A Second-Order Passive Noise-Shaping SAR ADC With 4× Passive Gain and A Two-Input-Pair Comparator

Abstract

Noise-shaping (NS) successive approximation register (SAR) analog-to-digital converters (ADCs) benefit from the energy efficiency of SAR ADCs, and achieve high resolution by oversampling and filtering the residue voltage. Passive NS-SAR ADCs have surpassed their active counterparts in terms of simplicity, low power, and power/voltage/temperature (PVT) robustness. To obtain an aggressive noise transfer function (NTF), however, it requires a large loop gain to compensate for the attenuation due to passive charge sharing. Earlier circuits used a passive gain of 4 to realize a first-order NTF. Higher-order NTF can be achieved with multi-port comparators that provide large active gains at the cost of higher input-referred noise. This paper presents a NS-SAR ADC structure with a passive gain of 4, and a low size-ratioed two-input-pair comparator. It achieves an NTF = (1–0.8z <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Simulations predict that the proposed structure provides higher signal-to-noise ratio than the state-of-the-art ones.