Enhanced Metastability Characterization Based on AC Analysis

The common way of characterizing the metastability properties of a circuit is by its metastability resolution constant tau and the aperture window. This approach is based on a model that represents the storage cell as a pair of crosscoupled inverters, each of which is, in turn, modeled by a constant-gain amplifier with a first-order low-pass filter at the output. The former reflects the inverter's signal regeneration capability, while the latter approximates its dynamic behavior. In this simple model there is no natural way of expressing, e.g., the load dependence of tau, therefore each change of the element's load capacitance requires a full recalibration. In this paper we propose decomposing the inverter into its constituent transistors and using their small-signal equivalent circuits for modeling. Metastability characterization is now based on a Spice AC analysis which yields a higher-order dynamic model of the circuit. Once the relevant parameters are known for a given element, the load dependence of tau can be expressed analytically, thus elegantly avoiding recalibration. We compare our approach with the extended nose short simulation (ENSS) method from literature and show that the results deviate by no more than 1 -- 2%.