I need to monitor 72 thermocouples so I've ordered a MUX80 and 3 more CB37's. I've been using a T7 with CB37 and with an LM34CAZ on AIN12 for the CJC but the MUX80 datasheet states that AIN12 is not connected to anything. So, how can I use the LM34CAZ with this setup, where must it be connected, and what would the AIN-EF Config-B value be?
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Assuming that all CB37s are at the same temperature (air temperature), you can connect the LM34CAZ to any analog input on any CB37. For example, say you put it on the AIN12 terminal on the CB37 connected to Mux80-X5, which is MUX108 or AIN108. Config-B is then set to the modbus address for AIN108, which is 216 (analog inputs start from 0 and use 2 registers each).
Also, with so many thermocouples make sure you consider complications #5 and #6 from the thermocouple app note:
https://labjack.com/support/app-notes/thermocouples
Thanks for the good advice. Now, where exactly is the "main" ground on the LabJack where a big fat wire may be connected?
Sounds like you are looking at these items from the Mux80 datasheet:
https://labjack.com/support/datasheets/accessories/mux80
D. Get rid of or at least reduce GND offset by minimizing connections (minimizing resistance) between the signal terminals and A/D chip, avoiding sourcing/sinking current to GND on the CB37, and adding a big fat wire from CB37-GND to U6-GND or T7-GND.
E. Use a star ground, and have a single solid connection from that star ground to U6-GND or T7-GND. For example, connect the negative leads of all signals to an external grounding post or grounding bar, and run a big fat wire from there to the main LabJack GND.
At the end of E, rather than saying "the main LabJack GND" it should (and now does) say "a GND on the main LabJack". So the suggestion is to use any GND terminal on the main device. Getting into further detail, the GND terminals next to AIN0 and AIN2 have the best route to the A/D chip and are sometimes slightly better than the other 3 GND terminals.