High-speed CMOS continuous-time switched-current sigma-delta modulators.

摘要

Oversampling SigmaDelta analog-to-digital converters (ADC) trade analog accuracy for digital complexity and are preferred for CMOS technology due to their high resolutions and less analog accuracy requirement. In the past, implementations of oversampling SigmaDelta ADCs have been dominated by switched-capacitor (SC) circuits. Due to the limitations of settling requirement, internal feedback, and signal-dependent sampling errors, switched-capacitor SigmaDelta ADCs can sample only at the rate of a few tens of MHz. Although advanced CMOS processes offer higher and higher fT for MOS transistors, conventional oversampling SigmaDelta analog-to-digital converters cannot benefit from these advantages.; This research proposes a new architecture for realizing high speed SigmaDelta modulators (the analog part of oversampling SigmaDelta ADCs). With the use of continuous-time open-loop integrators for loop filters and current switches as digital-to-analog converters (DACs) in the feedback paths, the proposed continuous-time switched-current SigmaDelta modulator can operate at much higher sampling rates then its switched-capacitor counterpart.; Nevertheless, the implementation of high-speed continuous-time switched-current SigmaDelta modulators involves several circuit design challenges, including switching noise, background noise, clock jitter, excessive loop delay, loop gain variation, etc. This research has successfully solved these problems with newly designed architectures and building blocks. With the differential structures and differential current switches, the noise problem was solved to enable the modulator to operate at high speeds. The proposed common-mode feedback circuits and high-speed current comparator enhance the performance of the modulator. The stability problems have been solved with new structures for delay compensation and by the use of feedforward gain compensation with a Gain Manager.; Prototypes of these modulator have achieved a 50MHz sampling rate for a 2mum process and a 400MHz sampling rate for a 0.6mum process, which are much higher than their switched-capacitor counterparts. The experimental results of this research have proven the superiority of continuous-time switched-current SigmaDelta modulators in high-speed sampling. The properties of higher bandwidth, no input sampling, less KT/C thermal noise, and insensitivity to clock jitter also enable continuous-time switched-current SigmaDelta modulators to track advances in CMOS technology.