The present invention discloses a time-interleaved analog-to-digital converter (ADC), which includes a first and a second sub-ADC and a calibration module. The calibration module includes a switch module and a calibration engine. The switch module selectively provides one of a set of reference voltage levels, which are provided by a resistor series of the first sub-ADC, onto an input signal line, which is shared by the first and the second sub-ADCs. The calibration engine calibrates pre-amplifying units of the first and the second sub-ADCs according to digital signals generated by the first and the second sub-ADCs.