Bandpass Filter changes my DC offset

[Jeff Plotzke]

I'm trying to take a waveform and do a bandpass filter with corner frequencies 7k and 9k to extract an 8k component. However, when I run the VI, it extracts the necessary frequency, but it re-centers my signal around 0. I've attached a screenshot of my problem.

Should this happen? I really want to keep it centered around what my 8k component should be. I could find the avg DC value on the original waveform and add it to the resultant waveform, but that wouldn't be mathematically correct and would only give me an approximation.

For details, I'm using the Filter Express VI configured as a 10th order Butterworth IIR Filter (I don't care about phase shifting -- I'm looking for steep corner frequencies)

So, I guess I'm looking for a way either to explain these findings and/or a way to get LabVIEW to keep my DC offset value.

Thanks!

~Jeff

[LAVA 1.0 Content]

Should this happen?

Yes, it really should. If you filter away 0 Hz component, which is the same thing as DC component, you end up with signal that has no DC component. So if you band pass filter your signal to include only frequencies of certain region and 0Hz doesn't belong to this region, you do not get any DC in you filtered signal.

If you want to keep you DC-component together with you 8k component, you can filter your signal with low-pass for 0.5 Hz and the band pass you had and add these two signals. Then you'll have you DC together with your 8k signal.

[Jeff Plotzke]

Yes, it really should. If you filter away 0 Hz component, which is the same thing as DC component, you end up with signal that has no DC component. So if you band pass filter your signal to include only frequencies of certain region and 0Hz doesn't belong to this region, you do not get any DC in you filtered signal.

Doh! Now that you say that, it makes perfect sense!

Thanks!

~Jeff