What is it about?

This paper presents a low-power supply-to-digital converter for bio-recording system where bio fuel cell provides both power and sensing data. It is achieved by supply controlled oscillator, supply-dependent activation buffers (SDAB), and encoders. A 22-nm prototype chip exhibits its feasibility with a power of 0.9-2.6 pW under 0.1-0.25 V.

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Why is it important?

In this rapidly developing information society, the demand for IoT devices is in creasing. There is a trade-off between timing accuracy, consuming power, and form factor in IoTs. To access timing information, crystal oscillators, DC-DC converters, on-chip stable clocks, or wireless communication which consumes large power are desired. However, there are some tiny IoT devices such as in-body sensors which are unable to mount batteries owing to safety requirements and area limitations. In such devices, energy harvesting, from the body, for example, is a promising alternative for wearable IoT devices. But in general, the power gained by such harvesting is still limited. Hence some sensing-and-recording-combined systems have limitations in power budgets and there is a demand for new recording techniques. One targeted application is a glucose-monitoring contact lens, which monitors blood glucose by using energy from lachrymal saccharide. One alternative method is to use supply-to-digital converters(SDC) whose digital code has sensing information. However, existing SDCs are insufficient in terms of power consumption, input supply voltage range, and required footprint to afford applications above, where their power budget is below hundreds of picowatts. This paper presents an energy-efficient, small-footprint, wide-input-range, supply to-digital converter for such IoT devices. The new circuit topology is introduced which uses supply-dependent-activation buffers with different V-thresholds in ad dition to a supply-dependent-frequency oscillator. Dynamic leakage suppression (DLS) logic cells, which operate by leak current, are used along with a smaller technology node to achieve lower power consumption. The measured results with the prototype chip using 22-nm CMOS successfully demonstrated its feasibility and achieved 0.9-2.6pW operation, 0.1-0.25V input range, and 0.00028mm2 footprint.

Perspectives

In this paper, we propose an AD converter for IoT devices utilized in special environments such as in vivo. Due to the system of the power source and the measured data being obtained from the same resource (biofuel cell), the circuit needs to be driven by the output of the measured data. Therefore, it can be called a supply-to-digital converter. It consists of several buffers with different thresholds in parallel, called SDAB. The circuit on/off leads to the detection of input voltage, and the fact that stable circuit voltage source is not required is an epoch-making feature. In addition, the circuit architecture is revolutionary in that the conversion can be performed while flexibly varying the driven area of the circuit in response to fluctuations in the input voltage of the measured data, which is different from conventional circuits. This is significant achievement since this transducer has increased the feasibility of a blood glucose monitoring system that operates on contact lenses.

Hiroaki Kitaike
Kyoto University

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This page is a summary of: Design of 0.9-2.6pW 0.1-0.25V 22nm 2-bit Supply-to-Digital Converter Using Always-Activated Supply-Controlled Oscillator and Supply-Dependent-Activation Buffers for Bio-Fuel-Cell-Powered-and-Sensed Time-Stamped Bio-Recording, January 2025, ACM (Association for Computing Machinery),
DOI: 10.1145/3658617.3698492.
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