How correctly connect USB 6251 to measure DC voltage

[nguyenmanhcuong]

Hi all

I have encounter a problem like in the topic http://forums.ni.com/ni/board/message?board.id=170&message.id=404676&query.id=250408.

I'm sorry for repeating of topic but I've read all of your advices and have not yet resolved problem. 

My task very simply: I use USB 6251, differential mode  to measure voltage of battery (Energizer) 1.5V (I connect AI0 to the positive pole and AI8 to the negative pole). But result shows >10V and Mean.vi shows bigger 4V (see the attached picture).

I don't understand what happed with my task?

Please help me!

Thank so much!

MC

[AdnanZ · Accepted solution]

You have a floating signal source. Have you connected resistors to remove bias currents? Look at Table 1 at this link and make sure you are connecting the wires that way:

Field Wiring and Noise Considerations for Analog Signals

You mention that you are using differential channel AI0 yet I can see that you are measuring AI4 on your front panel. Have you tested the connection in MAX? What sort of readings are you getting there? Be sure to select Continuous Acquisition and acquire 100 samples at 1000 Hz. Post a snapshot of the readings you get there.

[nguyenmanhcuong]

Thank Adnan Zafar

 You're right, I did not connect resistors to remove bias currents. But now I do another way and get good results: Connected positive pole to the AI0, negative to the AI GRD, set configure in put to "RSE".  

I do not understand why we must set the sample rate to 1000Hz? Can set to 10kHz or greater? 

Thanks

MC

[DavidMN]

Hi MC,

Yes you can set to 10 kHz or greater.  The USB 6251 is capable of sampling up to 1.25 MS/s.  I cannot speak for Adnan, but I believe he was just recommending a good ratio for sampling rate to number of samples for obtaining representative results (eg 10:1).