A sigma-delta modulator with optimized biquad-based loop filter for WCDMA application

A 4^th order single loop sigma-delta modulator with an optimized biquad loop filter is presented for WCDMA application. The single loop implementation relaxes the analog circuit requirements and hence decreases power consumption. Unlike the Butterworth or Chebyshev, the more general form of biquad filters provides more flexibility for the designer to optimize the power consumption. Minimum power consumption is achieved by minimizing the output swing of integrators that decreases Opamps swings and slew rate requirements. More power is saved by minimizing the capacitor spread factor in the switched-capacitor circuit. The loop filter is also designed to provide an aggressive noise shaping while stability of the loop is maintained by using multi-bit quantizer inside the loop and by careful design of the loop filter. The fourth order single loop modulator consumes 4 mA from 1.8 V supply while providing 72 dB of SNDR and 85 dB dynamic-range.