samples it, switches to the next channel, and so on. Multiplexing is a common technique for measuring several signals with a single ADC.

• Resolution – The number of bits that the ADC uses to represent the analog signal is the resolution. The higher the resolution, the larger the number of divisions the range is broken into, and therefore, the smaller the detectable voltage change. Figure 3 shows a sine wave and its corresponding digital image as obtained by an ideal 3-bit ADC. A 3-bit converter (which is actually seldom used but a convenient example) divides the analog range into 2³, or 8 divisions.

• Range – Range refers to the minimum and maximum voltage levels that the ADC can quantize. NI multifunction DAQ devices offer selectable ranges so that the device is configurable to handle a variety of voltage levels. With this flexibility, you can match the signal range to that of the ADC to take advantage of the available measurement resolution.

• Code Width – The range, resolution, and gain available on a DAQ device determine the smallest detectable change in voltage. This change in voltage represents 1 least significant bit (LSB) of the digital value and is often called the code width. The ideal code width is found by dividing the voltage range by the gain times two raised to the order of bits in the resolution. For example, one of our 16-bit multifunction DAQ device, the NI-6052E, has a selectable range of 0 to 10 or -10 to 10 V and selectable gain of 1, 2, 5, 10, 20, 50, or 100. With a voltage range of 0 to 10 V, and a gain of 100, the ideal code width is defined by the following equation:

10 V/(100 X 2¹⁶) = 1.5 µV
Critical Considerations of Analog Inputs – Although the basic specifications previously described may show that a DAQ device has a 16-bit resolution ADC and a 100 kS/s sampling rate, you may not sample at full speed on all 16 channels and get full 16-bit accuracy. For example, some products on the market today with 16-bit ADCs get less than 12 bits of useful data. To determine if the device that you are considering will give you the desired results, scrutinize specifications that go beyond the product resolution. The Accuracy Calculator provides detailed specification of how National Instruments DAQ devices will perform.

While evaluating DAQ products, also consider the differential nonlinearity (DNL), relative accuracy, settling time of the instrumentation amplifier, and noise.

• DNL – Ideally, as you increase the level of voltage applied to a DAQ device, the digital codes from the ADC should also increase linearly. If you were to plot the voltage versus the output code from an ideal ADC, the plot would be a straight line. Deviations from this ideal straight line are specified as the nonlinearity. DNL is a measure in LSB of the worst-case deviation of code widths from their ideal value of 1 LSB. An ideal DAQ device has a DNL of 0 LSB. Practically, a good DAQ device will have a DNL of ±0.5 LSB.

• Relative Accuracy – Relative accuracy is a measure in LSBs of the worst-case deviation from the ideal DAQ device transfer function, a straight line. Relative accuracy is determined on a DAQ device by connecting a voltage at negative full scale, digitizing the voltage, increasing the voltage, and repeating the steps until the input range of the device has been covered. When the digitized points are plotted, the result will be an apparent straight line (see Figure 4a). However, you can subtract actual straight-line values from the digitized values and plot these resulting points, as shown in Figure 4b. The maximum deviation from zero is the relative accuracy of the device.

• The driver software for a DAQ device translates the binary code value of the ADC to voltage by multiplying it by a constant. Good relative accuracy is important for a DAQ device because it ensures that the translation from the binary code of the ADC to the voltage value is accurate. Obtaining good relative accuracy requires that both the ADC and the surrounding analog circuitry are properly designed.
  
• Settling Time – Settling time is the time required for an amplifier, relays, or other circuits to reach a stable mode of operation. The instrumentation amplifier is most likely not to settle when you are sampling several channels at high gains and high rates. Under such conditions, the instrumentation amplifier has difficulty tracking large voltage differences that can occur as the multiplexer switches between input signals. Typically, the higher the gain and the faster the channel switching time, the less likely the instrumentation amplifier is to settle. In fact, no off-the-shelf programmable-gain instrumentation amplifier can settle to 12-bit accuracy in less than 2 µs when amplifying at a gain of 100. NI developed the NI-PGIA specifically for DAQ applications, so devices that use the NI-PGIA can consistently settle at high gains and sampling rates.

