Initial spike in voltages on RSE inputs when sampling multiple channels

[prjackso]

I find that I get spikes at the beginning of my voltage data sets collected using RSE inputs and aquiring on multiple channels.  This problem is independent of the data logger I use. 

Scenario:  I sample a voltage from a constant voltage source (say 1.5 volts) at 100 Hz and find that the first 7 or so samples are far from the input voltage (they start at about 2.5 V and quickly and uniformly approach the input value of 1.5 V).  After about 7-10 values, the acquired voltage is a constant 1.5 V.  This only occurs if I acquire voltages from multiple channels using a data logger.  If I sample only one channel with the same setup, the intial spike is gone and the acquired voltage is a contstant 1.5 V from the beginning.

I don't see this intial spike while sampling multiple channels with differential inputs.

Any advice is appreciated. 

-PRJ

[Tom_W_[DE]]

Hi PRJ-

I have been testing with my PCI-6040E E Series card and haven't been able to replicate the behavior you are seeing.  Are you sure that all of your connections are correctly made for RSE?  You need to keep in mind that differential connections are fundamentally different from RSE.  I would suggest that you verify for RSE that you are connecting your signal (+) to the numbered input channel and signal (-) to AIGND.  For differential you should connect to the input channel (X) and (X+8) for signal (+) and (-), respectively.

I would also check to make sure that you are not seeing ground loops in your systems as this can sometimes introduce skew into RSE readings.  A brief discussion about errors introduced with RSE readings of non-floating voltage sources is available here.

In general, differential measurements are advised because of their ability to provide better common-mode rejection and to remove the possibilities of ground loops.  NRSE mode is another alternative that can be used if sacrificing two input channels for a single measurement is not a good option.  More discussion about various connection techniques is available in this tutorial.

Hopefully this helps-