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DC/RMS value of waveform question!

Hello,

       This would probably a very common question, but I hav a DAQmx system sampling 1000 AI samples at 1000Hz. I would like to get the DC value of each and individual point of this waveform.

when i use the Basic DC/RMS.vi and write the data to a file using Write to Measurement File.vi, i do not observe 1000samples in 1 sec. 

however, i observe only 1 sample/sec, because the loop frequency now becomes 1000samples/1000Hz = 1 sample/sec.. 

 

is there a way that I can write all the 1000samples in 1 sec to file? Any other vi or logic?

 

any help would be appreciated... thanks

 

Now on LabVIEW 10.0 on Win7
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If you want to log each and every sample, then using the RMS function makes no sense. Just wire the waveform to the Write to Measurement File and you are done.
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i don't want the RMS value to be written, the DC value?... i have a signal conditioner which is supposed to produce +/-10V DC output, which it is producing,

however, the signal is a very high freq signal, which cannot be fed into DAQmx AI. if I see the output voltage on CRO, I observe the flickering output voltage as I see the AC part of the signal, however, if i observe it on a true RMS voltmeter, then the signal happens to rock steady with no flicker....

 

hence, i was thinking of either write either the DC value or the RMS value. 

 

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That may be a much more complicated question.

 

1. You have a high frequency signal.  Does this signal have a DC component or are you just interested in the total RMS value?  Is the time varying part of the signal periodic?  Do you know the period or frequency?

 

2. What kind of conditioning does your signal conditioner do? Does it rectify and filter the signal? Does it do a true RMS covnversion or is it average responding but calibrated for RMS for sinusoidal signals? Does it do the kind of signal processing you want?

 

3. Are you trying to measure the signal coming out of the signal conditioner or are you trying to measure teh original signal and produce an output after software signal porcessing which is similar to what the signal conditioner produces?

 

Lynn

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@johnsold wrote:

That may be a much more complicated question.

 

1. You have a high frequency signal.  Does this signal have a DC component or are you just interested in the total RMS value?

The signal does have a DC component in it, and that's what I am interested in...

 

 Is the time varying part of the signal periodic?  

Ideally, it should not be, but I am observing a periodic pattern in the output waveform,.

 

Do you know the period or frequency?

 Yes, it is around 20Khz, (that's what is the input to the signal) 

 

2. What kind of conditioning does your signal conditioner do?

This is a signal conditioner for AC lvdt, made by Analog Devices AD598. performs the calculation based on ratiometric type of signal conditioning, where, Vout = (Va-Ab)/(Va+Vb), Va and Vb are the voltages

out of secondary of LVDT, input freq to the LVDT is 10KHz at the pri, 3Vrms

 

Does it rectify and filter the signal? Does it do a true RMS covnversion or is it average responding but calibrated for RMS for sinusoidal signals? Does it do the kind of signal processing you want?

It does rectify internally, there is a precision full wave rectifier inside the signal conditioner, so, I am assuming there should not be any further signal processing required once the signal is outputted from the chip.

 

3. Are you trying to measure the signal coming out of the signal conditioner or are you trying to measure teh original signal and produce an output after software signal porcessing which is similar to what the signal conditioner produces?

I am trying to measure the signal out of the signal conditioner.

 

Lynn


 

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Hello LV_Enthu,

 

Is there a reason you are trying to write to file at such a high rate? I would recommend you save the information in a queue (buffer) then utilize the write to measurement file to save the information.  Even though the write to measurement file does not take one second to output all the data to file, the data measured by your Data Acquisition Card ( barring any unusual setup) will still have taken the each sample at the 1KHz you requested. Also, please put a screenshot of your LabVIEW code so we can better troubleshoot the issue in case the suggestion above is not one you are willing to undertake. 

 

Regards,

 

Izzy O.

Applications Engineer

National Instruments 

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Thank you for the description of your ssytem and the problem you are having.

 

The 20 kHz component is probably the second harmonic of the 10 kHz LVDT excitation and is the result of the rectification inside the signal conditioner.  Please read pages 8 and 9 of the AD598 data sheet. It describes the ripple to be expected on the DC output voltage as a function of the filter capacitance. For small values of capacitance the ripple can exceed 100 mV rms at 10 kHz. The speed of response and the amount of ripple will require a tradeoff.

 

Unless your sampling (at 1 kHz) is exactly synchronized with the 10 kHz excitation, you will have aliasing whihc cannot easily be removed after digitizing.

 

Can you increase the filter capacitance enough to reduce the ripple below the level where it is a problem without slowing the system response too much?

 

Can you sample at 50 kHz and average the samples?

 

Lynn

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One other thing: This has nothing to do with RMS.

 

Lynn

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Thanks, johnsold.. i will try out the options you mentioned and keep posted

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