Modeling static analog behavior for determining mixed-signal test coverage using digital tools

Abstract

Determining test coverage for digital circuits is a commercially solved problem. This solution enables Design-forTest (DfT) which is justified by the increase in test coverage achieved. Applying digital tools to analog and mixed-signal circuits requires modeling analog circuit behavior. By representing single-wire analog behavior using a digital bus, the multi-level nature of analog signals can be “understood” by the digital tool. Using test coverage as a metric, mixed-signal DfT can be applied and justified. In this contribution, we consider the example of a Successive Approximation Register (SAR) ADC, comprising a digital controller and an analog feedback loop. This mixed-signal circuit is modeled such that a standard digital tool can be applied for determining test coverage and even generating test patterns. By adding DfT, i.e. additional controllability and observability, we can demonstrate improved test coverage. This improvement can be weighted against the additional silicon expense and ultimately justifies the DfT applied.