Virtually all traditional signal quality analysis procedures operate on data in the frequency domain, and therefore require steady-state signals. However, real-life signals are, typically, not steady-state. In order to be able to measure the conversion quality of a sigma-delta modulator under non-steady-state excitation, in this chapter a time domain SINAD measurement method is introduced. The operation of this time domain method is first verified with steady-state signal measurements, and finally applied to non-steady-state signals. The simulations on various non-steady-state signals indicate that no significant conversion errors are generated, and that the traditional signal quality analysis procedures are adequate.