• Noise – Any unwanted signal that appears in the digitized signal of the DAQ device is noise. Because the PC is a noisy digital environment, acquiring data on a plug-in device takes a careful layout on multiple-layer DAQ boards by skilled analog designers. Simply placing an ADC, instrumentation amplifier, and bus interface circuitry on a one or two-layer board will likely result in a very noisy device. Designers can use metal shielding on a DAQ device to help reduce noise. Proper shielding not only should be added around sensitive analog sections on a DAQ device, but also must be built into the layers of the device with ground planes.

With sophisticated measurement hardware such as a plug-in DAQ device, you can get significantly different accuracies depending on the device. NI works hard to make extremely accurate devices, in many cases more accurate than stand-alone instruments. This accuracy is published in NI product specifications. Be leery of boards that are inadequately specified; the specification omitted may be the one that causes inaccurate measurements. By evaluating more analog input specifications than simply the resolution of the ADC converter, you ensure that you get a DAQ product that is accurate enough for your application.

Analog Outputs

Analog output circuitry is often required to provide stimuli for a DAQ system. Several specifications for the digital-to-analog converter (DAC) determine the quality of the output signal produced – settling time, slew rate, and output resolution.

• Settling Time – Settling time is the time required for the output to settle to the specified accuracy. The settling time is usually specified for a full-scale change in voltage. For more information on settling time, refer to the Analog Inputs section.

• Slew Rate – The slew rate is the maximum rate of change that the DAC can produce on the output signal. Settling time and slew rate work together in determining how quickly the DAC changes the output signal level. Therefore, a DAC with a small settling time and a high slew rate can generate high-frequency signals because little time is needed to accurately change the output to a new voltage level.

• Output Resolution – Output resolution is similar to input resolution; it is the number of bits in the digital code that generates the analog output. A larger number of bits reduces the magnitude of each output voltage increment, thereby making it possible to generate smoothly changing signals. Applications requiring a wide dynamic range with small incremental voltage changes in the analog output signal may need high-resolution voltage outputs.

Triggers

Many DAQ applications need to start or stop a DAQ operation based on an external event. Digital triggers synchronize the acquisition and voltage generation to an external digital pulse. Analog triggers, used primarily in analog input operations, start or stop the DAQ operation when an input signal reaches a specified analog voltage level and slope polarity. NI-DAQ driver software helps you quickly and easily configure your trigger settings for single or multiple measurement devices.


Multidevice Synchronization

NI developed the Real-Time Synchronization Interface (RTSI) bus for synchronizing measurement products. The RTSI bus uses a custom gate array and a ribbon cable to route timing and trigger signals between multiple functions on one DAQ board or between two or more boards. With RTSI bus, you can synchronize A/D conversions, D/A conversions, digital inputs, digital outputs, and counter/timer operations. For example, with RTSI bus, two analog input boards can simultaneously capture data while a third board generates an output pattern synchronized to the sampling rate of the inputs. NI-DAQ includes a routing and synchronization engine which automatically completes routes within your device and across the RTSI or PXI trigger bus. Along with synchronizing multiple DAQ devices, you can also use the RTSI bus to synchronize with National Instruments motion, vision, and CAN boards, and other instruments.

Digital I/O (DIO)

DIO interfaces are often used on PC DAQ systems to control processes, generate patterns for testing, and communicate with peripheral equipment. In each case, the important parameters include the number of digital lines available, the rate at which you can accept and source digital data on these lines, and the drive capability of the lines. If the digital lines are used for controlling events such as turning on and off heaters, motors, or lights, a high data rate is usually not required because the equipment cannot respond very quickly. The number of digital lines, of course, should match the number of processes to be controlled. In each of these examples, the amount of current required to turn the devices on and off must be less than the available drive current from the device. DIO can also be used in industrial applications, to verify that a switch is open or closed and to check the voltage levels as high or low. It can also be used for high-speed handshaking or simple communication methods.

