A 12-bit charge-redistribution SAR ADC for silicon drift detector readout ASICs

This paper presents a first prototype of a 12-bit analog-to-digital converter (ADC) suitable for X and γ-ray multichannel readout front-ends for SDD signals processing. The task of the converter is the digitization of output multiplexed data at a sampling frequency up to 5 MS/s and about 11-bit accuracy. The chosen architecture is the fully-differential bridge-capacitor charge-redistribution (CR) successive-approximation-register (SAR) with monotonic switching algorithm. The chip is fabricated in a standard CMOS 0.35 μm 3.3 V technology and the ADC area occupancy is 0.42 mm². The measured input-output characteristic shows monotonicity over the whole dynamic range while static parameters are -0.17/1.22 LSB and -2.2/2.2 LSB respectively for differential (DNL) and integral nonlinearity (INL). Dynamic performance consist in 68 dB SFDR, 66.5 dB SiNAD and an effective number of bits (ENOB) equal to 10.75 at 4 MS/s. An in-depth analysis of the circuit topology together with simulated and experimental results are here presented.