Design of self-checking fully differential circuits and boards

摘要

A design methodology for on-line testing analog linear fullyndifferential (FD) circuits is presented in this work. The test strategynis based on concurrently monitoring via an analog checker the commonnmode (Chi) at the inputs of all amplifiers, The totally self-checkingn(TSC) goal is achieved for linear FD implementations provided that thenchecker CM threshold is small enough with respect to the specifiednmargins of erroneous behavior in the circuit outputs. The designnmethodology is illustrated for a switched-capacitor biquadratic filternand the self-checking properties evaluated for a hard/soft-fault model.nA large checker threshold of 100 mV of CM is chosen since the filternimplementation does not minimize nonidealities (e.g., amplifier offsetsnor clock feedthrough) which result in significant CM components. Thencircuit outputs are accepted to deviate within a 10% band. With thenimplemented checker, the TSC goal is not achieved for some faults innnarrow regions of the frequency band. For the worst case, a hard faultnwhich results in a 31% deviation is undetected in only a narrow band ofnapproximately 310 Hz. The circuit can be made TSC with a checkernthreshold of 40 mV and an accepted output deviation of 15%. This is,nhowever, more demanding on the checker (which currently takes less thann3% of the total area and about 7.6% of the total power) and requires annimproved filter implementation to reduce CM components. Our solutionnconsists of relaxing a bit the TSC property of the functional block andnapplying a periodical off-line test to make the checker strongly codendisjoint (SCD). This is easy to implement since an off-line test is alsonrequired for the checker. The checker outputs a double-rail errornindication which ensures compatibility with digital checkers and makesnthe design of self-checking mixed signal circuits straightforward. Thencircuit-level mixed-signal approach is extended to the board level bynmeans of the IEEE Std. 1149.1 digital test bus