How do you measure ultra-low impedance in PCB power distribution networks?

Today's Discussion
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Measuring ultra-low impedance in a power distribution network (PDN) can be challenging, especially at high frequencies. The 2-port shunt-through method is commonly used for these measurements, but ground loops and cable losses can introduce errors, particularly when measuring impedances in the micro-ohm range.

I wanted to start a discussion on practical approaches for making accurate ultra-low impedance measurements.

2-port shunt-through impedance measurement setup using the Keysight E5061B VNA and the Picotest ground isolators J2102A-J2113A.
2-port shunt-through impedance measurement setup using the Keysight E5061B VNA and the Picotest ground isolators J2102A-J2113A.500×271 17.4 KB

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The 2-port shunt-through method measures impedance by analyzing the transmission coefficient (S21) through the device under test. However, the instrument grounds and test cables can create an additional current-return path, introducing errors in the measured impedance.

Minimizing this common-mode current is essential for accurate results, particularly when measuring ultra-low impedances.

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A few factors are important when setting up these measurements:

  • Minimize ground-loop errors: Use a suitable ground isolator to reduce common-mode current in the measurement setup.
  • Use high-quality cables and probes: Cable losses and inadequate shielding can affect measurement accuracy.
  • Verify the setup: Measure a known resistance to confirm the measurement setup, and use a four-wire Kelvin measurement to verify very low DC resistance.

The appropriate isolation method depends on the required impedance range and measurement frequency.

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For measurements as low as 20 µΩ, the test setup itself can significantly affect the results. The Picotest J2102A common-mode transformer supports measurements at and above 20 µΩ, with an operating frequency range starting at approximately 100 Hz. The J2113A semi-floating differential amplifier can be used for measurements at and above 1 mΩ, including down to DC.

Selecting the isolation method, cables, and probes based on the required measurement range helps reduce errors when characterizing a PDN.

Of course, the best approach depends on the impedance range, frequency of interest, and available test equipment.

For a detailed explanation of the measurement technique and ways to minimize ground-loop errors, check out this article:

Ultra-low Impedance Testing Using the 2-Port Shunt-Through Measurement Technique.