Accurate Impedance Measurements Below 1 mΩ

June 2, 2025 by Tim Ashworth

This blog post presents measurements of the real part of impedance of a low-resistance component measured with the MFIA Impedance analyzer using a low-impedance test fixture designed primarily to connect to DC-Link capacitors with spade connectors. This setup allows for accurate equivalent series resistance (ESR) measurements of a nominal 500 uΩ component with an accuracy better than 2%. It further shows the high repeatability when the component is removed and re-connected into the fixture.

Background

When measuring the ESR of low-impedance components such as DC-Link capacitors, it is required to measure the real part of the impedance at low frequencies below tens of kHz. DC-Link capacitors are continually being improved as the ESR is reduced through improved design and material choices. This development drive has pushed the ESR to levels well below 1 mΩ, and as such there is a clear requirement to be able to measure ESR at these low levels to an accurate and reproducible degree.
This blog post does not present measurements of a DC-Link capacitor; such measurements can be found in this blog post. Rather we present here measurements of a well-defined resistor with the same form factor as the connectors of a typical DC-Link capacitor in order to demonstrate the accuracy of the ESR measurement with the MFIA impedance analyzer.

The Reference Sample

To demonstrate the accuracy of the MFIA, we selected two 500 microohm components (part number 846-PSR100KTQFF0L50CT-ND). These resistors were soldered onto carriers with the same form factor as typical busbar connectors found on DC-Link capacitors (Figure 1 shows the fixture for low ESR measurements, wtih a short component inserted and the DUT lying next to the fixture). After mounting the components, the reference DUTs were then measured at DC by a micro-ohm meter (Keysight 34420A, recently calibrated). The two samples were measured as follows: Sample 1 has resistance of 527 uOhm,  sample 2 has resistance of 550 uOhm.  As a reference for zero measurement, we use a short piece with low inductance and the same form factor as the sample carrier.

Low-ESR Fixture connected to the MFIA impedance analyzer

Figure 1: Photo of the setup showing the low-impedance test fixture connected to the MFIA Impedance Analyzer. A short component with the same form factor as a busbar connector is inserted, and the DUT lies unconnected next to the fixture.

Short Compensation

When measuring at low-impedance, it is critical to have a well-defined short measurement as a baseline. To do this on the MFIA impedance analyzer (and the MFLI with MF-IA option) we use the compensation advisor of the LabOne instrument control software. With the short component inserted into the fixture, a short compensation was carried out over a frequency of 100 Hz to 100 kHz. Before removing the short component, we first measure the short component to establish a baseline. After this, the short component can be removed and the DUT can the inserted into the fixture for measurement.

Fixed Frequency Measurements

Using the plotter tool of LabOne, we measure the real part of the impedance (realZ) at 1 kHz. Figure 2 shows a value of 521 uOhm at 1 kHz for sample 1. This compares nicely with the measured value at DC of 527 uOhm. This value is -1.1% below the expected DC value.

Figure 2: Screenshot of the LabOne Plotter Module showing the real part of the impedane (realZ) to be 521 uOhm at 1 kHz. This compares well with the DC value of 527 uOhm.

Sweeper Measurements

We then measure the same same using the Sweeper module of LabOne. Figure 3 shows such a sweep from 100 Hz to 10 kHz. The cursor tools allows for direct measurement of the standard deviation between the cursors, in this case between 100 Hz and 10 kHz. The value of 534 uOhm also compared well with the expected DC value of resistance of 527 uOhm. The AC value is +1.3 % greater than expected; a very accurate measurement for an impedance analyzer.

Figure 3: Screenshot of the LabOne Sweeper Module, showing an average value of real impedance (realZ) to be 534 uOhm. This is in agreement with the DC measurement of resistance of 527 uOhm.

Measurements on the Second Sample

We repeated the same measurements on the second sample, sample 2 (expected DC resistance 550 uOhm). Figure 4 shows a screenshot of the fixed frequency measurement in the Plotter Module of LabOne at 1 kHz. The measurement of 549 uOhm is just -0.2% below the expected DC value of 550 uOhm.

Figure 4: Screenshot of LabOne Plotter Module showing the real part of impedance (realZ) at 1 kHz of sample 2. The measured value of 549 uOhm is just -0.2% short of the expected DC value of 550 uOhm.

Figure 5 shows the corresponding Sweeper measurement from 100 Hz to 10 kHz. The measured value averaged over this frequency range is 550 uOhm which matches the expected DC value of 550 uOhm to less than 1 uOhm.

Figure 5: Screenshot of LabOne Sweeper Module showing the realZ measurement of sample 2. The average value over the frequency range 100 kHz to 10 kHz is 550 uOhm, which matches the expected DC value of resistance of 550 uOhm.

Testing the Reproducibility 

To confirm the reproducibility of the ESR measured, sample 2 was removed and reseated ten times while the measurement ran. Figure 6 shows the resulting measurement; for each of the ten removals, the thick orange bar show the invalid measurement while no DUT was connected. The average ESR value is outlined with horizontal cursors and shows no obvious deviation after the ten remove/reseat cycles.

Figure 6: Screenshot of LabOne Plotter Module showing the ESR of sample 2 as the DUT is removed from the fixture and re-seated. The vertical orange bars represent the time when the DUT is not connected. The result show that even after 10 re-seats, the measured ESR value is stable at 553 uOhm.

Conclusion

This blog post presented measurements of the real part of impedance, also called the equivalent series resistance (ESR), of two low resistance components. The measured values were within 1.2 % of the resistance measured at DC by a third-party (and fully calibrated) micro-ohm meter. These results present measurements taken on two different MFIA instruments, and can be seen as typical performance of the MFIA at low impedance. When removing the DUT and re-seating it in the fixture, we show that the reproducibility is very high, with no obvious change in the measurement even after ten reseat cycles. This blog post confirms that the MFIA can bring high accuracy to low impedance measurements when the ESR is critical to your application.

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