SCB-68 Signal Interference?

[bsteinma]

Hi,

I have been using an SCB-68 DAQ board for acquiring signals from pressure transducers and there was a point when I was curious if the DAQ was wired correctly by the previous user. The issue I am having is that it looks like my signals are somehow electrically tied to each other. I have inserted both a diagram of how they are actually wired and a screenshot of the signals I am referring to. This has been happening for a while now but I trusted the wiring so left it alone. This is very noticeable when I change any settings on the amplifier being used. In the screenshot, the top two graphs are connected to an amplifier and the bottom two are connected to a different HP patient monitor amplifier and are then sent to the SCB-68 before being acquired with the DAQ assistant in LabVIEW. I found it interesting that there seems to be interference in the green signal (not connected to the same amplifier as the red and orange, but it's wires are geographically closer). Is this a normal issue, am I wired incorrectly, or is it a shielding issue that I could somehow fix? I wasn't sure if it could possibly be a problem with my VI in labview since the signals are all acquired with the DAQ assistant and then split, but it seems like the issue lies with shielding. Any input and advice will be extremely helpful. Also, if you need more clarification just let me know.

Much Appreciated,

bsteinma

[Kevin_Price]

I would primarily point you to a nice wiring guide on the ni website found here:

Field Wiring and Noise Considerations for Analog Signals

Just one other comment.  I notice that the 2nd (orange) graphed signal is more than 2 orders of magnitude larger than the 3rd graphed signal.  Odds are, your SCB-68 terminal block is cabled to a data acq board that multiplexes the channels.  As the multiplexer switches away from a channel with a larger voltage, it takes some time for the analog-to-digital converter to bleed off the charge it acquired.  If it hasn't yet finished at the time that the next channel is multiplexed in, then there will be some bleed-through.

In addition to what you can glean from the link above, here are two other possible strategies:

1.  When creating your list of channels to be acquired, you can list the same channel multiple times.  It will simply be read multiple times.  The later readings should show less "bleed-through" influence.

2.  Try ordering your channels to read smaller voltages first and larger ones last.  Without going into too many gory details right now, this *might* provide more charge bleed-off time from the higher voltage channel.

-Kevin P