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MCC 134 Common Mode Error



Dear Digilent community, 

I am currently running into a common mode error with a thermocouple attached to the mcc134 hat. When plugged into a handheld reader, the thermocouple gives a valid reading, however when attached to the hat, it gives a common mode error. I have also tried plugging in other thermocouples to the hat, with said other thermocouples working correctly. This thermocouple has worked with the hat in the past, just several days ago, with no changes to the code. The TC is currently attached to a metal vessel, and works with the hat when unattached. I suspect it is some grounding issue, however I am not sure how to handle this issue. Thanks in advance. The TC in question is https://www.mcmaster.com/3857K212/


In summary:

TC works with handheld thermocouple tester, while attached to the metal vessel

TC worked several days ago, on the same hat, on the same port, with the same code

Other TCs work on the same hat, on the same port

The TC in question does not work on any other ports either

I now get a common mode error when the TC is attached to the metal vessel, yet I can get actual readings when not attached to the metal vessel. 

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Hello @joshch

The MCC 134 has a common mode voltage spec of 0.8 V max. This is a voltage measured between any CHx+ or – input and any other input 

As you have many other thermocouples that do operate correctly, there is something abnormal about that particular thermocouple. Option 1 is to not use that thermocouple.

Since it works when not touching the metal vessel, the Option 2 is to place a thermally conductive, electrically isolated barrier between the TC and the vessel.

Such as Mylar tape or thermally conductive grease.



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