An analog-to-digital converter including a first stage and a second stage. The first stage receives a first reference voltage and a first analog input voltage, generates a first digital signal by quantizing the first analog input voltage, and generates a first analog output voltage based on the first digital signal and the first analog input voltage. The second stage receives a second reference voltage and the first analog output voltage, in which the second reference voltage is lower than the first reference voltage. The second stage further generates a second digital signal by quantizing the first analog output voltage, and generates a second analog output voltage based on the second digital signal and the first analog output voltage.