Question about filter VI in LabVIEW.

[cd384]

I am using LabVIEW 7.  I am trying to reduce the noise in a voltage signal that I am sending to LabVIEW.  When I use the filter VI (Functions> Signal Analysis> Filter), the output signal looks like the frequency response of the filter.  I need to simply get the filtered signal in the time domain.  How can I get this? 

Are there other ways to reduce the noise of a signal in LabVIEW?

Thank you.

[Tyler]

There are a couple things that stick out in my mind. First if you are using the the Express Filter VI, you need to make sure that you know at about what frequency the signal you are acquiring is at. That way you can appropriately set up the filter to get rid of any components you don't want. Since you are wanting to do a time domain, a straight DC voltage won't show any results. Most noise on acquired signals is 60Hz noise, so I would suggest using the appropriate filter to get rid of that noise. If this doesn't help get rid of the noise, it might be an issue with how you are acquiring your signal. In that case you need to let us know how you are acquiring the signal, what hardware your using, and the specs of the acquistion. That way we can come up with some suggestions on what we could do from the hardware end.

There are also some generic filter VIs located in All Functions>>Analyze>>Signal Processing>>Filters that you could try using to see if you have any success. Let us know how it goes. 🙂

Tyler H.

National Instruments

[cd384]

The signal I am aquiring is a dc voltage, but there is some ripple, as you can see in this picture:

I tried using a generic filter like you suggested (lowpass equiripple).  It gave me an error that I am not sure how to fix.  I think it has to do with converting data to go into and out of the filter.  Here is a picture of the error.  You can also see how the filter is setup.  How can I fix this?

Message Edited by cd384 on 03-01-2006 03:44 PM

[Wendy L]

Hello cd384,

The following KnowledgeBase describes this error and give some suggestions on how to resolve it. 
KnowledgeBase: How can I avoid seeing Error -1802 " Waveforms have different dt values." in LabVIEW?

Let me know if they work out for you.  Thanks!

[cd384]

Thanks Wendy,

That fixed the problem with the error, but I am getting NaN as the output to my graphs, instead of numbers.  Here is basically what I am doing.

Why is it giving me NaN?