A 1 V 103 dB 3rd-order audio continuous-time ΔΣ ADC with enhanced noise shaping in 65 nm CMOS

Abstract

As technology scales, integrating high resolution ADCs into high fidelity mixed signal systems becomes challenging in advanced CMOS processes. Cascading integrators to achieve high-order filter structures limits the modulation index and compromises on stability at the expense of added hardware and power consumption. To optimize the maximum stable amplitude (MSA) to accommodate a larger input dynamic range, the supply rails have to be expanded, which limited technology choices to those of a larger feature size. This work proposes a 25 kHz 3rd-order continuous-time ΔΣ modulator (CTΔΣM) utilizing a 5-bit SAR quantizer, enabling noise coupling (NC) to be possible in a typical nanoscale CMOS 65 nm technology with VDD of 1 V. Mismatches in SAR comparator and DAC array are mitigated with a proposed calibration scheme while CM mismatches are solved by a floating differential charge storage capacitor (FDCSC) coupling method. To allow sufficient time for SAR bit cycling and noise charge feedback settling, 1 Ts excess loop delay (ELD) is compensated with digital differentiation that minimizes both the power and complexity of the auxiliary feedback DAC. The prototype obtained DR/SNR/SNDR of 103.1 dB/100.1 dB/95.2 dB while dissipating 0.8 mW, hence achieving a FoMSNDR and FoMschreier of 0.34 pJ/level and 177.9 dB, respectively.