Abstract: An imaging system comprises a pixel array and readout circuitry coupled to the pixel array. The readout circuitry includes a ramp generator and a plurality of column unit cells, each comprising a column ramp buffer and a column comparator, and each column ramp buffer comprising an input node coupled to receive a ramp signal from the ramp generator, a transistor having a gate terminal coupled to the input node and a drain terminal coupled to a power line, an output node coupled between a source terminal of the transistor and the column comparator, and an alternating current (AC) coupling unit coupled between the input node and the transistor. The AC coupling unit comprises a capacitor coupled between the input node and the gate terminal of the transistor, and a reset switch coupled between the input node and the gate terminal of the transistor.