Inaccurate current measurement using Shunt resistor

[Margaret F. · Accepted solution]

Hi Jake,

Thank you for posting to the NI forums. It sounds like you may be having some sort of grounding issue. I think a good way of isolating the problem would be to remove the SCXI system and just target the DAQ card. We can do that by connecting your transducer to your 1180, then connecting the 1180 to the 1349. You can then cable the 1349 to the computer and plug it into the DAQ card. From here you can open up a test panel for the DAQ card in MAX and test an analog input channel. Let's see if this takes some variables out of the equation and helps get down to the root of the problem.

[Jake809]

Hello Margaret,

Thank you very much for the advice.  This seems to have been the problem.  WIth 1180 and 1349 out of the SCXI chassis everything seems to be behaving much better.  I first used channel ai0 to test a battery again, which held firm at the proper voltage.  With the transducer wired in, the output was a steady 4 mA, as expected.  What would you suggest I do to maintain these results while using the SCXI chassis? 

Thanks again,

Jake

[Jordan F]

Hi,

Do you have any other modules in the SCXI chassis while you are taking the measurements?  If you do, try just using the 1180 in the chassis by itself to see if you receive an acceptable measurement.  Otherwise, I would try another slot in the SCXI chassis to see if you get the same behavior.  Also, if you have another SCXI chassis, try to see if you can reproduce this problem.

[Jake809]

Yes, I have the DAQ card cabled to an SCXI-1530.  I read while waiting for help that devices capable of operation in parallel mode reserve channels even when operating in multiplexed mode, so I was trying to use reserved channels for general use.  Connecting my transducer to channels ai4-ai7 or ai12-ai15 solved the problem.  Thank you to everyone for your help.

Regards,

Jake