Mixed-Signal Noise Analysis

With the rapid growth in the wireless communications industry and IBM's rise as a leader in SiGe technology, design tools for mixed-signal design are becoming more and more important to IBM designers and customers. One of the most pressing problems in mixed analog/digital design is the problem of substrate coupling. Substrate coupling occurs when currents injected into the silicon substrate from the digital portion of a chip travel through the substrate and cause voltage fluctuations in the highly sensitive analog circuits located elsewhere on the chip. These unwanted voltage fluctuations can wreak havoc on the operation of the analog circuits and cause them to underperform their specifications or to fail outright.

Substrate Coupling Analysis tools allow designers to assess the impact of digital circuit switching on the neighboring analog circuits. A substrate coupling tool consists of three main modules. First, one must characterize the operation of the digital circuits and obtain an estimate of the circuits injected into the substrate. Second, one must construct an adequate electrical model of the substrate itself (typically a network of resistors and capacitors) and simplify this circuit so that one is capable of simulating its behavior. Third, and finally, one must compute the voltage fluctuations at the analog circuit contacts that arise from the switching digital circuits. A new substrate coupling analysis tool will begin development shortly, based on IBM's previous work in the areas of power supply noise analysis, parasitic extraction, reduced-order modeling and circuit simulation.