Mixed-signal circuits and boards for high safety applications

A design methodology for analogue on-line test is presented by means of a real circuit implementation. The test strategy is based on monitoring via a very small analogue checker the inputs of all operational amplifiers of a fully differential circuit. The self-checking properties of the functional circuit are evaluated for a hard/soft fault model. Since the analogue checker outputs a double-rail error indication, the compatibility with digital checkers is ensured and the design of self-checking mixed-signal circuits becomes very simple. The mixed-signal approach as extended to boards through the IEEE Std. 1149.1 digital test bus and a layout rule to avoid interconnect differential shorts