Input Referred Noise of VCO-Based Comparators

The input referred noise (IRN) of a VCO-based comparator is analysed in this brief. The analysis reveals the dependency of the IRN on four major design parameters quantitatively: the IRN is inversely proportional to the threshold set by the phase detector and to the transistor width, approximately inversely proportional to the square root of the number of inverters in each VCO ring, and parabolically depends on the overdrive voltage of the input transistors. Such dependency is validated by simulation and can serve as a design guideline when designing efficient high resolution VCO-based comparators.