Power/Performance Bits: Nov. 11

Smaller DACs and ADCs; separating rare earths.


Smaller DACs and ADCs
Researchers at the National University of Singapore invented a novel class of Digital-to-Analog (DAC) and Analog-to-Digital Converters (ADC) that use a fully-digital architecture.

This digital architecture means design time for sensor interfaces can be reduced from months to hours with a fully-automated digital design methodology, the team said. It also has the benefits of low complexity, reducing the silicon area and manufacturing cost by at least 30 times compared to conventional designs.

“Our research transforms the traditionally analog and mostly-manual design of data converters into fully-automated digital design, reducing the silicon area by an order of magnitude and the design time by two orders of magnitude, allowing semiconductor companies to be cost-competitive while reaching markets faster,” said Massimo Alioto, an Associate Professor from the Department of Electrical and Computer Engineering at NUS. “Being digital, our sensor interfaces are effortlessly ported across manufacturing technologies and applications, and can be immersed in digital circuits to avoid the traditional effort required by their integration on the same silicon chip.”

The new data converters are able to gracefully degrade signal fidelity when its supply voltage or clock frequency experience wide fluctuations, which would be a concern with IoT sensors running off harvested energy. This would allow continual sensor signal monitoring under adverse without voltage regulation, instead of causing catastrophic resolution degradation when the supply voltage is below its minimum rated value.

As an example, a DAC designed for 1 V was demonstrated to correctly operate at half this voltage, while degrading its resolution by only 1 bit when the supply voltage is reduced by a substantial 0.3V.

“The capability of having graceful resolution degradation under voltage and frequency overscaling suppresses the need for complex circuit solutions that accurately regulate the supply voltage and the clock frequency being utilised by data converters. In other words, our data converters are simpler to design, and also simplify the system that they are employed in,” said Alioto.

The team plans to work on turning traditionally analog and design-intensive silicon sub-systems into digital standard cell-based designs, including sub-systems such as amplifiers, oscillators, voltage and current references.

Separating rare earths
Researchers at the University of Pennsylvania propose a method for separating rare earth elements, which could aid in recycling of material recovered from discarded electronics.

While not truly rare in terms of amount in the earth’s crust, rare earth elements are rarely found in concentrated deposits and are expensive and environmentally challenging to extract.

The standard approach for separating mixtures of elements is to perform a chemical reaction that causes one of the elements to change phase, like going from liquid to solid, which allows elements to be separated using physical methods like filtration. This type of approach is used to separate rare earth metals; mixtures are placed into a solution of an acid, and an organic compound and individual metal ions slowly move out of the acidic phase and into the organic phase at varying rates based on the metal’s chemical properties.

However, rare earth metals have very similar chemical properties such as solubility and how they react with other elements, making separating them a time and energy-consuming process that also generates a substantial amount of acid waste. “It works well when you do it 10,000 times, but each individual step is poorly efficient,” said Eric Schelter, Professor of Chemistry at the University of Pennsylvania.

Rare earth metals do show different paramagnetism: how attracted they are to magnetic fields. The team found that using both a magnetic field along with a decrease in temperature caused metal ions to crystallize at different rates.

The researchers were able to selectively separate heavy rare earths like terbium and ytterbium from lighter metals such as lanthanum and neodymium. It worked particularly well for certain combinations, like a 50/50 mixture of lanthanum and dysprosium that returned 99.7% dysprosium in one step, a 100% improvement compared to the same method but without using a magnet.

Next steps involve looking for ways to improve the reaction’s efficiency while studying how magnetic fields interact with these chemical solutions. Robert Higgins, a postdoctoral researcher at University of Pennsylvania, sees this study and other fundamental chemistry findings as an important first step towards making rare earth metal recycling more efficient and sustainable. “The faster we can find new ways of performing separations more efficiently, the faster we can improve some of the geopolitical and climate issues that are associated with rare earth mining and recycling.”

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