Analog-to-digital conversion apparatus and method

A properly level-controlled digital signal can be obtained every sample. An input signal is supplied to amplifiers having their gains of one, two, four and eight. Outputs of the amplifiers are converted to digital signals at the same time in corresponding A/D converters having the same characteristic, respectively, and inputted to a data selection part. In the data selection part, when an input signal is high level, one A/D converter to which is inputted the input signal passing through the corresponding amplifier having its large gain, overflows so that the overflow-bit thereof becomes "1". The output digital signal of the A/D converter is selected and outputted, which is connected to the amplifier having the largest gain among the amplifiers connected to corresponding A/D converters each of which has its overflow-bit is "0". The digital signal is corrected in its scale and outputted.