Bitstream switching rate based calibration of delta-sigma modulators

This paper proposes a simple and effective criterion based on the switching rate of the output bit-stream of a delta-sigma modulator to estimate the performance and stability of the modulator. The switching rate and its relation to the NTF design are analyzed for both single and multi-bit delta-sigma modulators as well as lowpass and bandpass modulators. A calibration method is proposed using the switching rate as calibration parameter to adjust non-idealities such as loopfilter zero spread and excess loop delay. The loopfilter coefficients can be adjusted to calibrate the delta-sigma modulator back to the specification by comparing the measured switching rate with the desired design criterion. The proposed technique is based only on counting and requires no external stimuli or complex hardware. The switching rate is a very simple performance indicator for testing of a delta-sigma modulator without the need for an FFT, as well as a powerful input parameter for calibration.