An improved OBT strategy for untuned continuous-time analog filters

This paper presents an enhanced Oscillation Based Test (OBT) scheme that performs a relative comparison between two oscillation frequencies, eliminating the need of a fixed reference and allowing in this way the application of the OBT technique to non tuned circuits. Indeed, the fact of dealing with untuned filters prevents careless application of the traditional OBT. This is because, as a consequence of the process parameters dispersion, the use of one unique and fixed reference oscillation frequency for evaluating the test results is no longer practical. In order to overcome this problem a new approach based on the relative comparison of two oscillation frequencies is proposed. For evaluating the effectiveness of the idea, catastrophic fault models at both, device and circuit level, have been adopted. The results are encouraging, despite the circuit complexity introduced for adapting the classical OBT to untuned filters. Although a Gm-C filter was used for the proof of concept herein presented, it results clear that this strategy can be applied to any kind of continuous-time analog filters like MOSFET-C or R-AO-C filters, among others.