Can I use one waveform to subtract from the other one in labview?

[klhjkg]

Here is the thing.

I am using DAQ to aquire amplified signal to measure its noise of output.However DAQ would contribute a little noise to the whole system.My thinking is to capture two waveforms of noise from DAQ itself and the whole system(amplified circuit and DAQ) and then make the subtration calculation between them.Is it possible to make it with labview?How?

Or can I aquire two groups of signal using DAQ at the same time? If it can't be realised is there some other way?

Thanks

[MikeS81]

Hi klhjkg,

you can use the subtract function, but if the waveforms have different size, then the result has the size of the smaller one. I think it is like subtraction of arrays.

Mike

[klhjkg]

You mean I can use one waveform stored signal to subtract from the other one directly?

[smercurio_fc]

The direct answer to your question is that yes, you can subtract waveforms using the subtraction node.

The better answer is for you to take a look at these KB articles (as a start):

Field Wiring and Noise Considerations for Analog Signals
Reducing the Effects of Noise in a Data Acquisition System by Averaging

[smercurio_fc]

Here's another:

Five Tips to Reduce Measurement Noise

[klhjkg]

Thank you for your help.

But still I can't subtract waveforms and I have no idea how to do it.Since waveform is output terminal and you can only connect output node to it.But if I would like to make calculation I have to connect two input nodes to ADD or SUBTRACT.It means waveform can't output something to ADD or SUBTRACT.

[Dennis_Knutson]

You are going to have to show some code. As the picture below shows, you don't have to do anything special to add/subtract one waveform from another.

Message Edited by Dennis Knutson on 06-16-2008 01:47 PM

[MikeS81]

Hi klhjkg,
see the attached picture, please. It shows how you can substract two waveforms.

Hope it helps.
Mike

[smercurio_fc]

I don't know what you mean by

---

Since waveform is output terminal and you can only connect output node to it.

---

"waveform" is not explicitly "output" or "input". It's simply a datatype. As the previous responses showed, you just wire.

You mentioned you're getting this from "DAQ". Are you actually getting a single waveform, or is your DAQ setup to output an array of waveforms?



Message Edited by smercurio_fc on 06-16-2008 02:49 PM

[NIquist]

If I'm reading you right I don't think your original concept is valid.  If you have already done a good job with wiring, shielding, etc.  You won't be able to simply subtract a data collection of "just noise" from a collection of "noise plus signal" and expect the noise to cancel itself out.  It's random.  A better method would be to collect the same signal several times and then average the results.  Even that will only reduce repetitive components in the noise.

As smercurio said however, the most important consideration is to make sure your connection method, grounding, shielding, timing, etc is top notch to prevent noise from contaminating the signal in the first place.  I generally use differential signals through good quality twisted pairs with the shield connected on the DAQ side only (to prevent ground loops).