A Dual-Mode Noise-Shaping Charge-Sharing SAR ADC for Ultrasound Probes

Abstract

Analog to digital conversion is an essential function in ultrasound imaging systems. The Successive Approximation Register ADC (SAR ADC) working in the charge domain is a suitable ADC topology for in-probe digitization. Recently, there has been an increased demand for improved image quality and higher signal-to-noise ratio. The noise-shaping (NS) technique is a good solution for achieving higher ADC performance. A charge-sharing SAR ADC with NS technique has been designed and fabricated using 180nm TSMC BCD technology. The ADC test chip powered by 1.8V supply can work in Nyquist mode (NS loop disabled) and NS mode (NS loop enabled). The Nyquist mode can be used for B-mode imaging, while the NS mode can provide the higher SNDR needed for Doppler imaging. The Nyquist mode consumes 1.575mW and achieves 61dB SNDR for a 2.2~6.2MHz signal bandwidth. In comparison, NS mode for Doppler imaging consumes 1.58mW. And it achieves 67.7dB SNDR for a 3.2~5.2MHz signal bandwidth after calibration for DAC non-linearity, improving the Schreier FOM by 3.5dB to 158.7dB while only increasing chip area by 17%.