Measurement Computing WLS-TEMP - Manual (Page 14)

WLS-TEMP Specifications
14
Wiring configuration
Connect the thermocouple to the WLS-TEMP using a differential configuration, as shown in Figure 3.
Figure 3. Typical thermocouple connection
Connect thermocouples to the WLS-TEMP so that they float with respect to ground. The GND pins are isolated
from earth ground, so you can connect thermocouple sensors to voltages referenced to earth ground as long as
you maintain the isolation between the GND pins and earth ground.
When you attach thermocouples 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
device is powered on, and ±40 VDC when the device 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 need an excitation current to produce a voltage drop that can be
measured differentially across the sensor. The WLS-TEMP features four built-in current excitation sources (±I1
to ±I4) for measuring resistive type sensors. Each current excitation source is dedicated to one channel pair.
The WLS-TEMP makes two, three, and four-wire measurements of RTDs (100 Ω platinum type) and
thermistors.
Use InstaCal 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.
 In RTD mode, the WLS-TEMP cannot measure resistance values greater than 660 Ω. This 660 Ω resistance
limit includes the total resistance across the current excitation (±Ix) pins, which is the sum of the RTD
resistance and the lead resistances.
 In thermistor mode, the WLS-TEMP cannot measure resistance values greater than 180 kΩ. This 180 kΩ
resistance limit includes the total resistance across the current excitation (±Ix) pins, which is the sum of the
thermistor resistance and the lead resistance.