On-chip test for Mixed-signal ASICs using two-mode comparators with bias-programmable reference voltages

A design-for-testability implementation for analogue functional blocks of mixed-signal ASICs is presented. For the analogue blocks direct access via an analogue input pin for the automated test equipment is required. To this end existing OpAmp or OTA stages of the respective analogue blocks are converted into simple clocked comparators. The resulting two-mode comparators are used to observe specific internal nodes of the functional block under test. Depending on the comparator mode, the observed test response evaluation can either be static and/or quasi-dynamic. At least two reference voltages are required each with two different levels determined by a hysteresis. All necessary reference voltages are generated on-chip in the central biasing cell of the ASIC. Due to this Design-for-Testability implementation, an on-chip test evaluation can be performed without the need to bring an analogue signal on- or off-chip. From simulation and measurement results of a feasibility study performed on a general purpose test circuit realised in 0.35 mum technology, the applicability was demonstrated. It showed that good fault coverages in the analogue functional blocks can be achieved. Estimations about the biasing programming indicated that this technique is in particular suitable for mixed-signal ASICs larger than 15 mm(2) with a typical total power consumption of more than 50 mW typical for high voltage applications