3.2 A 0.028mm² 32MHz RC Frequency Reference in 0.18μm CMOS with ±900ppm Inaccuracy from −40°C to 125°C and ±1600ppm Inaccuracy After Accelerated Aging

RC-based frequency references can achieve medium inaccuracy (~1000ppm) with small chip area [1–6], and so can potentially replace bulky crystal- or MEMS-based frequency references in cost-sensitive IoT applications. However, due to the large and nonlinear temperature dependence of on-chip resistors, achieving lower inaccuracy requires complex temperature compensation schemes, thus increasing chip area [1–4]. A further challenge is their long-term frequency drift. Recently, it has been shown that P-type polysilicon (P-poly) resistors, which are widely used because of their low temperature coefficients, are highly susceptible to aging [4, 6]. Although the resulting drift (~5000ppm) of a P-poly-based RC frequency reference can be mitigated by periodically calibrating it against a duty-cycled reference oscillator based on more stable diffusion or via-metal resistors [4], this comes at the expense of much larger chip area.