The plug-in data acquisition board market consists of hundreds of different boards ranging from low-end digital I/O to high-end oscilloscopes. However, the sweet spot of the plug-in board market is the multifunction, medium-speed board that represents, by some estimates, $150M to $200M of sales annually.
As its popularity attests, the multifunction board is useful for the majority of typical test applications. The only question: Which combination of features will serve the most users at the lowest cost?
Each of the leading suppliers in this market offers from 25 to more than 75 multifunction boards. By their very nature, multifunction boards are difficult to compare to one another since each function can have a different set of parameters. The result is a dizzyingly large array of products from which to choose.
To help you select the right board for your application, there are several features and functions of today’s multifunction boards that need to be examined: A recent survey on multifunction-board features conducted by IOtech indicates current industry preferences. Users were surveyed via e-mail and the Internet, yielding 209 responses.
Buses
The largest differentiation among multifunction boards is whether they use the tried-and-true ISA bus or the new PCI bus. For new applications, the PCI boards are gaining popularity because they work with today’s new PCs and are faster than ISA.
In the survey, the majority of respondents preferred PCI vs 26% who endorsed ISA. But because of the large number of OEM applications for ISA boards, suppliers will likely continue to build ISA boards for the foreseeable future.
ADCs
The next important differentiations are resolution and sampling speed. According to the survey, the most desired resolution is 16 bit, preferred by 49% of the respondents as compared with 31% who chose 12-bit resolution. Although most board suppliers offer both 12-bit and 16-bit versions of their products, the 12-bit boards are likely to disappear because the cost of 16-bit analog-to-digital converters (ADCs) has dropped significantly in the past few years.
As for speed, most multifunction A/D systems now operate in the 100-kHz to 330-kHz range, fast enough for applications that involve the measurement of physical parameters such as temperature, sound, vibration, strain, and pressure. In the survey, 29% preferred the 100-kHz to 300-kHz range, 40% wanted slower speed, and 30% liked something faster. As higher-speed ADCs become available at lower cost, the sweet spot for these boards will likely inch up.
Analog Inputs
According to the survey, half of the respondents opted for eight to 16 differential analog input channels; 26% chose more channels; 24% preferred fewer. As a result, the sweet spot for boards remains in the eight- to 16-channel range where it has been since the beginning. Most multiplexing electronics come in multiples of eight so it saves almost nothing to offer boards with fewer channels.
There are two ways to offer a product with more than eight to 16 channels. The first method includes 32 or 64 channels of input on one board. This usually comes at the expense of I/O for other functions since the number of I/O pins on a board’s connector is limited.
The second technique permits the on-board channels to be expanded via external signal conditioning and expansion options which allow hundreds of channels to be measured by one board. Some suppliers support multiple A/D boards installed in the same PC for added channel count. As more channels are added, the per-channel scan rate does not reduce since each board has its own A/D that can sample concurrently if it is appropriately designed and supported in software.
Calibration
Today, calibration should be performed digitally, eliminating the need to remove the cover of a PC to adjust potentiometers. This also allows the board to be at its operating temperature when calibration is performed. Despite the convenience and the improved accuracy, some suppliers still are introducing multifunction boards with potentiometers, maybe because the new boards simply are derivatives of older-technology ISA boards.
Some boards allow you to modify the calibration without affecting the factory or laboratory calibration. This is useful, for example, if you want to perform an on-the-spot calibration immediately prior to a measurement without affecting the board’s true calibration.
Analog Outputs
Calibration was not covered in the survey, but analog output was. The most preferred number of analog output channels was two (33%), but many respondents desired four channels (30%) or more (22%). Interestingly, most major suppliers either do not offer any or only offer two analog output channels at 12 bits or 16 bits of resolution. Survey respondents also clearly selected 16-bit output resolution.
These are the easy parameters to compare. What’s more confusing is how the analog outputs are updated. If the application simply needs DC outputs, such as set points or DC biasing, then the update method usually is not important. However, if the application requires the generation of waveforms, then it is very important to explore the update scheme.
At the low end, some boards only allow a waveform to be loaded into RAM on the board, which limits the length of the output waveform. More advanced boards output a waveform from a PC RAM or hard disk, allowing waveforms of almost limitless length. PCI bus mastering facilitates this capability.
