In this paper, a functional fault model for analog circuit diagnosis is proposed. A faulty circuit is modeled as a fault-free module in serial or in parallel with a fault module. To extract the faults, we adopt an iterative deconvolution technique to deconvolute the impulse response of the fault module from the faulty response. Such impulse response fault modeling makes the faults independent of input signals and system function. As the test results show, the input independent property significantly improves the diagnostic resolution, and the system function independent property allows single-module fault tables to be applied to multi-module diagnosis.