A design methodology for power-efficient reconfigurable SC ΔΣ modulators

This paper presents a methodology to design reconfigurable switched-capacitor (SC) delta-sigma modulators Ms) capable of keeping their corresponding power efficiency figures constant and optimal for a set of resolutions and signal bandwidths. This method is especially suitable for lowbandwidth, medium-to-high resolution specifications, which are common in bio-medical application range. The presented methodology is based on an analytic model of all different contributions to the power dissipation of the ΔΣM. In particular, a novel way to predict the static power dissipated by integrators based on class-A and class-AB OTAs is presented. The power-optimal solution is found in terms of filter order, quantizer resolution, oversampling ratio and capacitor dimensions for a targeted resolution and bandwidth. As the size of the sampling capacitors is crucial to determine power consumption, three approaches to achieve reconfigurability are compared: sizing the sampling capacitors to achieve the highest resolution and keep them constant, change only the first sampling capacitor according to the targeted resolution or program all sampling capacitors to the required resolution. The second approach results in the best trade-off between power efficiency and simplicity. A reconfigurable ΔΣM for bio-medical applications is designed at transistor-level in a 0.18μm CMOS process following the methodology discussed. A comparison between the power estimated by the proposed analytic model and the transistor implementation shows a maximum difference of 17%; validating thus the proposed approach.