USB-TEMP User's Guide
Functional Details
4-2
To reduce the error, reduce the length of the thermocouple to lower its resistance, or lower the AWG of
the wire by using a wire with a larger diameter. With open-thermocouple detection disabled, 30 nA (max)
of input leakage current is injected into the thermocouple.
RTD and thermistor measurements
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 measures the sensor resistance by
forcing a known excitation current through the sensor and then measuring (differentially) the voltage
across the sensor to determine its resistance.
After the voltage measurement is made, the resistance of the RTD is calculated using Ohms law – the
sensor resistance is calculated by dividing the measured voltage by the current excitation level (±
Ix
)
source. The value of the ±
Ix
source is stored in local memory.
Once the resistance value is calculated, the value is linearized in order to convert it to a temperature
value. The measurement is returned by software as a 32-bit floating point value in a voltage, resistance or
temperature format.
Data linearization
An on-board microcontroller automatically performs linearization on RTD and thermistor measurements.
!
RTD measurements are linearized using a Callendar-Van Dusen coefficients algorithm (you select
DIN, SAMA, or ITS-90).
!
Thermistor measurements are linearized using a Steinhart-Hart linearization algorithm (you supply
the coefficients from the sensor manufacturer's data sheet).
USB connector
The USB connector pr5V power and communication. No external power supply is required.
LED
The LED indicates the communication status of the USB-TEMP. It uses up to 5 mA of current. Table 4-2
defines the function of the USB-TEMP LED.
Table 4-2. LED Illumination
LED
Illumination
Indication
Steady green
The USB-TEMP is connected to a computer or external USB hub.
Pulsing green
Data is being transferred.
Upon connection, the LED should flash three times and then remain lit (indicates a successful
installation).
Power
The two
+5V
terminals are isolated (500VDC) from the USB +5V.
Caution!
Each +5V terminal is an output. Do not connect to an external power supply or you may
damage the USB-TEMP and possibly the computer.
