Fault detection method and apparatus

A method of testing an input circuit comprising the steps of:

• generating a test signal which is periodic and has a predetermined phase and a predetermined amplitude;
• summing the test signal and an input signal to provide a summed signal;
• processing the summed signal with the input circuit to provide an output value;
• processing the output value to provide an output signal corresponding to the input signal;
• generating an extracted test signal from the output value corresponding to the test signal;
• comparing the extracted test signal with a reference signal representing said test signal;
• generating an error signal in dependence upon the result of said comparing step;
• whereby testing of the input circuit can be obtained during operation of the input circuit with the input signal.