Estimation of test metrics for the optimisation of analogue circuit testing

The estimation of test metrics such as defect level, test yield or yield loss is important in order to quantify the quality and cost of a test approach. For design-for-test purposes, this is important in order to select the best test measurements but this must be done at the design stage, before production test data is made available. In the analogue domain, previous works have considered the estimation of these metrics for the case of single faults, either catastrophic or parametric. The consideration of single parametric faults is sensitive for a production test technique if the design is robust. However, in the case that production test limits are tight, test escapes resulting from multiple parametric deviations may become important. In addition, aging mechanisms result in field failures that are often caused by multiple parametric deviations. In this paper, we will consider the estimation of analogue test metrics under the presence of multiple parametric deviations (or process deviations) and under the presence of faults. A statistical model of a circuit is used for setting test limits under process deviations as a trade-off between test metrics calculated at the design stage. This model is obtained from a Monte Carlo circuit simulation, assuming Gaussian probability density functions (PDFs) for the parameter and performance deviations. After setting the test limits considering process deviations, the test metrics are calculated under the presence of catastrophic and parametric single faults for different potential test measurements. We will illustrate the technique for the case of a fully differential operational amplifier, proving the validity in the case of this circuit of the Gaussian PDF.