Oscillation-based DFT for second-order OTA-C filters

We propose an easily implemented and low-cost design-for-testability scheme for OTA-C filters based on an oscillation-based test (OBT) methodology. The OBT method is a vectorless output test strategy easily applicable to built-in self-test. During test mode, the filter under test is converted into an oscillator by establishing the oscillation condition in its transfer function. The oscillator frequency can be measured using digital circuitry and deviations from the cut-off frequency indicate faulty behaviour of the filter. The proposed method is suitable for both catastrophic and parametric fault diagnosis and is effective in detecting single and multiple faults. The validity of the proposed method has been verified using comparison between faulty and fault-free simulation results of two-integrator loop, Tow-Thomas and KHN OTA-C filters. Simulation results for 2^nd order filters using a 0.25 mum CMOS technology show that the proposed oscillation-based test strategy has more than 96% fault coverage and, with a minimum number of extra components, requires a negligible area overhead.