This paper presents a technique that enables online testing of sensors through the superposition of the test stimulus onto the measurand. Perturbations due to the surrounding environment can very often introduce fluctuations in the test output that is a major concern for this type of sensor testing. This paper proposes a novel approach to tackle this problem by encoding the test stimulus using a pseudo-random sequence. The trade-off between the level of rejection of a perturbation and the overall test time is studied. In the case of the MEMS accelerometer considered in this paper, it is demonstrated that the rejection is more than 20dB for a test time of 2.55s. Furthermore, excessively strong perturbations can be monitored so that the test status is updated only if the accuracy of the test signal permits so. It is shown that the proposed solution is compatible with the technique that consists of injecting the stimulus through the sensor bias for the case study MEMS accelerometer.