OM-USB-TEMP-AI User's Guide
Signal I/O Connections
15
The OM-USB-TEMP-AI makes two, three, and four-wire measurements of RTDs (100 _ platinum type) and
thermistors.
Use
Insta
Cal to select the sensor type and the wiring configuration. Once the resistance value is calculated, the
value is linearized in order to convert it to a temperature value. A 32-bit floating point value in either
temperature or resistance is returned by software.
RTD maximum resistance
Resistance values greater than 660 _ cannot be measured by the OM-USB-TEMP-AI in the RTD mode. The
660 _ resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the sum of
the RTD resistance and the lead resistances.
Thermistor maximum resistance
Resistance values greater than 180 k_ cannot be measured by the OM-USB-TEMP-AI in the thermistor mode.
The 180 k_ resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the
sum of the thermistor resistance and the lead resistance.
Two-wire configuration
The easiest way to connect an RTD sensor or thermistor to the OM-USB-TEMP-AI is with a two-wire
configuration, since it requires the fewest connections to the sensor. With this method, the two wires that
provide the RTD sensor with its excitation current also measure the voltage across the sensor.
Since RTDs exhibit a low nominal resistance, measurement accuracy can be affected due to the lead wire
resistance. For example, connecting lead wires that have a resistance of 1 _ (0.5 _ each lead) to a 100 _ platinum
RTD will result in a 1% measurement error.
With a two-wire configuration, you can connect either one sensor per channel pair, or two sensors per channel
pair.
Two-wire, single-sensor
A two-wire single-sensor measurement configuration is shown in Figure 4.
Figure 4. Two-wire, single RTD or thermistor sensor measurement configuration
When you select a two-wire single sensor configuration with
Insta
Cal, connections to T#H and T#L are made
internally.
Two-wire, two sensor
A two-wire, two-sensor measurement configuration is shown in Figure 5.
