A 250-kHz 94-dB Double-Sampling Modulation A/D Converter With a Modified Noise Transfer Function

This paper presents a high-order double-sampling single-loop modulation analog-to-digital (A/D) converter. The important problem of noise folding in double-sampling circuits is solved here at the architectural level by placing one of the zeros in the modulator’s noise transfer function at half the sampling frequency instead of in the baseband. The resulting modulator is of fifth order but has the baseband performance of a fourth-order modulator. Through the use of an efficient switched-capacitor implementation, the overall circuit uses only four operational amplifiers and hence, its complexity is similar to that of a fourth-order modulator. An experimental 1-bit modulator was designed for an oversampling ratio of 96 and a bandwidth of 250 kHz at a 3.3-V supply in a conservative 0.8m standard CMOS process. Due to the doublesampling, the sampling frequency is 48 MHz, although the circuits operate at a clock frequency of only 24 MHz. The circuit achieves a dynamic range of 94 dB. The peak signal-to-noise ratio and signal-to-noise-plus-distortion ratio were measured to be 90 and 86 dB, respectively. The power consumption of the complete circuit including clock drivers and output pad drivers was 43 mW. The analog blocks (opamps, comparators, etc.) consume 30 mW of this total.