Investigations into current sensing test strategies

Mixed analogue-digital ASICs offer considerable advantages in terms of cost, flexibility and reliability. The majority of the approaches to mixed test require a careful partitioning of digital from analogue sections of a chip or are restricted to chips having particular structures. This satisfies controllability and observability requirements to permit the separate testing of each section. Such two stage testing has an inevitable cost penalty, however. Hence, there are significant benefits potentially available if unified techniques can be developed which permit the testing of both categories of circuit with the same equipment. Explorations have begun into such approaches to mixed testing. In carefully characterised test stimuli are injected at the inputs of a mixed circuit to excite a transient response from the circuit capable of propagation in both analogue and digital parts. Diagnosis is then made by analysis of this transient response. This paper explores two possible testing techniques, analogue supply current monitoring and the detection of analogue faults by monitoring digital circuitry