Use of clipping in sigma-delta modulators

High-resolution conversion processes utilising sigma-delta modulation (SDM) with practical sampling rates and single-bit quantisers require high-order loop filters. Third- and higher-order systems are particularly prone to instability when the input signal approaches full-scale (set by the quantiser output voltage), and such overload instability results in very low signal-to-noise ratios (SNR) observed at the modulator output. This paper shows by simulation how this type of instability in a fourth-order coder can be avoided by placing clippers at loop-filter integrator outputs, and how optimal choice of clipper levels can increase the SNR observed when the modulator is in overload, from less than 0 dB without clippers to 40 dB->80 dB. Results generated using clippers where integrator outputs are reset are compared with those of saturating clippers. Other techniques for avoiding overload instability, such as adaptive loop filters, are also considered