Things to Know About Impedance Measurement

Impedance is the commonly measured parameter in electrical engineering. Most of the electrical designs require the measurement of impedance as a function of frequency. Measuring impedances in the range of ohms is easier with good accuracy. The real measurement challenge comes into play when we need to measure very low impedances, in the range of milli and micro ohms. Various factors need to be studied to perform high fidelity low impedance measurements. Ultra-low impedance measurements are commonly performed in high performance microprocessor power distribution network (PDN) designs. Clean power is very important aspect in power supply PDN design, and the lower the PDN impedance the cleaner the power supply. Lower impedance filters the power supply noise and provides cleaner power supply to the processor. Technology scaling is the followed method to increase the speed of a processor. Processor power supply voltages are scaled for this. At the same time, a processor power consumption increases due to the requirements of more functionalities. The combination of these two cases makes the required processor current to go up to hundreds of amperes. In case of server processors, the current per processor are in the range of kilo amperes. A benchmark tool to assess the PDN impedance required is estimated using target impedance which is given as the maximum allowed change in the processor supply voltage to the maximum transient current expected. In case of modern processors this is in the range of micro ohms. Measuring such low impedances are the corner stone of power integrity verification for these high end processors. Every microprocessor power delivery should be designed properly to make sure that the antiresonances are well suppressed. This will make sure that the processor power supplies are clean. In case of driver circuits the power supply generated noise might result in error in data transmission. Measurements are the most important part to make sure that the design is validated well in advance. This also helps to mitigate some of the electromagnetic compatibility (EMC) issues, like radiated electromagnetic interference (EMI) originating from the PDN.

Vector network analysers (VNAs) are the main component used to measure ultra-low impedances due to their superior sensitivities. VNA has sensitivities 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. Two 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 micro ohms are impacted by the noise floor of the VNA and the cable shield resistances. Choosing the right methods, we can measure ultra-low impedances.

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