We describe a computer-aided approach to automatic fault isolation in
active analog filters which enhances the design-for-test (DFT)
methodology proposed by Soma (1990). His primary concern was in
increased controllability 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-based diagnosis approach to implement a diagnostic
algorithm. CLP(R) is a logic programming language which combines
symbolic and numeric computation. The diagnostic algorithm uses
different DFT test modes and results 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 values of component
parameters. The diagnosis is performed incrementally, in each step
reducing the set of potential candidates for the detected fault.
Presented case studies show encouraging results in isolation of soft
faults of a given low pass biquad filter.