Can the NI PXI-4072 and PXIe-4082 FlexDMM & LCR Meter Perform Impedance Measurements?

Updated Dec 12, 2018

Reported In

Hardware

  • PXI-4072
  • PXIe-4082

Driver

  • NI-DMM

Issue Details

I have a circuit that is comprised of inductive, capacitive, and resistive elements. Can I use the NI 4072 or the NI 4082 to perform an overall impedance measurement of this circuit?

Solution

No, the NI 4072 and the NI 4082 are not able to measure impedance. However, impedance can be calculated by measuring resistance, capacitance, and inductance independently, as the NI 4072 and NI 4082 were designed. For more information about the theory, see White Paper: Capacitance/Inductance Measurements with PXI DMMs

To ensure measurement accuracy, the load that is measured should be as close to purely inductive, capacitive, or resistive as possible. 

Resistance Measurements
The NI 4072 and the NI 4082 makes resistance measurements using a constant-current technique. A current is supplied to the resistance under test, and the DMM measures the resulting voltage as represented in the following figure.

Figure 1: Circuit Configuration of DMM Measuring a Resistor

Capacitance/Inductance Measurements
The NI 4072 and the NI 4082 uses a multi-tone AC current source as the excitation signal necessary to measure capacitance and inductance. The current waveform is a very stable, harmonically limited square wave which is applied to the device.

Figure 2: Circuit Configuration of DMM Measuring a Capacitance or Inductance
 
Note: Bold values denote vector quantities or complex numbers.

Additional Information

When the current and voltage are known, the NI 4072 and the NI 4082 calculates the capacitance or inductance using FFT peak analysis. If the residual series impedance (Zs) and the stray parallel admittance (Yp) introduce a significant error in the measurement, the NI 4072 and the NI 4082 can measure the magnitude of the error and reduce it using cable compensation techniques.

For further information regarding the test signals and model used to measure capacitive and inductive circuit elements, see the NI Digital Multimeters Help under the Devices>>NI 40x2>>DMM Measurements>>Capacitance/Inductance section.

For more information regarding cable compensation, see the Devices>>NI 40x2>>DMM Measurements>>Capacitance/Inductance>>OPEN/SHORT Compensation section.

Note: Change x to 7 or 8, to specify a NI 4072 or NI 4082 device respectively.