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Connecting RTD Inputs
Each DT9872 and DT9874 contains pluggable screw terminals for connecting RTD inputs.
Note: On the standard DT9874 instrument, channels 16 to 31 correspond to the RTD input
channels.
Internally, these signals are connected in differential mode.
the screw terminal blocks for RTD connections.
Figure 8: Screw Terminal Block Numbering for RTD Connections
Note: To make wiring easier, use the supplied screwdriver to attach your signals to the
screw terminal blocks. When you are finished, plug the screw terminal block into the screw
terminal header that corresponds to the channel to which you are wiring.
The DT9872 supplies each RTD channel with 425 μA of excitation current to prevent
self-heating. The resistance of the RTD circuit increases gradually, repeatably, and linearly
with temperature. As the resistance increases, the voltage drop across the RTD also increases.
The DT9872 reads this voltage drop and automatically converts the voltage to the appropriate
temperature based on the RTD type.
The DT9872 and DT9874 support Pt100 (100 Ω Platinum), Pt500 (500 Ω Platinum), and Pt1000
(1000 Ω Platinum) RTD types using Alpha coefficients of 0.00385 and 0.00392; you can mix and
match RTD types across RTD channels. Refer to the following web site for more information
on RTD types: http://www.omega.com.
To connect an RTD input, you can use a 4-wire, 3-wire, or 2-wire connection scheme, described
in the following subsections. For the best accuracy, use 4-wire RTD connections; this
connection scheme enables Kelvin sensing to minimize errors due to lead wire resistance.
4
3
2
Return
– Sense
+Sense
Current
Figure 8
shows the numbering of
1
Wiring Signals
45
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