measuring mV using analog input

[labview1958]

I am using an analog input of a Ni-DAQ 6025E card. I can measure a 5V signal. If it is 5V on the voltmeter, then it is 5V on the graph of amplitude Vs time. However a 0.5V signals comes as an oscilating signal of between o and 1.5V. Do I need to calibrate the graph? If so, how do I do that?

[Tareq]

Hi,

What is the signal you are trying to measure?

Try to set the high and low limits of the channel to minimize the range as possible. eg. If I'm expecting a signal between 0 and 1V and I set the range of AI channel between 0 and 5V, I'm getting 1/5 of the resolution that I can have. 

If you are not making averaging, try to average every 100 samples for example and see what happens to the signal.

Make sure that ground loop does not exist in your setup.

Hope it works now.

[Lorne H.]

LabVIEW1958,

Are you sure that your low voltage is exactly 0.5 Volts?

You will need to check this with an oscilloscope.

Using a voltmeter to measure the low voltage will give you an average of the voltages over time.  For example if you have a square wave with a 0 volt low and a 5 volt high, most handheld voltmeters will measure 2.5 volts.

I bring this up, because it is possible that your hardware is measuring your signal properly and that your voltmeter is filtering the noise in your signal so that it looks like a constant 0.5 volts even though it is varying from 0 volts to 1.5 volts.

Lorne Hengst
Application Engineer
National Instruments

[LV_Pro]

You should also look at the signal using the Measurement & Automation Explorer's (MAX) test panel for your card. It allows you to run the card with various settings and not have to worry whether your program has some bug. As the previous posts stated, a meter tends to filter out noise, otherwise the display would be flickering on all but the steadiest voltage. Depending on your sample rate you could be looking at a dc signal with an ac (noise) component, that your meter would show as a steady voltage of one value and your program (or MAX) will show as a voltage that is "oscillating around that meter's value. As they said, if you have access to an o-scope that will help a great deal. I had a recent project that I was brought in on where we weren't able to make accurate measurements of the DC voltage going to a motor. I put a scope on it and found that I had been misled, it wasn't a DC voltage it was a pulse width modulated (square wave) signal where the motor responds to the DC average of the signal. Required a different method to measure.

P.M.

.

[labview1958]

Assuming my signal is a varying signal between 0V and 1.5V, how do I get my LV program to read it as 0.5V, the same as the voltmeter reading?

[Lorne H.]

LabVIEW1958,

You would need to take a bunch of readings, try 100, and then average them.  This would create a software lowpass filter.  Or you could use one of the lowpass filter VIs in LabVIEW and set the cutoff frequency to a low value, like 1 Hz.

Lorne Hengst
Application Engineer
National Instruments

[labview1958]

I have attached the VI I am using to measure the analog voltage signal. I hope it is correct. Can it be improved? I am using AGND and A0 channel in my NI-DAQ 6025E card. What is multi and single in this cases?

[labview1958]

Is my LV correct?

[labview1958]

Is my LV the source of my problem? Is it correct?

[Otis]

Hi labview1958,

I would recommend doing what LV_Pro recommended and try using the device in Measurement and Automation Explorer (Start > All Programs > National Instruments > Measurement and Automation Explorer).  First test the device there (by using a test panel) and see if you are getting the measurements that you are expecting. 

I would also check out some of the related pages here to see if you can get some information that might help you better develop your system.

The program that was posted will probably work, but it's not really the best way to do what you are trying to do. 

If you have LabVIEW 7.0 or higher then you can use DAQmx which is much easier to use for new users.  I recommend that you simply use the DAQ Assistant Express VI to configure your system.  You can access that by going into your Block Diagram on a New VI, right click and go to Functions > Input > DAQ Assistant.  You can now quickly configure your measurements exactly as you like, test the way it functions and then use those signals in your application.

Best of luck getting this together,