Ultra-Ultra-Low Impedance (4 micro-ohm) Measurements
Published by Signal Integrity Journal, July 26, 2022
Companies like Intel, Nvidia, AMD, Qualcomm, and Broadcom are putting more cores on a single die. Ampere is an excellent example where they have designed a cloud processor solution with 128 cores on a single die.
It is well known that to meet voltage ripple specifications, designers cannot control the current on these processing solutions. However, we can manage the impedance in our power distribution network (PDN) designs. The challenge is that these processor solutions require significant amounts of current, upwards of 1000A, so PDN targets continue to decrease to even lower levels. That means the necessary impedance targets are well below 100 uOhm and even below 32 uOhm.
Steve Sandler demonstrated the solution to this problem in a 2018 EDICON University session  on how to measure below 100 uOhm, demonstrating 22 uOhms in that session as well. Some key takeaways are minimizing the shield resistance in the cables used in the measurement setup, increasing the CMRR, and controlling the ground loop.
So, what if our impedance target is below 22 uOhm, or even below 10 uOhm? How do we measure that?
Before measuring below 10 uOhm, we first need a known device under test (DUT) to check our measurement setup after calibration.
Ok, so let’s get a 10 uOhm DUT to check our measurement setup. On second thought, where do I find a 10 uOhm DUT? The unfortunate answer is that we need to make it ourselves.
Figure 1 depicts the DUT that was created. However, when it was first made, it was not known precisely what the impedance of the copper on this PCB mount was. So how do we check this?
Figure 1. Depiction of 4.5 uOhm known DUT
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