Measurement Computing USB-5203 - Manual (Page 18)

USB-5203 User's Guide Functional Details
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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-5203 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).
External components
The USB-5203 has the following external components, as shown in Figure 12.
1 Screw terminal pins 1 to 26 4 Data logging button
2 Screw terminal pins 27 to 52 5 CompactFlash ® memory card slot (with card)
3 LED 6 USB connector
Figure 12. External component locations
Screw terminals
Use the screw terminals for connecting temperature sensors and digital I/O lines. These terminals also provide
ground and power output connections. Refer to the "Sensor Connections" chapter for screw terminal
descriptions.