Ni 9205 configuration

[danfonsso]

Hello,

           I´m using the NI 9205 to read a signal from a voltage source (dc). I put the positive connector´s souce in the pin a0 and the negative connector´s source in the pin COM. But the signal that we are seeing in the computer is not continuos. I don´t know what is happening. I tooked a picture of the situation. I hope that someone help me. Thanks

[Oswald_Branford]

Hi Danfonsso!

From what i can see in your picture the value you are getting its between 2.22 and 2.26V. In this case it looks like the signal its not continous cause the scaling is taking that 0.02 to 0.04 as a big difference. What you need to do is try to adjust your Y-axis so it takes the 2.2 as a continous value and then you can see it as a DC line on the 2.2 V range. Try dissabling Autoscale Y and check if that helps.

Have a great day!!

[danfonsso]

Hello Oswald Branford,

                   First i would like to thank you for your answer. But it didn´t works. I run the task without to mark the auto-scale. I traied to creat a new scale but it didn´t works either. I´m not undestanding why is happining. Because i´m putting a continuos signal and i´m seeing a variable signal. Do you have another suggestion? or would you mind to explain to me your first solution a little bit more? thanks 

[Cássio]

Hi Danfonsso.

First you need disable Autoscale Y, how the Oswald told you. Because in this case, you see the little variation of voltage that is normal.

Regards.

[danfonsso]

Hello Cássio,

                      I´ve already done this. I disabled autoscale Y. But the signal still not continuos. The signal has some variation. Do you know how to correct this? thanks

[JamesBeams]

Danfonsso,

I believe by "continuous" in this case you are referring to the vertical variation of the signal, or "noise" and not the acquisition of the signal over time.

The source of this will depend on a couple of things.  First it would help to know the type of DC source you are using to produce the input to the module.  All DC sources are not perfectly DC and will have some small variation in them over time.  Second noise may be injected into the system if you are using long or unshielded wires to connect between your DC source and the analog input.  Third there will be a certain amount of measurement error of the ADC inside the 9205 module (maximum amount specified in the user manual).  All of these effects will result in one measurement being slightly different from the next. 

To make the signal "smoother" you can use either a digital or analog lowpass filter.  The simplest lowpass filter would be an RC filter.  LabVIEW has lowpass filters available as well, or if you are looking for a more accurate DC power source in the first place this experiment may tell you that your DC source is not as good as you had hoped it would be.