A signal processing apparatus includes an analog-to-digital conversion unit, a determination unit including a comparator, and a data calculation unit that performs calculation. The data calculation unit generates a first calculation result by performing calculation including processing for averaging a first digital signal and a second digital signal or processing for adding the first and second digital signals. The data calculation unit generates a second calculation result based on a difference between the first digital signal and first noise digital data. The data calculation unit switches whether to output the first calculation result or output the second calculation result, based on a comparison result of the comparator.