Implicit anti-aliasing is a remarkable property of continuous-time delta-sigma modulators 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. The authors 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. © 2004-2012 IEEE.