AC Voltages with SCXI-1104C

[Shahrukh]

Hello,

I have a PXI-1052 Chassis, using a PXI-6251 M-DAQ card, along with the signal conditioning chassis.  I need to measure some AC signals, so I purchased the SCXI-1104C module, which is specified to read upto 42Vp-p.  My signal is <10 Vp-p at 1000Hz. 

Can I configure MAX to read AC signals directly?

Thank you,

Shahrukh Alavi

[lab_boy]

Hey Shahrukh,

You can configure SCXI in MAX for AC measurements. Here is the getting started document. I am sure this will help.

Good luck

-Lab

[EEsRULEtheWORLD]

Hi, Shahrukh.

Your system does not need to be configured to read an AC signal.  The analog channels on your system will read AC and DC signals.  If your signal is DC, make sure it doesn't go above 60VDC or below -60VDC; if your signal is AC, make sure that the highest peak doesn't go above 42VAC and the lowest peak doesn't go below -42VAC.  Exceding these ranges will damage your device.

Coupling is strictly DC for these devices, meaning that you will not be able to remove the DC portion of the signal.  To get around this, you can implement a capacitor between the positive terminal of your signal and the positive terminal of your input channel that will allow you to remove the DC component, as shown here.  (Note:  The S series and Dynamic Signal Analyzer (DSA) devices have software selectable coupling configurations.) 

Check out page 3-3 of the SCXI 1104/C User Manual for more information regarding analog signal connections.

Let me know if you need any more information.  I hope you're having a great day!

[Shahrukh]

Hello,

Thanks for the replies.  I should have been more specific, is there a way to automatically read the RMS (or Peak) value of an AC signal?  I am currently calculating this value in software, but would prefer to do it directly (like a multimeter)

Thanks,

Shahrukh

[EEsRULEtheWORLD]

Hi, Shahrukh. 

To compute the rms value of a waveform, you must take the square root of the mean value of the square of the signal level. 

Cheaper DMMs will assume that the waveform is a sine wave and divide the peak value by sqrt(2).  Expensive DMMs will sample the waveform and calculate a "true" RMS reading.  These DMMs use nonlinear signal processing in the analog domain and performs the following calculation:

 

This is the same equation that is used with the RMS VI in LabVIEW.  Using this VI will give you the same results as the "direct" method that DMMs use.

The NI 4070 DMM takes it one step further and uses an onboard digital signal processor to compute the RMS value from digitized samples of the AC waveform.  The result is more accurate with fast-settling AC readings.