A 12-bit 40-MS/s SAR ADC With a Fast-Binary-Window DAC Switching Scheme

Abstract

This paper presents a 12-bit 40-MS/s successive approximation register analog-to-digital converter (ADC) for ultrasound imaging systems. By incorporating a fast binary window digital-to-analog converter (DAC) switching technique, the problematic most significant bit transition glitch was removed to improve linearity without increasing the input capacitance or using a calibration scheme. A hybrid DAC was also developed to overcome the yield problem that occurs when a tiny unit capacitance is used in the DAC. Moreover, a reference buffer was used to accelerate the DAC settling to achieve high-speed conversion. The prototype ADC was fabricated using a 130-nm CMOS technology. The ADC core occupied an active area of 0.1 mm2 and consumed a total power of 1.32 mW when a 1.2-V supply was used at a conversion rate of 40 MS/s. The measured peak signal-to-noise-and-distortion ratio and spurious-free dynamic range were 64 and 77.5 dB, respectively. The peak effective number of bits was 10.33, which is equivalent to a Walden figure-of-merit of 25.6 fJ/conversion step.