Design of a reference voltage buffer for a 10-bit 50 MS/s SAR ADC in 65 nm CMOS

This paper presents the design of a fast-settling reference voltage buffer (RVBuffer) which is used to buffer the high reference voltage in a 10-bit, 50 MS/s successive approximation register (SAR) ADC implemented in 65 nm CMOS. Though numerous publications on SAR ADCs have appeared in recent years, the role of RVBuffers in ensuring ADC performance, the associated design challenges and impact on power and FoM of the entire ADC have not been discussed in-depth. In this work, the speed limitation on precise settling of the digital-to-analog converter voltage (DAC) in a SAR ADC imposed by parasitic inductances of the bondwire and PCB trace is explained. The crucial design parameters for the reference voltage buffer in the context of the SAR ADC are derived. Post-layout simulation results for the RVBuffer are provided to verify settling-time, noise and PSRR performance. In post-layout simulation which includes the entire pad frame and associated parasitics, the SAR ADC achieves an ENOB of 9.25 bits at a supply voltage of 1.2 V, typical process corner and sampling frequency of 50 MS/s for near-Nyquist input. Excluding the reference voltage buffer, the ADC consumes 697 μW and achieves an energy efficiency of 25 fJ/conversion-step while occupying a core area of 0.055 mm².