A sigma-delta modulator includes a processing circuit, a quantizer, a truncater and a feedback circuit. The processing circuit receives an input signal and an analog information and generates an integrated signal by perform an integration upon a difference between the input signal and the analog information. The quantizer includes a successive approximation register (SAR) analog-to-digital converter (ADC) for receiving the integrated signal and generating a digital information according to the integrated signal. The truncater receives the digital information and generates a truncated information according to the digital information. The feedback circuit generates the analog information to the processing circuit according to the truncated information.