A new approach for the on-line testing of operational amplifiers embedded into analog and mixed-signal circuits is presented in this paper. A built-in detector is used for providing an on-line test signal, which is suitable for concurrent error detection in operational amplifiers. The fault detection strategy is based on monitoring via an analog checker the normal output/input and the on-line signal of operational amplifiers. In this context, the circuitry associated to a self-checking operational amplifier is presented. In order to evaluate the fault coverage and the performance degradation in the resulting operational amplifier, extensive simulations are performed considering catastrophic faults.