A 16 MHz CMOS RC Frequency Reference With ±90 ppm Inaccuracy From −45 °C to 85 °C

This article presents a 16-MHz RC frequency reference implemented in a standard 180-nm CMOS process. It consists of a frequency-locked loop (FLL) in which the output frequency of a digitally controlled oscillator (DCO) is locked to the frequency-phase characteristic of a Wien bridge RC filter. Since it is made from on-chip resistors and capacitors, the filter’s characteristic is temperature dependent. To compensate for this, the control signal of the DCO is derived by digitizing the filter’s output phase and combining it with the digital output of a Wheatstone bridge temperature sensor. After a two-point trim, this digital temperature compensation scheme achieves an inaccuracy of ±90 ppm from −45 °C to 85 °C. The frequency reference draws 220 $\mu \text{A}$ from a 1.8-V supply, with a supply sensitivity of 0.12%/V and a 320-ppb Allan Deviation floor for a 10-s stride.