With the proper digital signal conditioning accessories, you can use the low-current TTL signals to/from the DAQ hardware to monitor/control high voltage and current signals from industrial hardware or to drive external relays. For example, the voltage and current needed to open and close a large valve may be on the order of 100 VAC at 2 A. Because the output of a DIO device is 0 to 5 VDC at several milliamperes, a signal conditioning module, such as SCXI is needed to switch the power signal to control the valve.

A common DIO application is to transfer data between a computer and equipment such as data loggers, data processors, and printers. Because this equipment usually transfers data in one byte (8-bit) increments, the digital lines on a plug-in DIO device are arranged in groups of eight. In addition, some devices with digital capabilities will have handshaking circuitry for communication-synchronization purposes. The number of channels, data rate, and handshaking capabilities are all important specifications that should be understood and matched to the application needs.

Timing I/O

Counter/timer circuitry is useful for many applications, including counting the occurrences of a digital event, digital pulse timing, and generating square waves and pulses. You can implement all these applications using three counter/timer signals – gate, source, and output.

• Gate – The gate is a digital input that is used to enable or disable the function of the counter.
• Source – The source is a digital input that causes the counter to increment each time it toggles, and therefore provides the timebase for the operation of the counter.
• Output – The output generates digital square waves and pulses at the output line.

The most significant specifications for operation of a counter/timer are the resolution and clock frequency. The resolution is the number of bits the counter uses. A higher resolution simply means that the counter can count higher. The clock frequency determines how fast you can toggle the digital source input. With higher frequency, the counter increments faster and therefore can detect higher frequency signals on the input and generate higher frequency pulses and square waves on the output. The DAQ-STC counter/timer used on our E Series DAQ devices, for example, has 16 and 24-bit counters with a clock frequency of 20 MHz. The NI-TIO counter/timer used on NI 660x counter/timer devices has eight 32-bit counters with a maximum clock frequency of 80 MHz.

The DAQ-STC is a NI custom application-specific integrated circuit (ASIC) designed specifically for DAQ applications. In comparison with the off-the-shelf counter/timer chips generally used on DAQ devices, the DAQ-STC stands alone. For example, the DAQ-STC is an up/down counter/timer, meaning that it can use additional external digital signals to count up or down, depending on whether the level is "high" or "low." This type of counter/timer can measure positioning from rotary or linear encoders. Other special functions include buffered pulse-train generation, timing for equivalent time sampling, relative timestamping, and instantaneous changing of sampling rate.

The NI-TIO, found in the 660x devices, is also a custom ASIC designed specifically for timing applications. It incorporates all of the DAQ-STC counter/timer functionality as well as adding new features such as native encoder compatibility, debouncing filters, and two signal edge separation measurements.
Software transforms the PC and the DAQ hardware into a complete data acquisition, analysis, and display system. Without software to control or drive the hardware, the DAQ device will not function and perform properly. The majority of DAQ applications use driver software. Driver software is the layer of software that directly programs the registers of the DAQ hardware, managing its operation and its integration with the computer resources, such as processor interrupts, DMA, and memory. Driver software hides the low-level, complicated details of hardware programming, providing the user with an easy-to-understand interface or a stand-alone application program.

The increasing sophistication of DAQ hardware, computers, and software continues to emphasize the importance and value of good driver software. Properly selected driver software can deliver an optimal combination of flexibility and performance, while significantly reducing the time required to develop the DAQ application.

While selecting driver software, there are several factors to consider.

Available Functionality. Driver functions for controlling DAQ hardware can be grouped into analog I/O, digital I/O, and timing I/O. Although most drivers will expose this basic functionality, you will want to make sure that the driver can do more than simply get data to and from the device. Make sure the driver has the ability to:

• Test channels without any programming
• Acquire data in the background while processing in the foreground
• Use programmed I/O, interrupts, and DMA to transfer data
• Stream data to and from disk
• Perform several functions simultaneously
• Integrate multiple DAQ devices
• Integrate seamlessly with sensors and a variety of signal types
• Provide examples to help get started

These and other functions of the DAQ driver, which are included with NI-DAQ, can save the user a considerable amount of time.

