A 750-MHz 6-b Adaptive Floating-Gate Quantizer in 0.35-μm CMOS

A 6-b 750-MS/s flash analog-to-digital converter (ADC) uses nonvolatile analog storage for reference levels and achieves a signal-to-noise-plus-distortion ratio (SNDR) and a spurious-free dynamic range of 37.2 and 48.6 dB, respectively. The architecture comprises an array of adaptive floating-gate comparators that enables storage and programming of reference levels, eliminating the need for resistive ladders. Reference levels may be programmed either manually by the user or autonomously during normal analog-to-digital conversion. Autonomous programming achieves histogram equalization by adjusting reference levels for finer resolution and greater sensitivity at frequently visited signal values. When programmed manually, the ADC achieves 34.3-dB SNDR at 750 MS/s for input frequencies up to 2.07times Nyquist rate, with a differential full-scale input range of 1 V. We observe integral nonlinearity and differential nonlinearity of less than 0.27 least significant bit at the Nyquist rate. One-month continuous operation shows no signs of reference-level drift due to charge leakage and maintains a constant bit error rate of 2.93 times 10^-9 /sample.