DAQ Problem

[Colinw88]

Hi there

Im currently using the NI USB-6008 DAQ to read in a voltage that is created from a circuit, however when i test the voltage using a dmm it remains at a constant voltage however using labview its continually changing. The aim is use Labview arithmetic abilities to help calculate what value i should have from the circuitry. Is there any way for it just to either convert it so its a signal value and not constantly changing as it will make this alot easier to understand ( first time ever having to use labview and struggling a bit)

Many Thanks

[johnsold]

Can you describe how it is changing?  Posting your VI with some data saved as default would help us see what you are doing.

Most DMMs do some averaging which may smooth out variations which the DAQ device records.  Do you have an oscilloscope to check your signal?

Of course you can perform averaging in LV, but before taking that approach, you should undersatnd what is happening.

Lynn

[Colinw88]

The display on the oscilloscope in the VI shows it as oscillating constantly. I'm just using an example this now to help understand the approach i should make. I do have an oscilloscope but it wont be until tomorrow afternoon that i will have access to it. Hmm, i see what you mean Lynn. I can message u the details of what I'm doing if you would like, to help give you a better understanding. Colin

[johnsold]

Colin,

Please post your details here.  That way others who may have suggestions can see them and be able to contribute.

It sounds like the DMM is averaging enough to smooth out a signal with significant time variations.  You may be sampling too slowly to capture everything and be seeing aliasing.  The scope should give you a lot more information.

Lynn

[Colinw88]

Hi Lynn, for for late reply ive not been working on the project but im back on it. I took your advice and measured it on an oscilloscope and your correct. The DMM does do some form of averaging. 

Im currently using the 6008 connected to an amplifier. The amplifier is amplifying the signal of a strain gauge as the signal was too weak to get a clear reading for very small measurements. I created an average VI however the problem i have is that the value is constantly changing and this requires to get an accurate reading. So im struggling to think of any solution that can alter with the signal altering. Would you have any suggestions you could make?

Ive attached 3 picture files to show what i mean. I found the averaging circuit from someone elses post on this forum.

[johnsold]

Colin,

You have several issues which are affecting your results.

1. We cannot tell how you have the DAQ Assistant set up.  What is the sampling rate? How many samples do you acquire with each reading?

2. You indicated that you see some "oscillations" in the data when viewed on an oscilloscope or with a graph connected to the DAQ data.  Please post that also so that we can see what else is going on.  How large is the oscillation compared to the desired signal?

3. If your interferring signal is from the power line, averaging to reduce it works best if you average over a complete cycle of the power line frequency.  If your data includes a different part of a cycle each time you average, you can get the kind of fluctuation you see.

Lynn