Clocks, Triggers, Etc.
To perform multiple conversions at precisely defined time intervals, many data acquisition boards come equipped with one or more pacer clock circuits. Pacer clock circuits are triggered by either a software instruction or a digital pulse (or analog voltage) at the board's connector.
Many boards also contain general purpose counter/ timer circuits. These consist of several counters and a frequency source. While many manufacturers specify the number of counter/ timers available on the board, make sure these devices are not dedicated to the clocking and triggering of the analog-to-digital and digital-to-analog subsystems.
Bus Speed
Conceived and designed as a way to give peripheral components high-bandwidth access to a host processor in a PC, the Peripheral Component Interconnect (PCI) bus meets the demanding data rates of today's computer peripherals. It does this by providing a 132 Mbytes/s (theoretical), 95 Mbytes/s (typical) burst-rate highway. In replacing the 3- to 5-Mbytes/s ISA bus, this improvement provides important benefits to data acquisition technology, enabling simultaneous operation of subsystems on the board at full specified rates.
In addition, PCI data acquisition boards can feed acquired data directly to the PC's memory, eliminating the need for on-board memory and the resulting gaps in data. Furthermore, PCI boards are auto-configured upon installation to match the system's resources. Manual configuration of jumpers or DIP switches is a thing of the past.
Matching application requirements to data acquisition board features is a fairly straightforward process. But how do you know if the specifications will match real-world performance? Will the board be reliable over time? These questions can be answered by closely examining the manufacturer's specifications.
Accuracy is the mainstay of many a board manufacturer's advertising message. But with signal and data conversion rates on the rise, it is no longer enough to measure performance by specifying a board's dc or time-domain specifications (such as relative accuracy). For an analog-input subsystem, relative accuracy is determined by the maximum deviation from the theoretical value of the full range. This is considered a dc or time-domain specification, because it is typically measured at very slow speeds, using a single analog input channel.
Although these conditions may cover some applications, most multifunction data acquisition boards monitor a variety of analog inputs at rates from 1 to 1-million readings per second. At higher speeds and multiple-channel acquisitions, the relative-accuracy specification may no longer be a good indicator of the performance of a data acquisition board.
For example, in multichannel systems, a multiplexer is often used to switch from channel to channel. Assessing the accuracy from measurements made on just one channel ignores the errors caused by the input channel's settling time.
When frequency-domain performance is characterized, you can be sure that the acquired data will be as accurate as you need it to be. The effective number of bits (ENOB) specification is the way to clearly convey a data acquisition board's ac performance. Combining all critical, real-world performance considerations such as accuracy, settling time, and dynamic performance in one easy-to-comprehend specification, ENOB specifies the overall accuracy of the analog-to-digital transfer function. With ENOB, you can evaluate how closely a digitized output sine wave matches the ideal.
Figure 2 shows an ENOB measurement for the 12-bit DT3010 high-speed PCI data acquisition board. In this case, the board achieves a nearly ideal ENOB of 11.6 bits with a 10-kHz sine wave input.
Reliability Indicators
Two good indicators of quality and reliability are the FCC and CE certification standards. These certifications are more than a formality required by law; they indicate to a buyer that a board has met certain standards and will perform robustly in a real-world environment.
A final indicator of reliable performance can be found in the manufacturer's specifications: maximum input voltage. This voltage should be well above the normal voltage range of the board. If it is not, and an over-voltage condition occurs, the board may be damaged. A more subtle specification, the power-off overvoltage, specifies what happens when the input signals are present while the system power is turned off.
Planning For Future Changes
A data acquisition system should meet your needs today and provide flexibility for the future, while protecting your investment in hardware and software. Common upgrades to a data acquisition system are changing and adding boards. If the application software is not designed with an open systems approach, adding new boards can cause expensive, time-consuming reprogramming. Making sure your application programming interface (API) is hardware-independent will allow you to change boards with no, or just minor, reprogramming. One standard available is Data Translation's DT-Open Layers. All Data Translation software, for example the Dat Acq SDK (software development kit), is developed under this standard.
Also, make sure your data acquisition software will support multiple Windows operating systems. What will happen when you load Windows 98, and how will the manufacturer of the data acquisition software support that upgrade? The answers to these questions will tell you how difficult it will be to migrate from one of the early versions of Windows to either Windows 98 or Windows NT.
Another hardware issue involves the use of interrupts. Traditionally, computer peripherals request the host CPU's attention via the hardware interrupts on the CPU. However, the number of peripheral components on a typical system (modem, scanner, CD-ROM drive, etc.) has increased to the point where it frequently exceeds the fifteen interrupts available. To address this problem, Data Translation and some other board manufacturers have stopped using hardware interrupts on new boards, achieving the same function with a software feature.
Conclusion
Planning a data acquisition system for your demanding needs can be achieved. Keep your application in mind when choosing a data acquisition board. Look for some simple indicators of accurate, robust performance, and choose software designed for future upgradability.
