Digital micro ohm meters for measure low resistance with high accuracy
The most commonly measured electrical parameter is impedance which is measured as a function of frequency. Measuring impedances in the range of ohms with high accuracy is usually easier, however the real challenge comes when the measurement range moves towards micro ohm range. Various factors need to be considered to make high fidelity micro ohm impedance measurements.
Ultra-low impedance measurements are commonly performed to assess the quality of high-performance microprocessor power distribution network (PDN) designs. 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.
Conventionally, the PDN is assessed using a benchmark figure called as target impedance which is the ratio of maximum allowed change in the power supply voltage to the maximum transient current of the processor. Technology scaling drives the processor speed. 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.
Today, it is common to see 500 micro ohm power rails, but more recently the bleeding edge is below 100 micro ohm. Measuring such low impedance values is a big challenge to a PDN designer. Measuring 100uOhms is a significant challenge, even using the venerable 2-port shunt-thru measurement that has served as the staple of ultra-low impedance measurement. The dynamic range of the 100 uOhm, 2-port, measurement is 108dB. An impedance dip to 30uOhms reduces this to 118dB. This dynamic range presses the limits of the noise floor for even the best VNA. The addition of operating voltage noise, including ripple and load-induced transients increases the challenge. At these ultra-low impedance levels everything matters, from the quality of the instrument interconnects, to the quality of the ground loop isolator. The method to obtain high fidelity ultra-low impedance measurements up to 20 micro ohms are shown in https://www.picotest.com/measurements/2-portUltralowImpedance.html
References:
1. http://www.electronicdesign.com/boards/how-measure-ultra-low-impedances
Conventionally, the PDN is assessed using a benchmark figure called as target impedance which is the ratio of maximum allowed change in the power supply voltage to the maximum transient current of the processor. Technology scaling drives the processor speed. 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.
Today, it is common to see 500 micro ohm power rails, but more recently the bleeding edge is below 100 micro ohm. Measuring such low impedance values is a big challenge to a PDN designer. Measuring 100uOhms is a significant challenge, even using the venerable 2-port shunt-thru measurement that has served as the staple of ultra-low impedance measurement. The dynamic range of the 100 uOhm, 2-port, measurement is 108dB. An impedance dip to 30uOhms reduces this to 118dB. This dynamic range presses the limits of the noise floor for even the best VNA. The addition of operating voltage noise, including ripple and load-induced transients increases the challenge. At these ultra-low impedance levels everything matters, from the quality of the instrument interconnects, to the quality of the ground loop isolator. The method to obtain high fidelity ultra-low impedance measurements up to 20 micro ohms are shown in https://www.picotest.com/measurements/2-portUltralowImpedance.html
References:
1. http://www.electronicdesign.com/boards/how-measure-ultra-low-impedances
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