Impedance is the most commonly measured electrical parameter. They are measured as a function of frequency. It is the simplest measurement when the values are in ohms range. The real challenge comes when the measurement range is in milli ohm range. Various factors need to be taken into consideration to make high fidelity milli ohm impedance measurements.
Impedances in the range of micro ohm are classified as ultra-low impedances. High-performance microprocessor power distribution network (PDN) designs require the measurement of ultra-low impedance measurements. In these applications, the impedance must be measured over a wide range of frequencies starting from DC to hundreds of MHz. Clean power is an important aspect in PDN design. Lower PDN impedance is better for a good microprocessor power supply. Lower impedance filters the power supply noise and provides cleaner power supply to the processor.
A PDN design starts with the estimation of target impedance which is calculated as the ratio of maximum allowed change in the power supply voltage to the maximum transient current of the processor. Processor speed is governed by technology scaling. Constant field scaling provides the best speed for a transistor. In this, the device dimensions and the supply voltages are scaled down by a common factor. So, the processor power supply voltage is scaled down in every generation. At the same time, every newer generation processor consumes more power due to the additional functionalities added to them. This makes the required processor current to go up to hundreds of amperes. For modern server processors, the current per processor is in the range of kilo amperes. Due to these reasons, the modern processors’ target impedance is in the micro ohm range. Measuring such low impedance values is a big challenge to a PDN designer.
Ultra-low impedances are measured using vector network analysers (VNAs) due to their superior sensitivities. VNA has sensitivity in the order of microvolts and utilising two port shunt through measurement methods we can measure milli ohms and micro ohms. The two-port shunt through measurements are an adaptation of four wire kelvin measurements used for DC resistance measurements. 2-port shunt-through measurement comes with an inherent ground loop problem. Mitigating these are essential in measuring low impedances. Low impedance measurements in the range of milli ohms are impacted by the noise floor of the VNA, ground loop, and the cable shield resistances. Choosing the right methods, we can measure impedances as low as 20 micro ohms. The 2- port shunt through measurement has an inherent ground loop, which can be remedied by a high-quality common mode choke or a high-quality differential amplifier. The cable resistances play an important role in these measurements. Another important factor that affects this measurement is the noise floor and dynamic range of the VNA.