Recently, a design-for-test (DFT) methodology for active analog filters was
proposed with primary goal in increased controllability and observability. We
operationalize and extend the DFT methodology by using CLP(R) to model and
diagnose analog circuits. CLP(R) is a logic programming language with the
capability to solve systems of linear equations and inequalities. It is
well suited to model parameter tolerances and to diagnose soft faults,
i.e., deviations from nominal values. The diagnostic algorithm uses different
DFT test modes and voltage measurements at different frequences to compute
a set of suspected components. Ranking of suspected components is based on
a measure of (normalized) standard deviations from predicted mean values of
component parameters. Presented case studies on real circuit show encouraging
results in isolation of soft faults for a given low pass biquad filter.