Noise appearing in signal after filtering

[Signal_G]

I am filtering a signal from a pressure transducer and after it's filtered a small spike of noise shows up at the beginning of the sample that isn't in the original data.  I talked to friend here who has seen a similar thing.  Does anyone have any experience with this?  I just want to clean it up.

I have attached to screen shots examples.  The graphs on top and bottom are zoomed equally. 

[rpursley8]

When you apply the filter, if the first point is non-zero, you see this effect.  What I usually do is find the value of the first point, subtract that value from the whole waveform, apply the filter, and then add the value back to the waveform.

[Signal_G]

Thanks for the tip.  That seems like a reasonable work around.

I tried to do that but in changing the first value converts the dynamic data to an array.  When I change it back the filter doesn't work.  Any ideas there? 

[rpursley8]

I don't work with the Dynamic Data type much so I didn't foresee your problem.  If you convert the DDT to a waveform and then extract the data, you can then do the subtraction and then convert it back to a DDT.  It's inconvenient, but I don't really see how to do it within the DDT.

Message Edited by rpursley8 on 05-17-2010 11:39 AM