A 50MHz-BW 168.8dB-FoM 2x Time-Interleaved Bandpass Noise Shaping SAR ADC Using Passive Filter

Bandpass analog-to-digital converters (BP ADCs) are one of the main building blocks used to simplify RF receivers by directly digitizing IF signals. Continuous-time BP ΔΣ ADCs often have high sampling rate, which is essential for IF signal conversion, but their energy efficiency is quite poor due to the use of active filters [1]. In [2], a BP Noise Shaping SAR (NS-SAR) ADC structure based on a passive loop filter was proposed to achieve better energy efficiency. However, the discrete-time operation limits the sampling rate to low, and the single-channel structure requires twice the capacitors for the passive loop filter for BP NS compared to lowpass or highpass (HP) schemes due to the two sample delays in the transfer function. To increase the sampling rate, a time-interleaved (TI) BP NS-SAR ADC has been proposed that combines two HP NS-SAR ADC channels [3]. Each ADC channel in this structure does not require the two sampling delays, so the passive filter can be easily implemented with a reduced number of capacitors. However, the use of multi-input comparators significantly increases the noise floor, resulting in each ADC channel performing 2nd-order NS for high resolution. A 2nd-order passive filter based on charge sharing slows the overall speed due to settling and complexity, and the use of a high oversampling ratio further limits the achievable bandwidth.