With the introduction of NI-DAQ 7, National Instruments revolutionized the speed by which you can move from building a program to deploying a high-performance measurement application. DAQ Assistant, which is included with NI-DAQ 7, provides a graphical, interactive guide to configuring, testing, and acquiring measurement data. With a single click, you can even generate code based on your configuration, making it easier and faster to develop complex operations; and because DAQ Assistant is completely menu-driven, you will encounter fewer errors and drastically decrease the time to your first measurement.

Which Operating Systems Can You Use with the Driver? Make sure that the driver software is compatible with the operating systems you plan to use now and in the future. The driver should also be designed to capitalize on the different features and capabilities of the OS. For example, the remote desktop feature on Windows XP set it apart from other operating systems. You may also need the flexibility to port your code easily between platforms, say from a Windows PC to a Macintosh. NI-DAQ is available for Windows 2000/NT/XP/Me.

NI-DAQ, the most widely used data acquisition driver, protects your software investment because you can switch between hardware products or operating systems with little or no modification to your application.

If a driver is not available for the operating system of your choice, National Instruments offers the Measurement Hardware DDK. It is a driver development kit that includes development tools and a register-level programming interface for NI data acquisition hardware for applications that require nonstandard OS support.

Are the Hardware Functions You Need Accessible in Software? A problem occurs when a developer purchases DAQ hardware, then attempts to use the hardware with software, only to find that a required hardware feature is not handled by the software. The problem occurs most frequently when the hardware and software are developed by different companies. NI-DAQ handles every function listed on the data sheets of our DAQ hardware.

Does the Driver Limit Performance? Because the driver is an additional layer, it may cause some performance limitations. In addition, operating systems such as Windows 9x can have significant interrupt latencies. If dealt with improperly, these latencies can greatly reduce the performance of the DAQ system. NI-DAQ is a high-performance driver that has code written specifically to reduce the interrupt latencies of Windows and to provide acquisition rates up to 10 MS/s.

Prior to NI-DAQ 7, DAQ drivers were single-threaded, making it complex to perform concurrent operations without having to poll and set occurrences to avoid blocking other operations. NI-DAQ 7 eliminates this problem completely because the driver software is fully multithreaded, so you can perform simultaneous operations without blocking. You can now simultaneously perform analog input, digital output, and counter operations without worrying about adding code to handle the simultaneous acquisitions.

The answers to these questions will give you an indication of the effort that has gone into developing the driver software. Ideally, you want to get your driver software from a company that has much expertise in the development of the DAQ software as they do in the development of DAQ hardware.

Which Application Software Can You Use with the Driver? Ensure that the driver can be called from your favorite application software or programming language and is designed to work well within that environment. A programming language such as Visual Basic, for example, has an event-driven development environment that uses controls for developing the application. If you develop in the Visual Basic environment, be sure that the driver has custom controls, such as those in NI-DAQ, to match the methodology of the programming language.

To Program or Not To Program? – An additional way to program DAQ hardware is to use application software. The advantage of application software is that it adds analysis and presentation capabilities to the driver software. However, even with application software, it is important to know the answers to the previous questions because the application software also uses driver software to control the DAQ hardware. Application software also integrates instrument control (GPIB, RS232, and VXI) with data acquisition.

NI offers three application software prooducts – LabVIEW with graphical programming methodology, LabWindows/CVI for the traditional C programmer, and Measurement Studio for VisualBasic, C++, and .NET – for developing complete instrumentation, acquisition, and control applications. All products can be augmented with add-on toolkits for special functionality. National Instruments VI Logger is an easy-to-use yet very flexible tool specifically designed for your data logging applications.


Developing Your System – To develop a high-quality DAQ system for measurement and control or test and measurement, you must understand each component involved. Of all the DAQ system components, the element that should be examined most closely is the software. Because plug-in DAQ devices do not have displays, the software is the only interface you have to the system. The software is the component that relays all the information about the system, and it is the element that controls the system. The software integrates the transducers, signal conditioning, DAQ hardware, and analysis hardware into a complete, functional DAQ system.