Clocks, Triggers, Etc.
To perform multiple conversions at precisely defined time intervals, many data acquisition boards come equipped with one or more pacer clock circuits. Pacer clock circuits are triggered by either a software instruction or a digital pulse (or analog voltage) at the board's connector.
Many boards also contain general purpose counter/ timer circuits. These consist of several counters and a frequency source. While many manufacturers specify the number of counter/ timers available on the board, make sure these devices are not dedicated to the clocking and triggering of the analog-to-digital and digital-to-analog subsystems.
Bus Speed
Conceived and designed as a way to give peripheral components high-bandwidth access to a host processor in a PC, the Peripheral Component Interconnect (PCI) bus meets the demanding data rates of today's computer peripherals. It does this by providing a 132 Mbytes/s (theoretical), 95 Mbytes/s (typical) burst-rate highway. In replacing the 3- to 5-Mbytes/s ISA bus, this improvement provides important benefits to data acquisition technology, enabling simultaneous operation of subsystems on the board at full specified rates.
In addition, PCI data acquisition boards can feed acquired data directly to the PC's memory, eliminating the need for on-board memory and the resulting gaps in data. Furthermore, PCI boards are auto-configured upon installation to match the system's resources. Manual configuration of jumpers or DIP switches is a thing of the past.
Matching application requirements to data acquisition board features is a fairly straightforward process. But how do you know if the specifications will match real-world performance? Will the board be reliable over time? These questions can be answered by closely examining the manufacturer's specifications.
Accuracy is the mainstay of many a board manufacturer's advertising message. But with signal and data conversion rates on the rise, it is no longer enough to measure performance by specifying a board's dc or time-domain specifications (such as relative accuracy). For an analog-input subsystem, relative accuracy is determined by the maximum deviation from the theoretical value of the full range. This is considered a dc or time-domain specification, because it is typically measured at very slow speeds, using a single analog input channel.
Although these conditions may cover some applications, most multifunction data acquisition boards monitor a variety of analog inputs at rates from 1 to 1-million readings per second. At higher speeds and multiple-channel acquisitions, the relative-accuracy specification may no longer be a good indicator of the performance of a data acquisition board.
For example, in multichannel systems, a multiplexer is often used to switch from channel to channel. Assessing the accuracy from measurements made on just one channel ignores the errors caused by the input channel's settling time.
When frequency-domain performance is characterized, you can be sure that the acquired data will be as accurate as you need it to be. The effective number of bits (ENOB) specification is the way to clearly convey a data acquisition board's ac performance. Combining all critical, real-world performance considerations such as accuracy, settling time, and dynamic performance in one easy-to-comprehend specification, ENOB specifies the overall accuracy of the analog-to-digital transfer function. With ENOB, you can evaluate how closely a digitized output sine wave matches the ideal.
Figure 2 shows an ENOB measurement for the 12-bit DT3010 high-speed PCI data acquisition board. In this case, the board achieves a nearly ideal ENOB of 11.6 bits with a 10-kHz sine wave input.
Reliability Indicators
Two good indicators of quality and reliability are the FCC and CE certification standards. These certifications are more than a formality required by law; they indicate to a buyer that a board has met certain standards and will perform robustly in a real-world environment.
A final indicator of reliable performance can be found in the manufacturer's specifications: maximum input voltage. This voltage should be well above the normal voltage range of the board. If it is not, and an over-voltage condition occurs, the board may be damaged. A more subtle specification, the power-off overvoltage, specifies what happens when the input signals are present while the system power is turned off.
Planning For Future Changes
A data acquisition system should meet your needs today and provide flexibility for the future, while protecting your investment in hardware and software. Common upgrades to a data acquisition system are changing and adding boards. If the application software is not designed with an open systems approach, adding new boards can cause expensive, time-consuming reprogramming. Making sure your application programming interface (API) is hardware-independent will allow you to change boards with no, or just minor, reprogramming. One standard available is Data Translation's DT-Open Layers. All Data Translation software, for example the Dat Acq SDK (software development kit), is developed under this standard.
Also, make sure your data acquisition software will support multiple Windows operating systems. What will happen when you load Windows 98, and how will the manufacturer of the data acquisition software support that upgrade? The answers to these questions will tell you how difficult it will be to migrate from one of the early versions of Windows to either Windows 98 or Windows NT.
Another hardware issue involves the use of interrupts. Traditionally, computer peripherals request the host CPU's attention via the hardware interrupts on the CPU. However, the number of peripheral components on a typical system (modem, scanner, CD-ROM drive, etc.) has increased to the point where it frequently exceeds the fifteen interrupts available. To address this problem, Data Translation and some other board manufacturers have stopped using hardware interrupts on new boards, achieving the same function with a software feature.
Conclusion
Planning a data acquisition system for your demanding needs can be achieved. Keep your application in mind when choosing a data acquisition board. Look for some simple indicators of accurate, robust performance, and choose software designed for future upgradability.