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These ranges are selectable per node; all channels set to resistance/RTD mode are set to the same range.
The node also protects each channel from overvoltages. Refer to the
information about overvoltage protection. Figure 15 shows the NI WSN-3226 input circuitry in
resistance mode.
There are four RTD channels on the NI WSN-3226. Each channel has three terminals:
•
EX terminal—Sources the excitation current. Connect the positive lead of the RTD to this terminal.
•
AI terminal—Measures the RTD resistance. Connect the sense lead of the RTD to this terminal.
•
COM terminal—Connect the negative lead of the RTD to this terminal.
Figure 16 shows an example of connecting a RTD to the NI WSN-3226.
The NI WSN-3226 sources different current levels on the EX terminal depending on the selected range.
For more information about the current source value for each range, refer to the
The NI WSN-3226 compensates for lead resistance errors. For the best measurement results when
measuring a 3-wire RTD, use equal length wires between the RTD and the EX, AI, and COM terminals.
You should also keep the leads as short as possible.
RTD Types
RTD types are specified by material composition, nominal resistance at 0 °C, and TCR (temperature
coefficient of resistance, or alpha). The NI WSN-3226 can directly measure the temperature of platinum
RTDs of either 100 Ω or 1,000 Ω nominal resistance. These RTDs are commonly referred to as Pt100 or
Pt1000 RTDs, respectively.
NI WSN-3226 User Guide and Specifications
Excitation
Current
EX
Buffer and Lead
AI
Compensation
COM
Figure 15. Input Circuitry for One Channel in Resistance Mode
RTD
Figure 16. Connecting a RTD
Specifications
MUX
ADC
NI WSN-3226
EX
AI
COM
NI WSN-3226
18
section for more
Specifications
section.
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