An embodiment provides a continuous-time delta-sigma modulator for analog-to-digital conversion. The modulator includes a signal path generating including a ring voltage controlled oscillator driven by an analog input signal. The signal path produces digital values by sampling the ring voltage controlled oscillator. A calibration circuit measures nonlinear distortion coefficients in a replica of the signal path. A nonlinearity corrector corrects the digital values based upon determined nonlinear distortion coefficients. Preferred embodiment ADC ΔΣ modulators do not require any analog integrators, feedback DACs, comparators, or reference voltages, and do not require a low jitter clock.