A 7-Bit 700 MS/s 2b/Cycle Asynchronous SAR ADC With Partially Merged Capacitor Switching

Abstract

This letter introduces a 7-bit, 700 MS/s, 2b/cycle asynchronous successive approximation register (SAR) analogue-to-digital converter (ADC). To relax the settling requirement, the capacitive digital-to-analogue converter (CDAC) is designed with non-binary weighting to provide redundancy, implemented using a pre-charge reduction scheme that removes next-cycle pre-charge activity in a 2b/cycle SAR ADC. To reduce the area of this non-binary weighted CDAC, a partially merged capacitor switching scheme is proposed. The prototype ADC is fabricated in a 28 nm CMOS process with an active die area of 0.0077 mm². At a 700 MS/s sampling rate, the ADC achieves a signal-to-noise-and-distortion ratio of 37.6 dB and a spurious-free dynamic range of 49.1 dB at the Nyquist input frequency. The power consumption is 2.41 mW from a 1.0 V supply, resulting in a Walden figure of merit of 55.56 fJ/conversion step at Nyquist.