A study on op-amp nonlinearity in a single-bit CT ΔΣ modulator employing GBW compensation

This work investigates the nonlinear effect of the 1st operational amplifier (op-amp) in a CT ΔΣ modulator with finite gain-bandwidth product (GBW) compensation. A behavioral op-amp model employing a hyperbolic tangent expression has been created and verified to approximate the op-amp nonlinear characteristics. In analyzing transient simulations of ΔΣ modulators compensated for different GBWs, the influence of the 1st op-amp nonlinearity has been studied, indicating that more stringent linearity requirement on the 1st op-amp must be satisfied in the modulator compensated for lower GBW. This issue can be addressed by increasing the op-amp DC gain, or by employing a mixed feedforward/feedback (FF/FB) architecture, which outperforms the cascade of integrators with feedback (CIFB) topology concerning linearity due to the reduced internal swing, thus allowing both GBW compensation and relaxed linearity requirements.

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