FACTS Engineering Direct Logic 405, Manual

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INSTALLATION AND WIRING 2.6
GROUNDED VS UNGROUNDED THERMOCOUPLES
Grounded Thermocouple Assembly
A grounded thermocouple provides better response time than an ungrounded thermocouple because the
tip of the thermocouple junction is in direct contact with the protective case.
Ungrounded Thermocouple Assembly
An ungrounded thermocouple is electrically isolated from the protective case. If the case is electrically
grounded (for example, by the conduit) it provides a low impedance path for electrical noise to travel. In
this configuration, the ungrounded thermocouple provides a more stable and accurate measurement in
an electrically noisy area.
Exposed Grounded Thermocouple
In some cases, the thermocouple does not have a protective case and is connected directly to a device
at a slightly different potential voltage. In this configuration, grounding the thermocouple will assure that
the thermocouple remains within the ±5V common mode operating range of the module.
Because a thermocouple is essentially just a wire, an exposed grounded thermocouple also provides a
low impedance path for electrical noise. This noise is first filtered by a single ended analog filter ahead
of the precision analog to digital converter. Following the converter, the noise is next filtered by a Sinc3
(or (Sinx/x)3) response digital filter which has >100dB of 50 Hz and 60 Hz rejection.