SHA-Less Pipelined ADC With In Situ Background Clock-Skew Calibration

A 10-b, 100-MS/s pipelined analog-to-digital converter (ADC) without dedicated front-end sample-and-hold amplifier (SHA) converts from dc to the 12th Nyquist band with in situ, mostly digital background calibration for the clock skew in the 3.5-b front-end stage. The skew information is extracted from the first-stage residue output with two comparators sensing out-of-range errors; a gradient-descent algorithm is used to adaptively adjust the timing of the front-end sub-ADC to synchronize with that of the sample-and-hold (S/H) in the multiplying digital-to-analog converter (MDAC). The prototype ADC, implemented in a 90-nm CMOS process, digitizes inputs up to 610 MHz without skew errors in experiments; in contrast, the same ADC fails at 130 MHz with calibration disabled (with the default sub-ADC sample point set at the midpoint of the delay range). The prototype with calibration circuits fully integrated on chip consumes 12.2 mW and occupies 0.26-mm² silicon area, while the calibration circuits dissipate 0.9 mW and occupy 0.01 mm². A 71-dB spurious-free dynamic range (SFDR) and a 55-dB signal-to-noise and distortion ratio (SNDR) were measured with a 20-MHz sine-wave input, and a larger than 55-dB SFDR was measured in the 10th Nyquist band.