What is the correct way to ground my thermocouple, when using the AD849x or AD59x to measure the thermocouple?
When using the AD849x or AD59x thermocouple amplifiers (to measure a thermocouple), the thermocouple should have one, and only one, ground connection or source of common-mode voltage. We always recommend connecting the thermocouple to ground through at 100 kΩ to 1 MΩ resistor placed at the negative (inverting) input of the amplifier on the PCB. This solution works well regardless of the thermocouple tip style.
If there is no electrical connection at the measurement junction (insulated tip), the resistor is small enough so that no meaningful common-mode voltage is generated. If there is an electrical connection through a grounded or exposed tip, the resistor value is large enough so that any current from the measurement tip to ground is very small, preventing measurement errors.
Any additional ground connection is detrimental to performance because ground loops can form through the thermocouple, easily swamping the small thermocouple signal. Grounding the thermocouple through a resistor as recommended prevents such problems.
Grounding the thermocouple (through a resistor) at the inverting input of the AD849x/AD59x also provides thermocouple break detection… This will force the output to rail high in the case of an open thermocouple condition.
We now have some more discussion of thermocouple grounding in video form, located here.
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