Introduction to benchtop equipment and data acquisition  (Page 5/6)

3.3 acquiring data

You will now develop your VI to acquire the data that the ADC is sampling. The VI will acquire 1000 samples ata rate of 100,000 samples per second. Once the 1000 samples have been gathered, the data will be analyzed programmatically todetermine the peak-to-peak amplitude and the DC offset. The data set will be displayed on a graph, and will be written to ameasurement file that can be opened in Excel.

LabVIEW contains a useful set of functions for data acquisition known as DAQmx. DAQmx is NI’s driver configuration software for DAQ devices. We will use the DAQ Assistant function toopen communication with the DAQ hardware, read data from the device, and close communication.

5. On the block diagram, right click to bring up the Functions Palette. (Refer to Figure 4.) The FunctionsPalette contains all of the functions and structures that can be placed directly on the block diagram. Notice that the functions areorganized into sub-palettes by functionality.

6. Click Measurement I/O>>NI-DAQmx>>DAQ Assist.

7. When you place the DAQ Assistant on the block diagram, a Create New…window will open as shown in Figure 5. This menu will guide you as you configure your software tocommunicate with your hardware.

8. Select Analog Input>>Voltage. DAQmx will scan the system for available hardware and present theoptions; as shown in Figure 6.

9. Expand the cDAQ module that is labeled (SCXI-1520).

10. Select ai0, (the analog input channel that the function generator is connected to.)

11. Click Finish.

12. The DAQ Assistant window should now open. Configure the DAQ Assistant as shown in Figure 7.

• Set the input range from 5 to -5 volts.
• Change the Terminal Configuration to Differential. (Differential assumes that the voltage is measured between twochannels, rather than referenced to a common ground.)
• Set the Acquisition Mode to N Samples.
• Change Samples to Read to 1000 and the Rate to 100,000.

13. When you click OK, LabVIEW will build a subVI according to your specifications.

3.4 amplitude measurement of data

Similar to the measurements made on the oscilloscope, LabVIEW programs can analyze characteristics of asignal.

14. Right click on the Block Diagram to bring up the Functions Palette. (Note that if you click on the FrontPanel you will bring up the Controls Palette, which has entirely different choices.)

15. Select Express>>Signal Analysis>>Tone. The Configure Tone Measurements dialog box will now open. The Tone Express VI will display values related tothe amplitude and frequency of the signal.

16. Configure the dialog box as shown in Figure 8.

17. Select Amplitude and Frequency in the Single Tone Measurements box.

18. When you click OK, LabVIEW will build a subVI according to your specifications.

19. Place your cursor over the data output terminal on the DAQ Assistant icon. The cursor should change to awire spool. Click on the data terminal and connect the wire to the signals inputterminal on the Tone Measurements icon. This wirerepresents the data flow from the DAQ Assistant to the measurements subVI.