Measurement Computing CIO-EXP-GP - Manual (Page 28)

6.8 Connecting RTDs To Screw Terminals
The connections made to the screw terminal depend on the type of RTD you are using. The inputs of the
CIO-EXP-GP are designed to provide the excitation and signal conditioning required for RTDs. An RTD
can have two, three, or four wires which you must connect to the CIO-EXP-GP. This section shows the
three types of RTD connections and describes how to connect them to the input channels.
6.8.1 Two-Wire RTD Hookup
A two wire RTD has two leads, one to each side of the temperature sensitive resistor. The excitation
current is connected directly to the leads at the CIO-EXP-GP screw terminals.
EXCITATION CURRENT (+)
SENSE HIGH (+)
SENSE LOW (-)
EXCITATION CURRENT (-)
2 WIRE
RTD
Shorting Wires
Between Terminals
CIO-EXP-GP BOARD
A two wire RTD is less accurate than the 4 wire
type, and so is not the first choice for the best
measurements. The reason for the inaccuracy is
that there is a slight resistance associated with the
excitation current flowing in the sense leads and
this resistance is added to the RTD’s resistance.
The inaccuracy is determined by the wire gauge
and length. However, as a general rule, the
difference in accuracy between the 2- and 4-wire
RTDs is often less than 0.1% of full scale.
Figure 6-3. Two-wire RTD Hookup
6.8.2 Three-Wire RTD Hookup
A three wire RTD has three leads, one for each side of the temperature sensitive resistor and one for the
excitation current.
EXCITATION CURRENT (+)
SENSE HIGH (+)
SENSE LOW (-)
EXCITATION CURRENT (-)
3 WIRE
RTD
Note: EXP-GP uses non-standard
3 wire RTD hookup
The current return and sense signals of one side
are shared. In the case of the EXP-GP the shared
signals are unconventional. The CIO-EXP-GP is
a true clone of the original EXP-GP, and shares
the unconventional circuit configuration, which
is corrected on the CIO-EXP-RTD. The
unconventional configuration does not affect the
quality of the measurement but, if you are
familiar with RTDs and use a standard
connection, please be mindful of this difference.
Figure 6-4. Three-Wire RTD Hookup
6.8.3 Four-Wire RTD Hookup
A four wire RTD has four leads. One to each side of the temperature sensitive resistor and an excitation
current source and its return.
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