remove unwanted signal after data acquision

[lyn]

i'm doing data acquision using ACH0 and ACH1 of PCI-6025E. ACH0(reference sinewave) should give me a 50Hz sinewave and ACH 1(spike) should give me a series of spikes, however the spikes are riding on the sinewave-like signal.

1. why is the spike waveform riding on another signal? i had double checked the signal on the oscilloscope, and verify that it is just a series of spikes. and i don't face this problem if i acquire the 2 signals separately by using acquire waveform.vi instead of the acquire waveforms.vi. synchronising these 2 waveforms is a must for my application hence i need to use acquire wavefroms.vi that provided me synchronism.

2. what can i do to remove the sinewave-like signal?

3. i tried using
filter however it don't seem to work as when i view the frequency content (using FFT spectrum mag-phase.vi)of the 'spike signal riding on the sinewave-like' signal, i got an output waveform that i cannot interpret.

4. why can't view the frequency components of the signal measured in qn 3. Uisng the extract single tone information.vi it shows that the frequency of the spike signal is around 50Hz.

[LocalDSP]

Different questions and suggestions.

First, an attempt to solve the problem in the acquisition:

1) Overload: It looks like the sine tone you are acquiring is extremelly overloaded (it clips at 10 Volt). This could be disturbing your measurement. Can you by any way avoid this overload?

2) cross-talk between channels: you are running your board at a scan-rate of 200 kHz. This is over the specifications of the settling time of the board. Try to decrease your scan rate and see if this fixes your problem (or at least reduces it).

3) Is the problem still there if you only acquire channel 1? If yes, the problem is not cause by scanning issues but most likely be due to bad connectivity, grounding issues, common mode problems, single-end / Differential co
nnection. This depends on the way your signals are connected.

Removing the problem in post-processing:

4) if everything else fails, you can remove the sine tone from your "spike signal" as follow: On the sub-VI "Extract Single Tone Information.vi" create a constant "Residual signal" for the control "Export signal". The time signal exported (on top of the VI) will then be what you are looking for, that is your signal with the detected 50 Hz sine tone removed.

[eh]

I would check how you are wired. A good app not for this is AN025 http://zone.ni.com/devzone/conceptd.nsf/webmain/01F147E156A1BE15862568650057DF15?opendocument