Resistance Measurement with Multifunction DAQ

Issue Details

I would like to calculate the impedance of a resistor. Can I use a Multifunction DAQ device to calculate the value of an unknown or variable resistance?

Solution

You can calculate the impedance measurement by using a Multifunction DAQ by doing an indirect measurement. 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.

1. Connect one end of a known resistor to an analog output or fixed voltage (it could be a power supply) as well as the positive lead of a differential analog input.
2. Connect the unknown resistor in series with the known resistance. 
3. Connect this junction with the negative lead of the first differential input along with the positive lead of a second differential analog input.
4. Wire the other end of the unknown resistor to both the negative side of the second analog input and the common reference for the AO (COM is the diagram above).

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) = Voltage at second input (Volts) / Current (Amps)

This is a general way to measure resistance in multiple ranges as long as the voltage drop in each AI is within the spec of the module. However, there are other modules tan can measure resistance directly, an universal like the NI-9218 or NI-9219. In those cases, you can perform 2-wire or 4-wire measurements, but the range will be limited.

Additional Information

For Multifunction I/O devices, make sure that the Analog Output can provide enough current required by the resistors according to the Current Drive specification of the device you are using. Otherwise, you can use external power or a different Analog Output device as a workaround.