Abstract: An A-D converter in which an input analog signal is converted by a first A-D converting circuit into a digital signal to obtain a high-order digit output, the high-order digit output is re-converted by a D-A converting circuit into an analog signal and the difference between the re-converted analog signal and the input analog signal is amplified and then converted by a second A-D converting circuit into a digital signal. An ideal voltage which would be produced when the input to the D-A converting circuit is separately applied for each bit of the D-A converting circuit is provided from a reference voltage source. The difference between the output derived from the D-A converting circuit in response to the application of the input thereto for each bit and the voltage of the reference voltage source corresponding to the D-A converting circuit output is converted by the second A-D converting circuit into a digital signal, so that an error signal is obtained.