A device for generating a ramp signal with a changing slope is disclosed. The device may comprise a processor configured to generate a variable signal. The device may also comprise a phase-locked loop (PLL) circuit configured to receive the variable signal and a reference clock signal, generate a changing ramp clock signal based on the variable signal and the reference clock signal, and output the generated changing ramp clock signal as an input of an analog-to-digital-converter (ADC) circuit.