module reference causing bad voltage readings in Cdaq

[Biosolutions]

We are reading a differential across 12 super capactiors in a 9206 module of a cdaq.  Thee voltage level of the 12 caps is about 4.2v each, , giving each stack a 50 volt ouput.  Three of these stacks are then put in series making a total voltage of 150V.   A voltage divider across each cell so we can scale down the voltages by a factor of 6 but we are still well over the individual voltage limit of each module (25V vs 10v).  

We are reading a differential across each cell, thus, limiting us to about 4.2v at each channel (and with the divider, about 0.7V)    A representative schematic of this is shown in the attached PDF document which maps each cell voltage into the cdaq module (all daq connections shown in schematic).     I have only shown 2 of the stacks but Im not sure I need to draw the rest.

So the issue is that the reference level (cell 6) seems to be floating rather than giving the expected output, 4.2V  (see attached image connected.png).  We know it is 4.2V as it can be measured with a DMM across the cell.   I have attached a plot of what the data looks like,  cell 6 and 7 show the most variation rather than being a solid 4.2 V.  Its like they are sinusoldial....

What is more odd is that the attached figure (disconnected) shows that when the stacks are disconnected from the cdaq, but the wires left in the modules, there is drift on these same channels.    It dosent really make a lot of sense.

[FinchTrain]

Hi,

Floating can be caused by a number of sources. It can be difficult to troubleshoot without the ability to look at your physical setup in person. That being said, take a look at this NI white paper detailing Field Wiring and Noise Considerations for Analog Signals. It documents a lot of common setups as well as preventative measures to eliminate noise and floating. I'd be glad to help with follow up questions after you check out this article.

Field Wiring and Noise Considerations for Analog Signals

http://www.ni.com/white-paper/3344/en/

Regards,

Finch Train

[Biosolutions]

all i want is that you look at the schemtatic to look for something obvious

it should work.

the shielding and others contrains, ie the wiring 101 are obvious stuff  but thanks

[FinchTrain]

Hi,

As you mentioned, the diagram should work. I would encourage you to keep an open mind in this troubleshooting process. Troubleshooting involves an ongoing conversation and most of the time it is because the problem is not something obvious.

If you have read the voltage correctly with a DMM it is clear that the problem is in fact not within your circuit design and rather a plethera of possibilities involving an incorrect reference with your DAQ device. If you would like further assistance let me know and I would be glad to help, but sometimes it is the obvious that we simply overlook. I would encourage you to read through the article and I would be glad to voluntarily have an ongoing conversation.

Regards,

Finch Train