Data Acquisition Boards.

Present-day benchtop and rack-and-stack automated test systems pre-dominantly utilize IEEE 488 digital multimeters (DMMs) while PC-based test systems tend to use data acquisition boards as the primary measurement instruments. This is because PC plug-in DMMs have not been around very long, and many of the early plug-ins failed to meet the requirements of benchtop and system DMMs.

SoftCal data acquisition boards can be calibrated by executing a software routine within the host CPU. This is possible because the data acquisition board has onboard programmable digital-to-analog converters or digital potentiometers instead of manual potentiometers and onboard high-precision references. The data acquisition board is initiated by loading pre-defined factory hardware settings, which are typically stored onboard. The user has the ability through a host-side routine to recalibrate the board as necessary.

These high-performance data acquisition boards are designed to work in a very wide range of test and measurement and control applications. Combined with powerful software,data acquisition boards turn personal computers into powerful measurement systems that may be used to automate experiments, construct product test stands, monitor and control production equipment, or be embedded in products ranging from airport security systems to jet fighter test simulators.

Measurement Computing Corporation offers the largest selection of PC-based data acquisition and measurement products. From PCI, USB, PC/104, ISA, and PCMCIA bus to RS-232, RS-422, RS-485, and GPIB interfaces; Measurement Computing has everything you need for your PC-based data acquisition and control applications.

Ideally, any measurement by a DAQ device would be converted immediately into digital data and transferred to the computer with no loss or modification of the signal. However, the components used in measurement devices are not ideal, and these components introduce various inaccuracies into the signal path. Typically, these errors consist of settling time errors due to the MUX and the PGIA switching between various signals, gain and offset errors introduced by the PGIA, and digitization errors introduced by the ADC. System noise is introduced in all parts of the analog signal path and it also adds to the uncertainty of the measurement. The total error of your measurement depends on the sum of these errors.

Previously, hardware and software for high-end systems were not interchangeable among products from different vendors. Plug-in data acquisition boards for a RISC workstation were scarce and very expensive. Conversely, plug-in data acquisition boards for PCs, compatibles, and Macs are plentiful, relatively inexpensive, and compatible with each other.

It is possible to select a wide range of different products and configurations that will acquire and process data equally well. These range from field-mounted individual devices-such as DIN-rail signal conditioners or smart transmitters-to plug-in data acquisition systems that can be installed in a PC.

Signal conditioning: The signal from any sensor must be converted from its analog form into a digital signal that the host computer or controller can understand. This can be done by a transducer or transmitter in the field, a local signal conditioner, or-if close enough-by the analog-to-digital (A/D) converter on a data acquisition board. In addition, signals from nonlinear sensors, such as differential pressure flowmeters and thermocouples, must be linearized. If the environment is particularly noisy, these conversions should be done as close to the sensor as possible.

Multiple PCs with data acquisition boards are used to monitor the quality of waste water at an automotive manufacturing plant. The system measures the quality of water in bath tanks used to wash car bodies prior to painting. The close on-line monitoring of the tanks has reduced the amount of expensive, environmentally sensitive chemicals used in the process.

Engineering waveform recording boards, designed with large memory buffers and a high-speed bus interface to withstand the non-real-time nature of PC systems, becomes the heart of the art. A sufficient buffer is essential to account for the periods when a PC system is busy handling other tasks, as well as a high-speed bus interface to offload that buffered data. With these design features, data acquisition boards will simultaneously acquire, buffer and transfer data to prevent a break in the analog record.

Resolution: This determines the smallest value change the system can detect. Most systems have 12 or 16-bit resolution; a 12-bit system resolves 1 part in 4,096, while a 16-bit system resolves 1 part in 65,536. The resolution in engineering units is determined by dividing the measurement span by 4,096 (or 65,536). For example, if you are measuring a voltage signal of 0-1,000 V with 16-bit resolution, the smallest change the system can see is 0.2 V (or 0.015 V).

There are hundreds of data acquisition board configurations available for PCs and Macs. Many other boards are available with more inputs, higher resolution, and faster scanning speeds. Now, let's look at some of the higher performance boards.

Analog inputs: Typical voltage inputs are 0-1 V, 0-10 V, 50 mV, 5 V, and 10 V. Other inputs include 0-20 mA, 4-20 mA, frequency, and resistance. Most of the big four process control measurements-flow, temperature, pressure, and level-fit one or more of these input ranges. Most data acquisition products have selectable or programmable analog inputs that accept the signal, and send it to a 12- or 16-bit A/D converter.

Using data acquisition boards for complex measurements requires the addition of signal-conditioning hardware. Functions such as low-pass filters, current sources, isolation amplifiers, and attenuators can be added to enhance the capabilities of the measurement system. Signal conditioning can reduce measurement noise, provide stimulus for passive components such as resistors and RTDs, and allow the measurement of a variety of sensors.

Reading outputs from all signal lines is considered a most reliably and high-speed data acquisition method. However, in order to obtain a two-dimensional image, signal outputs for each of vertical and horizontal directions must be read out together with their timings. Therefore, an electronic apparatus for such an operation becomes huge.

ICP DAS PCI and ISA data acquisition boards offer cutting edge data acquisition capabilities for your industrial control needs. The ICP DAS PCI, PIO, PISO, and ISO data acquisition boards are available in a bevy of digital and analog configurations, which are communicable via standard PC-104 slots, as well as capable of interfacing with other modules and devices via alternate communication protocols.

There are four main methods for interfacing instrumentation to computers for data acquisition or for remote operation of the instrument. An instrument may take the form of an oscilloscope, multimeter, energy meter, power meter, display, printer, chart recorder, spectroscope, or laser, or an extensive system containing a number of these components.

The Controller Area Network (CAN) is a serial communication way which efficiently supports distributed real-time control with a very high level of security. It provides the error process mechanisms and message priority concepts. These features can improve the network reliability and transmission efficiency. Furthermore, CAN supplies the multi-master capabilities, and is especially suited for networking intelligent� devices as well as sensors and actuators within a system or sub-system. DeviceNet is based on the CAN bus and is one of the world's leading device-level networks for industrial automation.

According to Measurement Computing's Stevens, data acquisition boards offer more bang for the buck by incorporating a higher level of functional integration, driven primarily by the chip vendors. "On the converter front, signal conditioning functions [bridge excitation, DAC deglitching, etc.] that have traditionally required discrete circuit design are now readily available and allow direct connection to sensors and field electronics," he said.

Throughput is often the most important factor in choosing a data acquisition interface. The Nyquist Theorem specifies that an input signal should be sampled at least twice as fast as the input's highest frequency component. For example to accurately measure a 1kHz signal, the minimum A/D throughput is 2kHz. This avoids signal aliasing. Aliasing occurs when high frequency components appear in the digital values as erroneous lower frequencies.