Such a low power consumption promises to extend the battery life of a variety of temperature monitoring devices for wearables, implantables, and environmental monitoring systems such as those for smart homes and IoT applications. The technology behind the new sensor could, say the researchers, also enable a new class of energy harvesting devices.
“Our vision is to make wearable devices that are so unobtrusive, so invisible that users are virtually unaware that they’re wearing their wearables, making them ‘unawearables,’” says Patrick Mercier, an electrical engineering professor at UC San Diego Jacobs School of Engineering and the senior author of a paper reporting the findings. “Our new near-zero-power technology could one day eliminate the need to ever change or recharge a battery.”
According to Hui Wang, an electrical engineering Ph.D. student in Mercier’s lab and the first author of the study, “We’re building systems that have such low power requirements that they could potentially run for years on just a tiny battery.”
In their pursuit of lower power miniaturized devices, the researchers focused on boosting the energy efficiency of individual transistors in the sensor integrated circuit – specifically those used as current sources for the conversion of temperature to a digital readout. To do this, they exploited transistor “leakage” current – the miniscule amount of current that flows in a transistor even when it is switched “off” – to build and power an ultra-pow-power current source.
They then used the current source in a new way to digitize temperature directly and save power. Typically the process is done indirectly, measuring a current-induced voltage across a resistor and then converting that voltage to digital using an analog-to-digital converter.
The researchers’ system instead uses two ultra-low-power current sources – one to charge a capacitor in a fixed amount of time regardless of temperature, while the other charges a capacitor at a rate that varies with temperature. As the temperature changes, a built-in digital feedback loop causes the system to adjust so that the temperature-dependent current source charges in the same amount of time as the fixed current source by reconnecting the former to a capacitor of a different size.
The size of that capacitor is directly proportional to the actual temperature. For example, if the temperature falls, the temperature-dependent current source will charge slower, and the feedback loop compensates by switching to a smaller capacitor, which dictates a particular digital readout.
The temperature sensor chip itself measures 0.15 x 0.15 mm2 and operates at temperatures from -20°C to 40°C. According to the researchers, its performance is comparable to that of the state of the art even at near-zero-power, although its response time is slower – i.e., approximately one temperature update per second – than existing sensors, but sufficient for devices that operate in environments where temperature do not fluctuate rapidly.
The team next plans to improve the accuracy of their sensor. In addition, they are optimizing its design for potential integration into commercial devices. A provisional patent is pending.