Continuous-time ΔΣ modulators with noise-transfer-function enhancement

A technique is presented to design high-order continuous-time (CT) DeltaSigma modulators using noise-transfer- function (NTF) enhancement. This is achieved by coupling the quantization noise into the forward path of the CT DeltaSigma modulator, using a passive CT filter. This introduces a real pole-zero pair into the NTF. Thus, the order of the NTF is increased, without affecting the signal transfer function (STF). The proposed NTF-enhancement technique is applied to a CT DeltaSigma modulator with a feedforward architecture, where all feedforward paths are summed within the last integrator of the DeltaSigma loop filter, thereby eliminating the need for an analog summation amplifier at the quantizer input. Behavioral simulation results confirm the improved noise-shaping and stability characteristics of the proposed feedforward CT DeltaSigma modulator with NTF enhancement, compared to classical CT DeltaSigma modulators.