Jump to content

As of July 17, 2015, the LabJack forums here at forums.labjack.com are shut down. New registrations, topics, and replies are disabled. All forums are in a read-only state for archive purposes.

Please visit our current forums at labjack.com/forums to view and make new posts. To post on the current forums, use your labjack.com login account. Your old LabJack forums login credentials have been retired. There are no longer separate logins for labjack.com and LabJack forums.


Photo

Stream Analog Input "jumping" problem


  • Please log in to reply
2 replies to this topic

#1 Iris

Iris
  • Members
  • 1 posts

Posted 13 June 2013 - 03:40 PM

I was using the U6 for analog input streaming to measure a DC voltage. But during my experiments, I noticed some "jumps" of the constant voltage reading, meaning the variations between readings can be up to more than 2 quantization steps. So in order to find out the source of this "jumping" problem, I grounded AI0 with the GND pin from the U6 unit. And the expected measurement results should be a constant 0V, however, I got the same type of 3 or 4 quantization levels of jumpsin this GND measurement. I've attached the output of the GND measurement (Pic. 1), and it is very obvious to see in the picture that the readings jump to more than 2 quantization steps quite often. I used the U6 easy Stream.vi from the downloaded package. The streaming had only one AI on, and sampling rate was at 5000KHz. The standard deviation of this specific set of 5000 data points is 0.319mV, and this is typical out of 50 sets of the same measurements. I did a quick FFT on this data (Pic. 2), and I didn't see any dominant frequency component other than the white noise floor. I was wondering what is causing this jumps of more than 2 quantization steps? Or is there anything I can do to solve this "jumping" problem? Thanks!

Attached Thumbnails

  • read.png
  • fft.png


#2 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 13 June 2013 - 03:59 PM

The U6 provides excellent noise levels for 16-bit analog inputs, compared to most 16-bit devices, and U6-Pro provides excellent noise levels for 24-bit analog inputs, compared to most 24-bit devices. But, you do get some noise, which is expected:

http://labjack.com/s...-and-resolution

Looks like you are using Range=+/-10V and I am guessing you are using the default resolution of 0 which equates to 1. At those settings we note a noise level of +/-3.0 counts:

http://labjack.com/s...users-guide/3.2

All your points are in that band except for the 6 outliers, so seems as expected.

#3 LabJack Support

LabJack Support
  • Admin
  • 8677 posts

Posted 13 June 2013 - 04:01 PM

Also see Appendix B which is based off command-response measurements, but agrees well with stream data where applicable:

http://labjack.com/s...uide/appendix-b


0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users