Analyses of Static and Dynamic Random Offset Voltages in Dynamic Comparators

When mismatches are present in a dynamic comparator, due to internal positive feedback and transient response, it is always challenging to analytically predict the input-referred random offset voltages since the operating points of transistors are time varying. In this paper, a novel balanced method is proposed to facilitate the evaluation of operating points of transistors in a dynamic comparator. Thus, it becomes possible to obtain an explicit expression for offset voltage in dynamic comparators. We include two types of mismatches in the model: 1) static offset voltages from the mismatch in $muhbox{C}_{rm ox}$ and threshold voltage ${rm V}_{rm th}$ and 2) dynamic offset voltage due to the mismatch in the parasitic capacitances. From the analytical models, designers can obtain an intuition about the main contributors to offset and also fully explore the tradeoffs in dynamic comparator design, such as offset voltage, area and speed. To validate the balanced method, two topologies of dynamic comparator implemented in 0.25- $mu{hbox {m}}$ and 40-nm CMOS technology are applied as examples. Input-referred offset voltages are first derived analytically based on SPICE Level 1 model, whose values are compared with more accurate Monte Carlo transient simulations using a sophisticated BSIM3 model. A good agreement between those two verifies the effectiveness of the balanced method. To illustrate its potential, the explicit expressions of offset voltage were applied to guide the optimization of “Lewis-Gray” structure. Compared to the original design, the input offset voltage was easily reduced by 41% after the optimization while maintaining the same silicon area.