Over the years, engineers starting their careers have asked me how to specify and create a data-acquisition system. The path can seem rocky until they analyze what they must measure and what they want to do with the measurement information. This and following columns outline how to think about such projects, for which I assume one or more "unknown" voltage signals.
You must know what to measure when you start a project. Although data-acquisition equipment can adapt to many needs, I recommend designing or configuring what you need to solve a specific problem. After solving that problem you can decide whether to add other capabilities.
A B&K Precision Model 5491B digital multimeter can take as many as 25 samples per second and provide true rms values to a PC. The company also provides instrument control software.
Must you measure analog, digital, or both signal types? At first, analog signals seem straightforward because most of the time you will just digitize them. Digital signals could involve counting pulses or timing periods between signal transitions. And because digital signals do not always make a clean transition between logic states, you might need to treat them as analog signals, and sample them accordingly. The sampled information lets you observe analog-like behavior such as jitter, glitches, noise, and runt pulses not seen if you capture only logic-level transitions. Also, by sampling and analyzing digital signals you can determine how well they meet signal-integrity specs.
The need to sample a signal leads to the next specifications: resolution and accuracy. Manufacturers' data sheets provide this information, so get your calculator ready. For a 1V signal, a 16-bit analog-to-digital converter (ADC), for example, provides a resolution of 15.3µV, the value of its least-significant bit. A 5½-digit DVM offers a 10µV resolution, the value of its least-significant digit. The DVM data sheet notes an accuracy of 0.012 percent, or 120V for the 1V signal. Electrical noise created within the 16-bit ADC and surrounding circuits will limit accuracy to about 14 bits, or 61µV. Match accuracy to the measurement requirements for your data. There is no need to specify accuracy in tens of microvolts when tens of millivolts will suffice. Accuracy is almost always less than a specified resolution.
Next, consider the frequency of your signals. To test slowly changing DC or near-DC signals in, say, battery or photovoltaic-cell tests, a digital voltmeter (DVM) with a USB interface to a PC might suffice. DVMs can provide root-mean-square (rms) measurements and can average a signal over a set period, too.
When you sample AC signals, things get more complicated because you must determine the bandwidth over which you need to capture signals and then determine the appropriate sampling frequency.
Be sure to check back in with Design News next week for Specifying & Creating Data-Acquisition Systems, Part 2.
I think this is spot on advice and a great reminder for those of us who have been at it for awhile too! As a test engineer for many years, one thing I learned is that while it is tempting to jump in and start designing a system without doing the homework it takes to understand the complete system requirements, that approach will hurt you down the road. Nothing is more painful than getting P.O.s approved and equipment purchased only to find out that the equipment you selected is inadequate to the task, and you have to inform your boss... Ouch!
My husband likes to say that enthusiasm is the first stage of a project and I think that is true. We need to capitalize on the enthusiasm by taking that time to define the needs of the system so we don't find out during stage three that the test equipment we ordered because it is so cool can't measure the parameters we need at the resolution and accuracy the customer spec requires...
This is great stuff - I can't wait to read future columns!
Nice opening to the series, Jon. Would the device that acquires the data also analyze the data, or would that necessarily be done by a different device that connects to the data acquistion tool? Would it then also be connected to a reporting mechanism that would send alerts when the data indicates things are out of whack or trending in a negative direction?
I think that it is very useful to have the information fed into a PC type device. This lets you play it back. During playback you can also alter sampling periods and other parameters so that you can refine what and how you measure. You could also have the PC generate alerts.
Jon, there are lots of data acquisition cards are available in market. We have to just plug these cards to our system using a GPIB card or any other parallel data connecting mechanisms like RS 232 link. Based on functionality and application NI had a set of data acquisition cards, which all are in plug and play mode. If we want to connect more devices, then we have to create a UI with lab view software.
In the article the following statement(s) is/are made:
For a 1V signal, a 16-bit analog-to-digital converter (ADC), for example, provides a resolution of 15.3V, the value of its least-significant bit. A 5½-digit DVM offers a 10V resolution,
I believe the correct units should be 15.3 uV and 10uV for the resolution in these two cases. It would be appreciated if the coorect units are being used before publishing.
Aside from the typos, this article presents a good starting primer on how to specify a DAC system.
However, I would go still farther. When I specify a DAC system I also go on to consider the entire workflow of the data collection and analysis process. It doesn't do a lot of good to collect a mountain of accurate data at an appropriate resolution without a straight forward means of storing it, archiving it and analyzing it. I always stop and consider what I'm going to do with the data, how complex the post processing is going to be and, probably most importantly, how often am I going to repeat this process.
If the effort is a one-off, then you don't need to get too fancy. A stand alone box like a Fluke Hydra Databucket might be suitable. A PCMCIA card or USB thumb drive and sneaker net are fine for moving data around. If all you're doing is plotting the data a looking for a min/max, then Excel is a perfectly useful tool (as long as the dataset has less then 32K samples per 'column').
If the post processing is much more complicated, or you'll be collecting and analyzing data repeatedly, then a more automated means of storing and analyzing data is a must. NI LabView and NI's hardware is a good default, though I've found a range of lower cost alternatives are also available. Tools like this allow the engineer to collect and fully package the data in near real-time.
As far as I'm concerned, this is an integral part of specifying a DAC system too.
Thanks for your comments, Christopher. I will talk more about the software-and-memory side of data-acquisition systems later in the series. You're right; it requires thought about what you want to do with the data.
Hello Geralda. Thanks for pointing out the errors in the text. I'll contact our Web editor and get the units squared away. Sometimes blog front-end software does not properly convert typed symbols and foreigh-language characters properly. The correct units appeared in the original text, and will appear properly in the issue of Design News that carries this information in my Measurements column. Again, thanks.
Hello, Mydesign. Good point about using off-the-shelf data-acquisition cards and modules. For many people, these devices will work just fine. I think we should help engineers and other technical people understand the basics of data acqusition, regardless of the type of equipment they use. Then, when they need something with additional capabilities, they can converse with manufacturers and understand basic terms and specifications. To properly use tools, we should understand how they work.
Hi, Rob. In many cases, the data-acquisition equipment connects to a computer, either a desktop PC, or some sort of embedded system. The larger computer will handle data analysis, plotting, storage, and so on. A smaller system might perform control operatiuons and report only some of the information. Some DAQ modules can connect to the Internet--as can PCs--and create alerts if a measurement exceeds certain limits, for example. These devices can provide some closed-loop control, too. The choice of how and where you handle the data depend on a specific application.
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