Compensation Techniques for Frequency and Gain Temperature Drift in Open-Loop VCO-ADCs

This manuscript presents two approaches to improve the temperature effects on ring oscillators used in open-loop VCO-ADCs. The first approach is a circuit design optimization, which is based on an analysis of the ring oscillator against temperature, considering both gain and frequency deviations. It is shown that ring oscillators can be biased in regions with stable frequency or gain. The second technique is an oscillator stabilization circuitry for differential VCO-ADCs. We propose a common-mode feedback circuit that tracks the VCO frequencies to tune the common bias point. Therefore, intrinsic VCO variations are also compensated by this feedback. A proof of concept chip demonstrating both techniques has been implemented in 0.13μm. The chip measurements show a frequency and gain drift of 6% and ±0.25dB respectively between -40°C to 100°C. These measurements are comparable to the results achieved by digital temperature calibration without requiring an on-chip temperature sensor.