Impedance is an essential parameter measured in electrical engineering, and many electrical designs require impedance measurement as a function of frequency. While measuring impedances in the range of ohms is relatively straightforward with good accuracy, the real challenge arises when measuring very low impedances in the range of milli and micro ohms. Performing high-fidelity low-impedance measurements requires careful consideration of various factors.
Ultra-low impedance measurements are commonly performed in high-performance microprocessor power distribution network (PDN) designs. Clean power is critical in power supply PDN design, and lower PDN impedance leads to cleaner power supply. Lower impedance filters the power supply noise and provides cleaner power to the processor. However, the combination of technology scaling and the requirement for more functionalities has led to a significant increase in processor power consumption, with required processor currents ranging from hundreds of amperes to kilo amperes in server processors.
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 the case of modern processors, this is in the range of micro ohms. Measuring such low impedances is critical to power integrity verification for these high-end processors. Proper design of every microprocessor power delivery ensures that the antiresonances are well suppressed, leading to clean processor power supplies. In the case of driver circuits, power supply-generated noise may result in errors in data transmission. Measuring low impedances is crucial to ensure that the design is validated well in advance and mitigate some of the electromagnetic compatibility (EMC) issues, such as radiated electromagnetic interference (EMI) originating from the PDN.
Vector network analyzers (VNAs) are the primary components used to measure ultra-low impedances due to their superior sensitivities. VNAs have sensitivities in the order of microvolts, and by utilizing two-port shunt-through measurement methods, milli-ohms and micro-ohms can be measured. Two-port shunt-through measurements are adaptations of four-wire Kelvin measurements used for DC resistance measurements. However, two-port shunt-through measurements come with an inherent ground loop problem that must be mitigated to ensure accurate low-impedance measurements.
Low-impedance measurements in the range of micro-ohms are also impacted by the noise floor of the VNA and the cable shield resistances. Choosing the right methods can help measure ultra-low impedances with the necessary accuracy. Therefore, ultra-low impedance measurement is a critical aspect of high-performance microprocessor power distribution network (PDN) designs and other electrical engineering applications. Proper measurement and analysis of low impedances help ensure that the design is validated well in advance and mitigate some of the electromagnetic compatibility (EMC) issues.
Testing impedance
Impedance is a fundamental electrical parameter that is essential for the proper functioning of many electronic systems. It is measured as a function of frequency, and it is the simplest measurement when the values are in the ohms range. However, measuring impedance in the milli and micro ohms range is a real challenge that requires careful consideration of various factors to achieve high fidelity measurements.
One of the applications that require ultra-low impedance measurements is high-performance microprocessor power distribution network (PDN) designs. In these designs, the impedance must be measured over a wide range of frequencies, starting from DC to hundreds of MHz. Clean power is an important aspect of PDN design, and lower PDN impedance provides a cleaner power supply to the processor by filtering the power supply noise. A PDN design starts with the estimation of target impedance, which is calculated as the ratio of the maximum allowed change in the power supply voltage to the maximum transient current of the processor. For modern processors, the target impedance is in the micro ohm range due to the constant field scaling that provides the best speed for a transistor, and every newer generation processor consumes more power due to the additional functionalities added to them.
Measuring such low impedance values is a big challenge for a PDN designer, and vector network analyzers (VNAs) are commonly used for this purpose due to their superior sensitivities. VNAs have sensitivity in the order of microvolts, and by utilizing two-port shunt through measurement methods, we can measure milli ohms and micro ohms. The two-port shunt through measurements are an adaptation of the four-wire Kelvin measurements used for DC resistance measurements, and it comes with an inherent ground loop problem that needs to be mitigated. Low impedance measurements in the milli ohms range are impacted by the noise floor of the VNA, ground loop, and cable shield resistances. Choosing the right methods, we can measure impedances as low as 20 micro ohms.
The 2-port shunt through measurement can be remedied by using a high-quality common-mode choke or a high-quality differential amplifier to mitigate the ground loop problem. Additionally, the cable resistances play an important role in these measurements, and it is important to choose the right cables with low resistance. Another important factor that affects this measurement is the noise floor and dynamic range of the VNA. Therefore, it is essential to choose the right VNA with high dynamic range and low noise floor to achieve accurate measurements. In conclusion, impedance measurement is an important aspect of electrical engineering, and by considering the various factors involved, accurate measurements can be achieved even in the ultra-low impedance range.
