Resistance Measurement with Multifunction DAQ

Updated Apr 20, 2018

Reported In

Hardware

  • CompactDAQ Chassis
  • C Series Voltage Input Module

Issue Details

I would like to calculate the impedance of a resistor. Can I use a Multifunction DAQ device (such as an NI-9205) to calculate the value of an unknown resistance?

Solution

Yes, you can calculate the impedance measurement by using a Multifunction DAQ. The basic wiring diagram to do so is shown in the image below. Please keep in mind this diagram can be used on different DAQ modules but would have to properly connect with the correct pin maps.
Wire the system as follows:
  1. Connect one end of a known resistance to an analog output as well as the positive end of a differential analog input.
  2. Attach the unknown resistor in series with the known resistance. 
  3. Connect this junction with the negative end of the differential input as well as the positive end of another differential analog input.
  4. Wire the other end of the unknown resistor to both the negative side of the second analog input (AI9) as well as to the analog input ground.

Calculate the unknown resistance:
  1. Measure the voltage at the first analog input (drop over the first resistor).
  2. Measure the voltage at the second analog input (drop over the second resistor).
  3. Calculate current by using the formula: Current (Amps) = Voltage at first input (Volts) / Known Resistance (Ohms)
  4. Calculate resistance by using the formula: Unknown Resistance (Ohms) = Current (Amps) / Voltage at second input (Volts)

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