Abstract: The disclosed invention is a sigma-delta modulator based ADC and a dithering method thereof. An analog dither circuit of an input stage in a quantizer, which is configured to perform dithering, receives an integrator output signal from an output of an integrator and a plurality of digital dither signals from digital dither generator, and employs these signals to produce multi-level analog dither signals via the operation of a plurality of transistors as switches coupled with a plurality of resistors coupled in series. The multi-level analog dither signals can prevent, reduce, or eliminate limit cycle in the sigma-delta modulator based ADC. Furthermore, disclosed invention does not require fixed load capacitance, resulting in no extra power consumption and solve the problem as the kickback noise variation.