Circuit-Level Linearity Study of Ring Oscillators for VCO-Based ADCs

Dueto technology scaling negatively impacting performance of classic analog blocks, in recent years, architectures of analog-to-digital (ADC) based on voltage-controlled (VCO) gain an increase in popularity. In this paper we expand on the linearity problem of controlled ring oscillators that limits the accuracy of ADCs. We present, for two of the most used delay cells in ring oscillators, how frequency characteristics is more linear when related to ratio of current to voltage control, than when directly controlled in current or in voltage. Four circuit topologies are proposed to implement this control strategy and two of oscillators were fabricated in a 130 nm CMOS technology with maximum oscillation frequency deviation from best fit line of 1.5% for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0-1.5[V]$</tex> voltage control signal range and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10-150[\mu A]$</tex> current control signal range.