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Chapter 4
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 Platinum), Pt500 (500 Platinum), and Pt1000
(1000 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-Wire RTD Connections
The 4-wire configuration offers the best accuracy with long connection wires, compared to the
3- and 2-wire configurations. The 4-wire connection scheme eliminates errors due to lead wire
resistance (R
is automatically cancelled as long as the sense wires are connected.
Figure 14
3-Wire RTD Connections
The 3-wire configuration eliminates one wire from the 4-wire RTD connection. Lead wire
resistance (R
drop is essentially equal and opposite to the voltage drop across +Sense.
Figure 15
48
) and thermal heating. Wire impedance of up to 100 anywhere in the hookup
L
shows a 4-wire RTD connection.
Return
– Sense
4
R
R
L
L
Figure 14: 4-Wire RTD Connection
) errors in the return wire from –Sense may be introduced unless the voltage
L
shows a 3-wire RTD connection.
RTD Channel
425
A
Current
+Sense
3
2
1
R
R
L
L
RTD
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