An analog-to-digital converter having a successive-approximation register digital-to-analog converter is described. For example, the threshold calculating unit calculates the clip threshold by identifying the maximum value of address values generated by the address generating circuit in each adjustment unit period and subtracting a certain offset value from the identified maximum value. 6 times the flow rate of the acrylic acid in the gaseous reaction mixture and the rectifying column used as a separation column is operated with the feed at the top and without reflux. An accumulator conveyor, traveling at a substantially similar speed or a designated variable speed as the first conveyor or conveyors, is aligned in the same plane as, is adjacent to, and may mate with the first conveyor at a predetermined location for contiguous movement in the same direction as the first conveyor at this predetermined location.