OM-USB-5203 User's Guide
Sensor Connections
11
Thermocouple connections
Connect thermocouples to the OM-USB-5203 such that they are floating with respect to GND. A thermocouple
consists of two dissimilar metals that are joined together at one end. When the junction of the metals is heated
or cooled, a voltage is produced that correlates to temperature.
The OM-USB-5203 makes fully-differential thermocouple measurements without the need of ground-
referencing resistors. A 32-bit floating point value in either a voltage or temperature format is returned by
software. An open thermocouple detection feature is available for each analog input which automatically detects
an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) and one or more sensor input channels
to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the OM-USB-5203 using a differential configuration, as shown in Figure 3.
Figure 3. Typical thermocouple connection
Connect thermocouples to the OM-USB-5203 such that they are floating with respect to GND. The OM-USB-
5203ground pins are isolated from earth ground, so you can connect thermocouple sensors to voltages
referenced to earth ground as long as the isolation between the GND pins and earth ground is maintained.
When thermocouples are attached to conductive surfaces, the voltage differential between multiple
thermocouples must remain within ±1.4 V. For best results, use insulated or ungrounded thermocouples when
possible.
Maximum input voltage between analog input and ground
The absolute maximum input voltage between an analog input and the isolated GND pins is ±24 VDC when the
OM-USB-5203 is powered on or off.
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to minimize
the error introduced by thermal EMFs.
RTD and thermistor connections
A resistance temperature detector (RTD) measures temperature by correlating the resistance of the RTD
element with temperature. A thermistor is a thermally-sensitive resistor that is similar to an RTD in that its
resistance changes with temperature – thermistors show a large change in resistance that is proportional to a
small change in temperature. The main difference between RTD and thermistor measurements is the method
used to linearize the sensor data.
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. The device has four built-in current excitation sources (±I1 to ±I4)
for measuring resistive type sensors. Each current excitation terminal is dedicated to one channel pair.
The OM-USB-5203 makes two, three, and four-wire measurements of RTDs (100
Ω platinum type) and
thermistors.
