A sigma-delta modulator with bitstream-controlled dynamic element matching

When dynamic element matching (DEM) techniques are applied to generate a precision reference for a (single-bit) sigma-delta modulator, intermodulation occurs between the DEM residuals and the bitstream, which increases the in-band quantization noise. This can be prevented by deriving the sequence of DEM steps from the bitstream. This technique has been implemented in a second-order sigma-delta modulator with a dynamic bandgap voltage reference, which was realized in a 0.7 μm CMOS process. Measurements show complete elimination of intermodulation products in the signal band, corresponding to an 8 dB reduction in quantization noise compared to conventional cyclic DEM.