A One-time Self Calibration Technique for Open-loop FIA in Noise Shaping SAR Achieving Ultra Gain PVT-robustness

Open-loop floating inverter amplifier (FIA) is widely used in noise shaping filter in SAR ADCs. However, its gain suffers from PVT variation, which primarily affects the shaping factor. Traditional trimming method adjust its gain by increasing or decreasing the effective reservoir capacitor. While adding additional I/O and cumbersome testing, the mere adjustment of reservoir capacitor also faces numerous limitations. This work presents a onetime self-calibrated scheme for FIA in noise shaping SAR ADCs, with neglectable dynamic power. Implementation of calibration adopt a coarse auxiliary cDAC, with most of remaining circuit reusing in main SAR ADC, indicating the cost advantage our design. Simulation results shows that for a 16× designed gain, the original gain fluctuated from 9.8× to 19×. Proposed calibration performed well at stabilizing gain at 16±0.5× under most of PVT corners. The scheme could refer to any other N× gain as well.