AD59x Correct Thermocouple Grounding

Document created by analog-archivist Employee on Feb 23, 2016
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I do not understand how to correctly ground my thermocouple?

 

When using the AD59x, the thermocouple should be grounded at one place, and
only one
place. The grounding can occur at the negative input of the AD59x, the positive
input of
the AD59x, or at the thermocouple tip.
Where the thermocouple is grounded will change the behaviour of the AD59x
during an
open thermocouple condition. If the negative input is grounded, the part’s
output voltage
will rail high during an open thermocouple event. If the positive input is
grounded, the
part’s output voltage will rail low during an open thermocouple event.
Behaviour is
undefined if grounding at the thermocouple tip.
Special Case: Non-isolated thermocouples
For good measurements, it is critical to only have one ground point. The vast
majority of
thermocouples have an electrically isolated sheath around the sensing end, so
there is no
electrical contact between the material being sensed and the thermocouple.
However
some types of thermocouples (such as those designed to respond very quickly)
may allow
an electrical connection between the material being measured and the
thermocouple. In
this case, if the material being measured is grounded, this connection counts
as the
thermocouple’s “ground” and the AD59x should not be grounded at one of the
input pins.
With long thermocouple connections the “ground” at the tip may be considerably
different than the ground at the AD59x. In these cases it is recommended to run
the
AD59x with a dual supply, so that if the “ground” at the thermocouple deviates
somewhat from the AD59x’s ground, it will still be in the common mode range of
the
part.

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