Therefore, when developing a DAQ system, be sure to thoroughly evaluate the software. The hardware components can be selected by determining the requirements of your system and ensuring that the hardware specifications are compatible with your system and your needs. Carefully selecting the proper software -- whether it be driver level or application software -- can save you development time and money.

Sensors and High-Voltage Signals

Transducers sense physical phenomena and produce electrical signals that the DAQ system measures. For example, thermocouples, resistance temperature detectors (RTDs), thermistors, and IC sensors convert temperature into an analog signal that an analog-to-digital converter (ADC) can measure. Other examples include strain gauges, flow transducers, and pressure transducers, which measure force, rate of flow, and pressure, respectively. In each case, the electrical signals produced are proportional to the physical parameters they monitor.

The electrical signals generated by the transducers must be optimized for the input range of the DAQ device. Signal conditioning accessories amplify low-level signals and then isolate and filter them for more accurate measurements. In addition, some transducers use voltage or current excitation to generate a voltage output. Figure 9 depicts a typical DAQ system with National Instruments SCXI signal conditioning accessories.
Signal conditioning accessories can be used in a variety of important applications.

• Amplification – The most common type of conditioning is amplification. Low-level thermocouple signals, for example, should be amplified to increase the resolution and reduce noise. For the highest possible accuracy, the signal should be amplified so that the maximum voltage range of the conditioned signal equals the maximum input range of the ADC.

• 
  For example, SCXI has several signal conditioning modules that amplify input signals. The gain is applied to the low-level signals within the SCXI chassis that are located near the transducers, so the module sends only high-level signals to the PC, minimizing the effects of noise on the readings.
• Isolation – Another common signal conditioning application is isolating the transducer signals from the computer for safety purposes. The system being monitored may contain high-voltage transients that could damage the computer without signal conditioning.

  An additional reason for isolation is ensuring that the readings from the plug-in DAQ device are unaffected by differences in ground potentials or common-mode voltages. When the DAQ device input and the signal being acquired are each referenced to "ground," problems occur if there is a potential difference in the two grounds. This difference can lead to what is known as a ground loop, which may cause inaccurate representation of the acquired signal; or if the difference is too large, it may damage the measurement system. Using isolated signal conditioning modules eliminates ground loops and ensures that the signals are accurately acquired. For example, the SCXI-1125 module provides isolation up to 300 V_rms of common-mode voltage and the NI-DMM (digital multimeter) provides isolation up to 300 VDC/300 V_rms.

• Filtering – The purpose of a filter is to remove unwanted signals from the signal that you are trying to measure. A noise filter is used on DC-class signals, such as temperature, to attenuate higher frequency signals that can reduce your measurement accuracy. For example, many SCXI modules use 4 Hz and 10 kHz lowpass filters to eliminate noise before the signals are digitized by the DAQ device.

  AC-class signals, such as vibration, often require a different type of filter known as an antialiasing filter. Like the noise filter, the antialiasing filter is also a lowpass filter; however, it requires a very steep cutoff rate, so it almost completely removes all signal frequencies that are higher than the input bandwidth of the device. If the signals were not removed, they would erroneously appear as signals within the input bandwidth of the device. Devices designed specifically for AC-class signal measurement – the NI 455x, NI 445x, and NI 447x dynamic signal acquisition (DSA) devices, the NI 61xx simultaneous-sampling multifunction I/O devices, and the SCXI-1141 module have built-in antialiasing filters.

• Excitation – Signal conditioning also generates excitation for some transducers. Strain gauges, thermistors, and RTDs, for example, require external voltage or current excitation signals. Signal conditioning modules for these transducers usually provide these signals. RTD measurements are usually made with a current source that converts the variation in resistance to a measurable voltage. Strain gauges, which are very low-resistance devices, typically are used in a Wheatstone bridge configuration with a voltage excitation source. The SCXI-1121 and SCXI-1122 have onboard excitation sources, configurable as current or voltage, that you can use for strain gauges, thermistors, or RTDs.

