Analysis and Characterization of Variability in Subthreshold Source-Coupled Logic Circuits

This article explores the effect of device parameter variations on the performance of subthreshold source-coupled logic (STSCL) circuits. A test chip has been fabricated in a standard CMOS 90 nm technology to study the matching properties of STSCL circuits. Both process variations and device mismatch have been included in this study. The performed analysis shows that while the STSCL topology is very robust against global variations mainly thanks to the adoption of an on-chip bias generator circuit, special design techniques are required to compensate for the effect of device mismatch. Proper device sizing as well as creating intentional mismatch in the biasing condition of STSCL gates are two effective approaches that have been investigated to overcome the variation related issues. Both die-to-die (D2D) and within-die (WID) variations in the delay of STSCL gates have been characterized and validated through measurements. A comprehensive analysis of timing jitter in STSCL-based ring oscillators is also presented.