Digital I/O
Regarding digital I/O, the biggest difference among boards is the number of lines offered. This ranges from eight lines on the low end to 40 lines on the high end.
More advanced boards permit the digital I/O lines to be expanded from the built-in number to hundreds through the use of external expansion options. This allows features such as optical isolation or relay closure to be added to the board. Nonetheless, the survey shows that most users (53%) are satisfied with eight I/O lines or less.
Another desirable feature is the capability to input digital signals synchronously with analog input, permitting the time correlation of digital signals to analog measurements. Only a few high-end boards presently support this feature.
It also is beneficial to perform pattern generation where a file of digital bytes or words can be output synchronously with the acquisition of analog and digital data. Again, only a few high-end boards presently support this feature.
Counter/Timers
Every multifunction board needs on-board timers and counters to pace the A/D conversion. The survey showed that users typically wanted one or two counter/timer channels, and 43% desired more. Some boards confusingly specify the counter/timers needed for A/D conversion as if they are available to the user when, in fact, they are required to run the ADC. Advanced boards allow you to scan the status of the counter inputs synchronously with the scanning of analog and digital inputs, permitting time correlation of counter readings with analog and digital inputs.
Software Support
Based on the survey, the desired software support can be ranked in this order: LabVIEW, Visual Basic, C++, DASYLab, and TestPoint. The desired operating systems are Windows 98, NT, and 2000. Linux was fourth but preferred by a surprisingly high number of respondents (13%), indicating the growing popularity of this operating system.
In the past, software support was a distinguishing feature among plug-in boards. However, today most boards include drivers for Windows 98/NT or offer them for a nominal price. In addition, most boards are supported by LabVIEW and a few other graphical programming environments such as TestPoint and DASYLab. As a result, software support rarely is an important criterion for board selection.
However, some boards do offer out-of-the-box data acquisition software that allows you to plug in a multifunction board and begin acquiring data immediately. Some suppliers include this software with their boards, and others provide it at a nominal charge.
The Sweet Spot
With so many software and hardware features to consider, you may find it difficult to compare boards for the best value. This survey was performed to determine where the sweet spot is for multifunction A/D boards. This is important because, given competition among board suppliers, the sweet spot is where test professionals will likely find the most desired features at the lowest cost. The survey results are given in Table 1 (see below).
Table 1
Responses
Percent
Bus
ISA
54
26%
PCI
114
55%
Compact PCI
14
7%
Other
27
13%
209
100%
A/D Resolution
7
3%
12 bits
64
31%
16 bits
101
49%
>16 bits
35
17%
207
100%
Number of Differential Input Channels
<8
48
24%
>8, <16
101
50%
>16
52
26%
201
100%
Speed
83
40%
60
29%
>300 kHz
62
30%
205
100%
Number of Analog Output Channels
0
32
16%
2
65
33%
4
59
30%
>4
43
22%
199
100%
Analog Output Resolution
13
7%
12 bits
53
29%
16 bits
100
55%
>16 bits
16
9%
182
100%
Analog Output Update Rate
DC
19
10%
0 to 100 kHz
76
42%
100 kHz to 300 kHz
61
33%
>300 kHz
27
15%
183
100%
Digital I/O Channels
0
22
11%
1 to 8
85
43%
9 to 24
67
34%
>24
24
12%
198
100%
Counter/Timer Channels
0
31
18%
1 to 2
68
40%
2 to 4
51
30%
>4
22
13%
172
100%
Operating System Support*
Windows 98
110
37%
Windows NT
85
28%
Windows 2000
42
14%
DOS
16
5%
Linux
40
13%
Other
7
2%
300
100%
Language/Application Support*
Visual Basic
83
25%
C++
73
22%
Delphi
11
3%
LabVIEW
115
34%
DASYLab
25
7%
TestPoint
18
5%
Other
10
3%
335
100%
* Multiple answers from respondents accepted.
About the Author
Tom DeSantis is the founder and president of IOtech. He holds a bachelor’s degree in electrical engineering and has more than 20 years of experience in the test and measurement industry. IOtech, 25971 Cannon Rd., Cleveland, OH 44146, (440) 439-4091,
Published by EE-Evaluation Engineering
All contents © 1999 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.
October 1999