• Linearization – Another common signal conditioning function is linearization. Many transducers, such as thermocouples, have a nonlinear response to changes in the phenomena being measured. NI makes NI-DAQ, LabVIEW, and Measurement Studio, which are application software packages that include linearization routines for thermocouples, strain gauges, and RTDs.

You should understand the nature of your signal, the configuration that is being used to measure the signal, and the affects of the environment surrounding the system. Based on this information, you can determine whether signal conditioning will be a necessary part of your DAQ system.

Extensions of PC Technology

New and improved technology in computers related areas has taken the world of data acquisition into new places. Data acquisition is no longer limited to a personal computer or an instrument, but is now available in other packages.

PXI - PCI eXtensions for Instrumentation
PXI systems offer rugged, compact, affordable solutions when developing a data acquisition application. PXI leverages standard PC technologies such as Windows, allowing the user to develop applications with LabVIEW, LabWindows/CVI, or other programming languages. Tighter timing and synchronization between multiple devices can be achieved with PXI’s integration of the PCI bus with a 10 MHz clock signal, low-skew PXI Star trigger, and RTSI communication. Data acquisition devices ranging from standard multifunction DAQ boards, including M-series, S-Series, and E-series boards, to more specialized instruments such as Digital Multimeters, High Speed Digitizers, Arbitrary Function Generators, and RF Upconverters and Downconverters are all available when designing a PXI system.

PXI Tutorial
PXI Specifications Tutorial

Real Time
Real-time systems deliver deterministic performance, increased reliability, and embedded operation. Deterministic performance is needed for applications such as dynamometer control and electronic control unit testing, where operations must consistently complete within a fixed amount of time. In addition to deterministic performance, real-time systems offer a high degree of reliability because they are dedicated to executing one application at a time, as long as the system receives power, the application continues to run, making real-time systems ideal for critical components such as safety shutdown. Lastly, real-time systems do not require user-interaction; therefore, you can deploy them as stand-alone or embedded systems for tasks such as in-vehicle and remote data logging. Development of a real-time data acquisition program occurs on a desktop computer and then is targeted to the real-time controller.

With a real-time operating system, you can use specific real-time hardware, such as National Instruments RT Series DAQ boards, which contain all of the necessary components to easily create real-time systems. Each board consists of two components -- a processor board and a DAQ daughter card. Like a PC, an RT Series processor board includes a:

• CPU
• PC chipset
• BIOS
• RAM

Unlike a PC, the processor board does not have a disk drive, monitor, keyboard, mouse, or other standard PC I/O devices (such as a serial or parallel port). Thus, it must work in conjunction with a host computer for application development, debugging, user interfaces, disk storage, and so on. The National Instruments RT Series boards are available in two different plug-in platforms – PCI and PXI.

Personal Digital Assistants (PDAs)
Measurement applications can now happen in the palm of your hand using personal digital assistants (PDAs). PDAs have gained widespread use and satisfy an increasing demand for reducing equipment size while increasing system mobility and modularity. You can take advantage of the benefits of this technology by using LabVIEW. With the LabVIEW 7 PDA Module and the LabVIEW development system, you can run VIs on Microsoft Pocket PC and Palm OS PDA devices. Then, with a PCMCIA DAQCard, you can use a Pocket PC to acquire and analyze data from anywhere you can take your PDA.

Reader Comments | Submit a comment »

Figure 6
In response to the previous comment about the number of data points in Figure 6: Actually, looks like they have an identical number of datapoints within the frame of +4 and -4, on the X axis. I can only assume that the editor cut off the lower attenuation in the graph, so that you could see the NI performance more clearly in that range.
- Dec 21, 2007

Graph misleading? Maybe...
Figure 6 lists a comparison between an NI DAQ and a competitor, showing the NI to have impressively less noise performing a digitization about a single point. However, I wonder why only 7 data points were taken for the NI, while the competitor's was subjected to the analysis of 43 points. Just a thought.
- Bryan Cattle, Princeton University. bcattle@princeton.edu - Jul 11, 2005