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 D-Si, the accuracy of such frequency references is limited to about 1000 ppm (-55 degrees C to 125 degrees C, onepoint 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 D-Ox. Since the temperature dependence of D-Ox is much less than that of D-Si, 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-mu m CMOS. With ideal temperature compensation, the proposed frequency reference achieves an inaccuracy of 90 ppm (-45 degrees C to 85 degrees C, two-point trim). Even with 0.1 degrees 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.