Ultra-Low Power Analog Circuit Design for Resistance-to-Digital Converter and Voltage/Current References: Achieving High Precision and Energy Efficiency

Abstract

In the dynamic landscape of the semiconductor field, the application and significance of ultra-low power circuits have become paramount, particularly in the domains of the Internet of Things (IoT) and biomedical applications. The relentless pursuit of higher speeds in traditional semiconductor designs is giving way to a new paradigm where energy efficiency takes precedence. In IoT applications, where devices are often constrained by the absence of continuous power sources, ultra-low power circuits enable prolonged operation through energy harvesting or miniaturized batteries. This not only extends the device lifespan but also aligns with the energy-conscious demands of IoT ecosystems. In the realm of biomedical applications, the importance of ultra-low power circuits is underscored by the need for minimally invasive, long-term implantable devices. These circuits facilitate precise sensing, control, and communication within the human body, ensuring both longevity and safety. As the semiconductor industry navigates this transformative shift, the development and integration of ultra-low power circuits emerge as a crucial frontier, shaping the future of semiconductor technology with profound implications for IoT and biomedical advancements. In ultra-low power IoT applications, as well as biomedical implants, need low power consumption, low supply voltage, and high accuracy. Keeping all these constraints in mind, we worked on designing an ultra-low power Resistance-to-Digital Converter (RDC) with improved Figure-of-Merit (FoM) and low-temperature sensitivity for miniature low-powered sensing system. The proposed RDC system consumes only 162nW, with FoM as low as 0.845pJ/conversion cycle. The RDC can measure wide range of resistances measuring from 50kΩ to 1MΩ. Due to this, the RDC can be configured to use as different sensors like pressure sensors, temperature sensors, touch sensors, etc. For almost all IoT and biomedical systems, biasing circuits are essential, as these are in general always-on circuits. It makes sense to design voltage and current references in the ultra-low power domain. Consequently, we focused on designing a 2.3nW, sub-Bandgap voltage reference. It generates a reference voltage of 336mV without incorporating any resistors and operates for a high-temperature range of -40◦C to 150◦C and a supply range of 0.7V-4V. In the above temperature and supply ranges, the proposed circuit’s power consumption stays fairly linear, eliminating the exponential power consumption issue at such high temperatures. Designed in 65nm CMOS process, it achieves an impressive line sensitivity of 0.0066%/V for a supply range of 0.7V to 4V and a PSRR of 89dB at DC and 1V supply. As we all know, the voltage reference always preferred to be process independent, and the proposed circuit shows a ±3σ-inaccuracy of 4.295% without additional trimming circuits. It used gate-leakage-based resistance to save significant silicon area of high-value resistance. However, we need to design voltage and current references separately, and low-level current references require high-value resistances, and other CMOS current references suffer from high process variation. Consequently, we focused on designing ultra-low power trim free voltage/current reference without using resistance and amplifiers. The whole system works for as low as 37nW. The proposed voltage/current reference outputs a voltage and current of 820mV and 3.23nA, respectively. Without trimming, the process variation of the proposed voltage/current reference is 1.34%(σ/µ) / 1.75%(σ/µ). Designed in 90nm CMOS process, it achieved proper trim-free, low-power operation without using resistors, Which reduces area significantly. These attributes make it a desirable choice for various ultra-low power wearables and IoT applications.