ENHANCING DESIGN-FOR-TEST FOR ACTIVE ANALOG FILTERS BY USING CLP(R)

We describe a computer-aided approach to automatic fault isolation in active analog filters which enhances the design-for-test (DFT) methodo logy proposed by Soma (1990). His primary concern was in increased con trollability and observability while the fault isolation procedure was sketched only in general terms. We operationalize and extend the DFT methodology by using CLP(R) to model analog circuits and by a model-ba sed diagnosis approach to implement a diagnostic algorithm. CLP(R) is a logic programming language which combines symbolic and numeric compu tation. The diagnostic algorithm uses different DFT test modes and res ults of voltage measurements for different frequencies and computes a set of suspected components. Ranking of suspected components is based on a measure of (normalized) standard deviations from predicted mean v alues of component parameters. The diagnosis is performed incrementall y, in each step reducing the set of potential candidates for the detec ted fault. Case studies show encouraging results in isolation of soft faults of a given low-pass biquad filter.