Dual stage or Differential Thermocouple Acquisition with SC-2345 and TC-01?

[bentnail]

I am currently using an SC-2345 with the TC-01 signal conditioning modules for collecting 5 channels of thermocouple data. The application requires accurate and high-speed (at least 10 samples/sec on all channels) DAQ. The thermocouples are submersed in water, as is the device I am testing. I think I have what is probably a ground-loop problem, but it is proving very difficult to solve (the result is a 0.2 to 0.5 degree celsius offset when I plug in the grounded signal cable of the device I am testing, and the offset increases with the number of thermocouples I have plugged in). An isolation transformer to power the PC and other devices I am testing does not work.

I know that the SC-2345 with
the TC-01 modules will only allow you to collect in NRSE mode because of the on-board CJC for each module. If I use an SCB-68 to collect my thermocouple data in DIFF mode, I do not have this offset issue (thus I assume it is a grounding problem because of the NRSE mode), but the data is much noisier and far less accurate than the SC-2345 with TC-01.

Can an SCC-AI isolated analog input be used for dual-stage conditioning with the TC-01s? Is their a way to maintain the filtering and CJC features of the TC-01s and acquire in DIFF mode or somehow isolate the inputs?

[RIO-Todd]

You can't use the SCC-TC modules within a dual stage configuration. I have linked a document below that includes which modules can be used within a dual stage configuration.

I believe you're right in thinking that it is a ground loop problem. Within the SCC-TC module, AISENSE is tied to AIGND of the DAQ device. Therefore, if you are connecting a ground referenced signal to the negative input of the SCC-TC module, you will be connecting the ground of the device with the ground of the DAQ device. This could definitely cause a ground loop. The SCC-TC user manual (linked below) shows this connection in figure 3.

Unfortunately, there isn't a way to use the SCC-TC series modules in differential mode. Our SCXI signal conditioning hardware can be used in different
ial mode, but the SCC signal conditioning hardware must be used in NRSE mode.

Which SCC Modules Can Be Cascaded or Used in Dual Stage Mode?
http://digital.ni.com/public.nsf/websearch/7B801B3C9E0B82A386256C25007640CA?OpenDocument

SCC-TC Series Thermocouple Input Modules User Guide
http://digital.ni.com/manuals.nsf/websearch/B0009EED5EC0D8B086256C020070CA12?OpenDocument&node=132100_US

Regards,

Todd D.