Quasi-equivalent feedforward system in asymptotic ΣΔ modulation

It was previously shown that sigma-delta (ΣΔ) modulators of "asymptotic" type theoretically yield an equivalent feedforward system where the recursive nonlinear mechanisms are extracted from the feedback loop and reduced to a memoryless function. With time-varying inputs, we show in this paper, partially by mathematical derivations and partially by experiment, that this system is quasi-equivalent to the original modulator in a sense that we explain. This reduction of the nonlinear mechanisms should permit more refined modeling of the ΣΔ errors in future research, with a better account of the original nonlinearities of asymptotic ΣΔ modulation.