ac voltage measurements with ni 9219

[Engr_tech]

Hello

i am trying to measure an ac voltage through lab view using NI 9219 module.for test purpose  i have connected an Ac voltage source HI to pin #4 and Lo to Pin # 5. by setting up the DAQ assistant i plotted the output value of a 5 Vp-p sine wave.

i am expecting a sine wave but i can only get a straight line of few milli volts in the output graph(lab View).i am not sure if the NI9219 internally converts it to dc Voltage or  i have some problems with the connection diagram.if NI 9219 has internal ADC converting the input to DC how can i de activate it so i can get an AC waveform 

Please comment.

Thanks,

Ali

[NIquist]

Well your module is limited to a sample rate of 100 samples / second so your top measuement frequency is going to be 50 HZ.  The 9219 is really designed for high accuracy (24-bit) and that means speed is compromised.

It should work at low frequencies but unless you post your code we can't know what you're doing wrong.  

[Engr_tech]

well i am using the NI example Contineous voltage Input .Vi. i connected a fucntion generator to the input of the module with sine wave 5v p-p.

so you mean NI 9219 cannot measure the Ac voltage haveing frequency higher than 50 HZ.

[NIquist]

Well you have to sample a pure sine wave at a rate of at least twice its frequency otherwise you will not have enough resolution to properly reconstruct the original analog signal.  http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem  More complex signals require even higher sampling rates (and fancy filters) to avoid aliasing issues.

The 9219 is a rather specialized module designed for strain gauges, thermocouples and other low-frequency application.  Have you read its spec sheet and manuals???  https://www.ni.com/en-us/shop/model/ni-9219.html

[djs_at_eaton]

Not only do you need to sample at least 2x faster than the highest expected frequency, but that rate assumes you sample for a relatively long time before you can expect a decent representation of the waveform to appear.  For "real world" situations a better rule is 10x the highest expected frequency.

The NI 9219 has a sample rate of 100 per second, so 50Hz is the fastest waveform you could theoretically expect to reconstruct.  Practically, you'd be limited to about 10Hz.

[lroma]

Hello, I am trying to sample a voltage signal with NI USB 9219. Unfortunately I cannot improve the frequency of the signal received, with stands at just 1.95 Hz. I am also facing a second problem. Instead of having a continuous accumulation of data ( I used the loop command), my graph is onlyshowing those recorded during the last 5 seconds.

Thank you.

[Dennis_Knutson]

Please attach your code. Your mention of 'loop command' is confusing since there is no such command. A graph is not supposed to show any accumulation of data. Only a chart will do this without any extra programming.

[lroma]

Cheers. Attached there's a picture sowing the code. What I mean by accumulation of data, is simply a test in which data are recorded for an unspecified length and graphed accordingly. What my graph is showing instead, is just results of the last 5 seconds. I have tried to set 50 Hz as data recording frequency, but Labview gives me an error.

[Dennis_Knutson]

The pdf file is pretty pointless since it does not show how you configured the DAQ Assistant. As I already said, a graph does not accumulate data. It will just show the results from one acquisition at a time. A chart does accumulate data up to the history length that you set.

[lroma]

Hello. I have managed to modify the extension of recorded data by switching graph/chart. However data are still recorded with a low frequency. I have attached my code. Thank you.