An analog-to-digital converter (ADC) device includes an ADC circuitry and a digital slope ADC circuitry. The ADC circuitry is configured to generate first bits and a first voltage according to an input signal. The digital slope ADC circuitry is configured to generate a second voltage at a node according to the first voltage and to gradually adjust the second voltage to generate second bits. After the second bits are generated, the digital slope ADC circuitry is further configured to perform a noise shaping function according to a first residual signal of the node.