Delta-sigma modulator topologies with high immunity to pattern noise

This paper addresses the problem of pattern noise encountered in single-stage delta-sigma modulators in the presence of a DC input signal. By utilizing state-space matrices, we explain the cause of these undesired cyclic patterns and show how a modulator´s susceptibility to tonal behavior can be determined by inspection of the system matrix A. By example of two frequently used single-stage architectures, a 2nd and a 3rd order system, we demonstrate that simple topological modifications can render a given system immune to cyclic sequences. The two examples prove that this immunity can be achieved without visibly degrading the modulator´s noise shaping capability. In fact, the modification applied to the 2nd order system even improves the quantization noise suppression.