Efficient Identification of Unstable Loops in Large Linear Analog Integrated Circuits

Abstract

Stability analysis is one of the key challenges in analog circuit design. As feature sizes continue to shrink and the effect of parasitics becomes more dominant, we are forced to deal with stability analysis of increasingly complex multiloop structures with potentially hundreds of loops-a task that can no longer be dealt with using traditional methods. An automated stability checker tool that detects sources of potential ringing behavior within a reasonable turnaround time has thus been made necessary. Such a tool would not just help in debug but could also serve as a postlayout validation tool. We thus present an efficient loop finder algorithm to identify sources of ringing in large linear analog circuits. At the heart of our automated stability checker are two newly developed computationally efficient algorithms-the first to detect all poles within a given region of interest with a high degree of confidence and the second to extract second-order approximations of node impedance transfer functions given these pole locations. In this paper, we discuss these algorithms in detail, propose various optimization heuristics to further speed up the pole discovery algorithm, and then go on to develop a parallel implementation of both these underlying algorithms. It is demonstrated that these approaches together allow us to outperform the original loop finder algorithm based on direct eigen methods by two to four orders of magnitude and thus enable stability analysis of even larger extracted industrial designs than was previously possible while providing reasonable turnaround time.