OM-USB-5203 User's Guide
Sensor Connections
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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.
RTD maximum resistance
Resistance v
alues greater than 660 Ω cannot be measured in 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
Resis
tance values greater than 180 kΩ cannot be measured in thermistor mode. The 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.
Two-wire configuration
The easiest way to connect an RTD sensor or thermistor to the OM-USB-5203 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.
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 InstaCal, connections to C#H and C#L are made
internally.
Two-wire, two sensor
A two-wire, two-sensor measurement configuration is shown in Figure 5.
Figure 5. Two-wire, two RTD or thermistor sensors measurement configuration
When you select a two-wire, two sensor configuration with InstaCal, connections to C#H (first sensor) and
C#H/C#L (second sensor) are made internally.
When configured for two-wire mode, both sensors must be connected to obtain proper measurements.
