add root mean square function

[kawaiicat]

hi, Neal

first of all, thanks your help.

i got  a question here, the "stimulate" is a function for self test like?
i got the same output as ur jpg attach.
However , i guess when i really meausre a waveform, i dont' use that "Simulate" and i directly connected to a function generator which is sine wave 1kHz, 1VPP, i use 2V/div, it gives me 0.36 for rmss
and i use 200mV, it gives me 3.57Vrms

so the scale will make the rms difference, am i right?

[Neal M]

Hi kawaiicat,

The simulate button only changes the DMM Initialize vi to make a simulated device, so the RMS behavior should be exactly the same.  To answer your second question, the RMS values should only be dependent on the signal and not the scaling since the RMS is defined as:


Which I got from the LabVIEW help.

That being said, I did not notice, however, that the RMS value given in my picture was actually incorrect.  Not only that, but it did depend on the V/div I set.  It should be near 7V since the signal is -10V to 10V.  Once I noticed this, I did some digging and discovered that the RMS value given is taken off of a waveform that has been scaled to fit the display.  The RMS value that we have been reading is not in Volts, but in Divisions.  To get to the correct RMS value, we have to multiply by V/div.  Here is a picture of how I unbundled the cluster containing the V/div information and then multiply to get the correct RMS value. 



Please try this and let me know how it works out.

Neal M.
NIC AE
www.ni.com/support
SR#: 1028611

Message Edited by Neal M on 10-02-2007 03:32 PM

[kawaiicat]

hi

finally i modify an example and add the function that i wanted.

however, i find the waveform is distort alot.
i use the 4070 card and want to measure the differential voltage.

you can see the file.

Message Edited by kawaiicat on 10-03-2007 04:11 AM

[Neal M]

Hello again kawaiicat,

I am slightly confused about what the new issue is.  From your jpgs it appears that the -37V to 37V signal (80.jpg) could be correct, however the -8V to 8V signal (FF.jpg) is incorrect.  If this is the correct issue, I would reccomend changing the range value to one closer to the actual signal (e.g. from 70V to 20V).

I also noticed that the rate was set to 900kS/s.  I would like to point out that in the 4070 data sheet,  it shows that at that rate you are only reading with 3 digits (~10 bits) of resolution:

If your application requires higher speeds and resolution, then we can start looking at some of National Instruments' other products that can accomplish this.  I hope this helps.

Neal M.
NIC AE
www.ni.com/support

[supervins]

Dear All,

my name's Vincenzo.

I have some problems. I'm able to acquire voltage signals from NI-USB-6211, 8192 samples at 125K Hz in a while. Then concatenating this array i'm able to write in tdms file.

My problems are: How could I do a mobile average and a mobile root mean square of any array into the while?

I have done a mobile average with Fir Filter and this is nice, but I don't know how to do the average RMS?

Could you help me?

thanks in advance for your kindness.