USB-TEMP User's Guide
Sensor Connections
3-4
The USB-TEMP
GND
pins are isolated from earth ground, so connecting thermocouple sensors to
voltages referenced to earth ground is permissible as long as the isolation between the GND pins (9, 19,
28, 38) 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 USB-TEMP is powered on, and ±40 VDC when the USB-TEMP 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 USB-TEMP features 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 USB-TEMP makes two, three, and four-wire measurements of RTD’s (100
Ω
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
Ω
cannot be measured by the USB-TEMP in the RTD mode. The
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.
Thermistor maximum resistance
Resistance values greater than 180k ohms cannot be measured by the USB-TEMP in the thermistor mode.
The 180 k
Ω
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 USB-TEMP 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
Ω
(0.5
Ω
each lead) to a 100
Ω
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.
