A Time-Interleaved SAR ADC With Bypass-Based Opportunistic Adaptive Calibration

This article reports a power-efficient 8x time-interleaved (TI) 2.4-GS/s 10-bit successive-approximation-register (SAR) analog-to-digital converter (ADC). To optimize the circuit design in terms of power efficiency and conversion rate, several enhancement techniques are presented. First, a pre-defined bypass window, introduced by the customized non-binary DAC, is used to modestly reduce the power consumption. Several conversion cycles are skipped as the input signal falls within the bypass window. Second, to enhance the operation speed, two alternate comparators are adopted in each ADC channel, and an opportunistic adaptive comparator offset calibration is proposed to eliminate the conversion rate degradation caused by the dedicated calibration cycle. The comparator offset is calibrated only when the bit bypass is triggered with the calibration step size adaptively set to acquire both fast convergence and small algorithm noise. In addition, the reference voltage of each ADC channel is provided by a pre-charged reservoir to avoid inter-channel crosstalk without the introduction of power-hungry-distributed reference buffers. A test chip is fabricated in a 28-nm fully depleted silicon on insulator (FDSOI) process with a core area of 0.11 mm(2), including the reference charge reservoirs. Clocked at 2.4 GS/s, the proposed ADC measures a 49.02-dB signal-to-noise-and-distortion ratio (SNDR) at Nyquist while consuming only 9.8 mW from a 0.9-V supply, thus resulting in Walden and Schreier figures of merit (FOMs) of 17.7 fJ/conversion-step and 159.9 dB, respectively.