Abstract: Relations between signal-to-noise ratio and power dissipation of feed-forward cascade ΣΔ modulators and those of switched-capacitor integrators composed of Class-A operation trans-conductance amplifiers are analyzed in detail.Based on the analysis, optimal values of integrators in ΣΔ modulators with both restrictions of signal-to-noise ratio and power dissipation are given in expressions, including:sample capacitor、resistor of switch、ratios of width and length of input transistors and so on.This optimal selection is good for low-power high-resolution ΣΔ ADC designers to have an excellent design with both restrictions, which is also good to give reference to detailed circuit design and reduce design cycle.