Implicit anti-aliasing is a remarkable property of continuous-time delta-sigma modulators (CTDSMs) employing a time-invariant loop filter. It turns out that weak nonlinearity of the operational transconductance amplifier (OTA) used in the input integrator, in conjunction with the DAC pulse shape and OTA parasitics can greatly degrade the alias rejection of the modulator. We report and analyze this hitherto undiscovered mechanism. The theory is borne out by measurement results from a fourth-order single-bit filtering delta-sigma modulator designed in a 65-nm CMOS process. The alias-rejection of the modulator, which is expected to be about 160dB, is degraded to around 70dB by weak OTA nonlinearity and OTA parasitics.

In the second part of the talk, we extend this theory to Bandpass CTDSMs. We derive the expression for alias rejection when the OTA used in the first integrator is realized as a two stage feed-forward compensated structure. Simulation results confirm the theory.