Spurious tone free digital delta-sigma modulator design for DC inputs

A method for spurious tone free delta sigma modulator design is presented in this paper. The method is based on an analysis of modulator state space orbits and it is applicable to digital modulators working with DC inputs. Modulator is designed to maintain a very long controllable sequence lengths. This results in smooth distribution of the quantization noise power and prevents it from concentrating into spurious tones. The sequence length is controlled by modulator scaling and applying initial conditions each time DC input changes. A list of suitable initial conditions and corresponding sequence lengths is given for MASH and error feedback modulators. This method is extended further to a more flexible case in which reloading initial conditions is no longer necessary. It is shown that when modulator resolution is increased by one bit and initial conditions applied at startup, the modulator spontaneously enters very long orbits each time DC input changes. In return for the small hardware increment the spectrum remains smooth and is further decreased by 8 dB.