Robust DfT Techniques for Built-in Fault Detection in Operational Amplifiers with High Coverage

An operational amplifier (op amp) is a fundamental block used extensively both as a stand-alone device and as a major block embedded in an SoC. To fully characterize an op amp, sophisticated analog and digital testing is required, which is expensive. Fault detection techniques have proved to reduce package cost and test cost by detecting faulty devices early in the test sequence. In this paper, a simple Design for Test (DfT) technique called intentional offset injection is proposed to detect various faults in the op amp. As our proposed method is completely digital, pure digital circuitry can be used, thereby avoiding expensive analog testing. The op amp can be tested with the proposed fault detection method during wafer probe test right after the continuity tests and the faulty devices could be discarded, thereby circumventing time-consuming analog testing. Additionally, our detection scheme can be used for power-on selftest after deployment and for online health monitoring during normal operation. We show that the proposed detection method can provide high fault coverage of 95% with modest area requirements. In this work, we also introduce a detector called digital window comparator which is used to monitor faults in the biasing circuit as well as in the Widlar current reference providing increased fault coverage.