FEEDFORWARD COMPENSATION TECHNIQUE FOR COMPARATOR DELAY DISPERSION FOR LEVEL-CROSSING ADCS

The accuracy of measuring the crossing moment of a given signal to a specific reference voltage plays a crucial role in determining the bandwidth of the signal that could be converted by a level-cr ossing ADC. This timing accuracy is mainly determined by delay dispersion o f the comparator and the accuracy of the time-to-digital converter (TDC) comprising the levelcrossing ADC. With the pico-second resolution TDC, the delay dispersion of the comparator became the bottle nick of the des ign of a high speed levelcrossing ADC. This paper presents a comparator with low delay dispersion for level-crossing analog-to-digital conversion app lications. The comparator comprises a technique to compensate the delay dispersion caused by variable input overdrive. The whole circu it is composed of three main blocks, namely, conventional comparator, fixed delay block and variable delay block. The variable delay block is c ontrolled such that it implements the inverse overdrive-delay characterist ics of the conventional comparator. Therefore, the overall delay dispersion f the circuit is effectively reduced. Using the proposed technique i n a level-crossing ADC would enable the sampling of a signal with 6.15X hi gher frequency than the case of using a conventional comparator keeping the same SNR.