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In a ΔΣ modulator design process, the amplifier nonlinearity problem is usually neglected. However, nonlinearity problems will seriously affect the signal to noise ratio, but rarely discussed in literatures. In this paper, we model the problem and elaborate the importance of the coefficient selected. We design a folded-cascode op-amp with source-degeneration to suppress nonlinearity. The PSD analysis of the modulator output shows that the third and fifth harmonics are both less then -100 dB with the effective resolution up to 15 bit.