Analysis of Integrator Nonlinearity in a Class of Continuous-Time Delta–Sigma Modulators

We analyze the effect of nonlinearity in the first integrator of a single-loop multibit continuous-time delta-sigma modulator, when the loop filter is of the cascade of integrators with feedforward kind. When a multibit quantizer is used in the loop, we show that integrator nonlinearity results in an increased in-band noise (IBN) floor. We develop analytical relations for the IBN spectral density for several opamp topologies, assuming that the shaped quantization noise is approximately Gaussian. Our results agree well with macromodel and transistor level simulations.