A 0.4-V 988-nW Tiny Footprint Time-Domain Audio Feature Extraction ASIC for Keyword Spotting Using Injection-Locked Oscillators

This work presents an injection-locked oscillator (ILO)-based feature extraction (FEx) system. It combines voltage- and time-domain signal processing to implement a power-efficient programmable gain amplifier (PGA) and a small-footprint, high-selectivity ILO-based voltage-to-time converter bandpass filter (VTC-BPF) bank and rectifier. The VTC-BPF enables direct analog-to-time conversion, eliminating the need for a separate VTC stage, and allows compact full-wave rectification via an xor gate. Fabricated in 65-nm CMOS, the prototype occupies 0.15 mm, 3.5 smaller than recent designs, while consuming 988 nW, making it 9 more power-efficient than existing fully time-domain solution. To the best of our knowledge, this is the first silicon-verified analog FEx operating at 0.4 V, achieving 86.3% accuracy on a 12-class keyword spotting (KWS) task.