A 346 µm 2 VCO-Based, Reference-Free, Self-Timed Sensor Interface for Cubic-Millimeter Sensor Nodes in 28 nm CMOS

We present a 346 μm² reference-free, asynchronous VCO-based sensor interface circuit demonstrated in 28 nm LP bulk CMOS. This design is specifically for sensor node interfaces which do not have the power or volume available for the high accuracy current sources, voltage sources, or low jitter timing references needed for traditional ADCs. By using a straightforward VCO design, it achieves wide resolution, voltage scalability, and process portability while consuming only ~1/100th the area of prior approaches and avoiding costly reference circuitry. In the design measured for this paper, resolution can be scaled from 2.8 to 11.7 bits and VDD from 500 mV to 1.0 V. The design contains a single-point calibration scheme that works across temperature, voltage, and resolution settings. Minimum power consumption is 11.7 μW at 0.6 V VDD and minimum energy per conversion step is 41.2 fJ/b at 0.6 V VDD and 9.42 bits of effective resolution.