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OM-USB-TEMP-AI User's Guide
Power terminal (+5V)
The
output terminal is isolated (500 VDC) from the USB +5V.
+5V
Thermocouple connections
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-TEMP-AI makes fully differential thermocouple measurements without requiring ground-
referencing resistors. A 32-bit floating point value in either a voltage or temperature format is returned by
software. An open thermocouple detection (OTD) feature is available for each thermocouple input. This feature
automatically detects an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) on one or more sensor input channels
to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the OM-USB-TEMP-AI using a differential configuration, as shown in Figure 3.
The OM-USB-TEMP-AI
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, we recommend the use of 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 ±25 VDC when the
OM-USB-TEMP-AI is powered on, and ±40 VDC when the OM-USB-TEMP-AI is powered 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 OM-USB-TEMP-AI features two built-in current excitation
sources (±I1 and ±I2) for measuring resistive type sensors. Each current excitation terminal is dedicated to one
channel pair.
Figure 3. Typical thermocouple connection
pins are isolated from earth ground. You can connect thermocouple sensors to
GND
14
Signal I/O Connections
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