It is usually difficult to design a high performance Sigma-Delta (ΣΔ) modulator due to system noises.In this paper,a disturbance observer (DOB) is utilized to estimate the system noises and eliminate their effects on ΣΔ modulators.The applied DOB is introduced with a Bode’s ideal cut-off (BICO) filter used for the Q-filter.The proposed DOB with the BICO filter used in ΣΔ modulators can achieve better noise-shaping ability,resulting from the less phase loss of the BICO filter.Finally,the simulation results show that the proposed BICO filter scheme is a useful additional tool for improving the performance of ΣΔ modulators.