A frequency-locked loop based on an oxide electrothermal filter in standard CMOS

The thermal diffusivity of silicon D
_Si
has been used to realize fully-CMOS frequency references. However, due to the temperature dependence of Dg, the accuracy of such frequency references is limited to about 1000 ppm (-55 °C to 125 °C, one-point trim) due to the inaccuracy of the on-chip temperature compensation circuitry. As an alternative, we propose a frequency reference based on the thermal diffusivity of silicon dioxide Dox. Since the temperature dependence of Dqx is much less than that of Dg, the resulting frequency reference will be much more stable over temperature. To investigate this idea, a thermal-diffusivity-based frequency-locked loop (FLL) was realized in 0.18-μm CMOS. With ideal temperature compensation, the proposed frequency reference achieves an inaccuracy of 90 ppm (-45 °C to 85 °C, two-point trim). Even with 0.1 °C inaccuracy, which can be achieved by BJT-based temperature sensors, 200 ppm can still be achieved. This demonstrates the feasibility of high-accuracy oxide-based frequency references in standard CMOS.