A/D board sampling error

Discussion in 'General Electronics Chat' started by samik, Dec 8, 2010.

  1. samik

    Thread Starter Member

    Nov 9, 2009
    20
    0
    Hi all,
    I faced this problem while sampling simultaneously from the first three analog input channels of the A/D board ( UEI ,Powerdaq,PD2-MFS-8-800-14DG). Whenever the first channel ( AIN 0) signal was going above 5~6 volt, the signal from the channel next to it (AIN1), was showing a lower value than expected( after sampling). I connected an oscilloscope in parallel with the channels and the sampled value was less than oscilloscope value for AIN1 ( for example if true value is 1.8 v, it was giving me 1.6 V). But whenever I swapped the channels this problem was solved. I am using Matlab for this and defined same range( +10,-10 V) for all the channels.
    The datasheet of the A/D board is attached herewith. I will be glad if anyone can throw any light on this.
    Thanks
    Samik
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    In the manual, table 5.3c, (p.48), which signal source type are you using?
     
  3. samik

    Thread Starter Member

    Nov 9, 2009
    20
    0
    I am using differential mode.......thanks Ron H
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    What is the common mode voltage of your differential input signal?
     
  5. samik

    Thread Starter Member

    Nov 9, 2009
    20
    0
    Dear Ron H
    The ai0 channel is connected toa pressure transducer which is already grounded, so the common mode voltage is 0. (row I,column II in table 5.3 c)
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Is there a current path between your signal source ground and your A/D ground? In other words, is the transducer ground connected to the A/D ground? Somewhere, there has to be a path for A/D input current to return to A/D ground.
     
  7. samik

    Thread Starter Member

    Nov 9, 2009
    20
    0
    The A/D board is connected to computer ground , and both transducer gnd and comp gnd are at same level.
     
Loading...