The present invention relates to a converting device for converting an analog voltage into a digital number and to an imaging system comprising the same. The invention further relates to a method for converting an analog voltage into a digital number. According to the invention, one or more charge pumping steps are performed that change a voltage over a capacitive element that has been set in dependence of the voltage to be converted. During each charge pumping step, one or more substantially identical charge packets may be transferred to of from the capacitive element. The magnitude of the charge packets belonging to different charge pumping steps may be different allowing multi-slope operation. The digital number representing the analog voltage is calculated based on the net charge that has been injected into or removed from the main capacitive element as a result of having performed the one or more charge pumping steps.