Design and implementation of an 11-bit 50-MS/s split SAR ADC in 65 nm CMOS

Abstract

This paper presents the design and implementation of an 11-bit 50-MS/s split successive approximation register (SAR) analog-to-digital converter (ADC) that features a comparator with input-referred offset cancellation, an improved split capacitor digital-to-analog converter (CDAC), and a CDAC mismatch calibrator. In order to reduce the input loading capacitance of the split CDAC, an extra unit capacitor is added to the most significant bit (MSB) array and the input is only sampled onto the bottom plates of the MSB array. Capacitance mismatch between the lowest-bit capacitor of the MSB array and the capacitors of the least significant bit (LSB) array is digitally calibrated. In the design of the comparator, kickback noise is suppressed by an intermediate stage. A switch is inserted between the regeneration nodes in order to rapidly equate their voltages before regeneration phase kicks start. An input-referred offset cancellation circuit, which adjusts the body voltages of the input triple-well transistors, was also developed. The ADC was designed and fabricated in a 65 nm complementary metal oxide semiconductor (CMOS) process. The chip consumes 2.48 mW and achieves a 58.95-dB signal to noise and distortion ratio (SNDR) at 50-MS/s sampling rate. Its figure of merit (FOM) is 95 fJ/conversion-step.