Methods and apparatus for determining non-linearity in analog-to-digital converters are disclosed. An example apparatus includes a signal interface to receive an output of an analog-to-digital converter (ADC), the output corresponding to a periodic signal transmitted to the ADC; a signal transformer to determine at least one of a harmonic phase or a harmonic amplitude corresponding to the output; and an integral non-linearity (INL) term calculator to determine the INL of the ADC based on a characteristic of the periodic signal and the at least one of the harmonic phase or the harmonic amplitude.