Selecting Data Acquisition Software Displays 1 of 5
What should you expect from Data Acquisition (DAQ) software displays?
If you want to do the job right, you need the right tools. We have all heard that before. But how do you pick the right tools?
For data acquisition, it is just as important to have good software as it is to have good hardware. You will spend much of your time viewing DAQ software displays, so here are the display basics that you should expect from your software.
Data Displays: Current Value Table (CVT) and History
Modern DAQ systems can capture data much faster than the user can see or interpret the data. Look for a data acquisition software system that records all the data but only shows enough of the data to be useful. Sometimes we only need to see a simple value for each channel. In other words, we need just the “Current Value” of the channel at the time the display is updated. At other times we need to view contiguous data values for a single channel. This is a snapshot of a finite set of “History” values at the full sample rate.
Current Value Table (CVT) Display
This type of display shows one value at a time for each channel in the display and is designed to allow the user to choose how often the display is updated, usually no more than twenty times a second. A simple table or tabular format , is the most common way to view CVT data (see Figure 1.)
History displays periodically grab a list of time-correlated values. For instance, if we displayed twenty thousand samples a second, the user would just see a blur. To be useful, the user needs to see snapshots of data. The history display grabs slices of data for a small segment of time and shows the data graphically. For example, an oscilloscope or spectrum display shows small periods of data at a time. Each update of the display shows the history values stored for the selected channel. This is useful to understand the high speed behavior of data.
The same history data can be represented as a frequency domain analysis called an FFT. The FFT Display takes the history data and mathematically converts it to the frequency domain in real time to create a spectrum display. The spectrum display splits the frequency range into a set of groups called bins. Each bin shows the energy found in that group of frequencies. In the example shown in Figure 3, most of the energy of the current signal is at 41.02 Hz.
Note: the FFT algorithm only shows meaningful frequency at up to half the sample rate used, which is called the Nyquist Frequency. Higher frequency values mathematically fold back to appear as lower frequency values. This is why it is so important to use a hardware filter in front of the analog-to-digital converter to eliminate information in frequency that is higher than half the sample rate.
To summarize, the user of data acquisition software should be aware that there are two fundamental ways to look at the acquired data: CVT and History. CVT data displays are ideal for viewing data that doesn’t change drastically from moment to moment. History displays are ideal for viewing and analyzing rapidly changing data.