Influence of Excess Loop Delay on the STF of Continuous-Time Delta-Sigma Modulators

This paper analyzes the effect of signal transfer function (STF) peaking in continuous-time (CT) Delta-Sigma- Modulators (DSMs) arising from excess loop delay (ELD) and its compensation. Starting from a generic model, the origin of different contributors to STF peaking are revised, namely noise transfer function (NTF) peaking due to aggressive noise shaping, instability from uncompensated delay as well as forward loop filter zeros in cascade-of-integrators with distributed feedforward (CIFF) modulators. The shift of forward loop filter zeros as a result of ELD compensation is shown intuitively for a 3rd order example, while system-level simulations of various 2nd, 3rd and 4th order CT single-loop modulators confirm increased STF peaking in the above stated scenarios.