The present invention relates to an A/D converter for converting an analog input voltage to a digital signal having a plurality of bits.
A/D converters are installed in various types of electronic equipment to convert analog input voltage to a digital signal. In such A/D converters, comparators are supplied with reference voltages that have become finer due to lower power supply voltages and higher output signal resolution (increased number of bits). For instance, a reference voltage must be generated by dividing the power supply voltage by 1024 to obtain a digital output signal having 10 bits. If the power supply voltage is low, it becomes difficult to generate the reference voltage.
A current mode A/D converter for converting analog input voltage to a current and then comparing the converted current with a reference current with a plurality of comparators has been proposed. In such a current mode A/D converter, the reference voltage does not need to be prepared by dividing the power supply voltage. Thus, even if the power supply voltage is lowered, the current A/D converter is applicable for higher resolution of the A/D conversion operation, that is, for increased number of bits in the output signal.
Japanese Laid-Open Patent No. 8-79078 describes a serial-parallel A/D converter that performs A/D conversion through voltage comparison for higher ranks and A/D conversion process through current comparison for lower ranks. Thus, the serial-parallel A/D converter is applicable for lower power supply voltages.
Japanese Laid-Open Patent No. 9-98033 describes a circuit similar to the current-voltage conversion circuit used in the current mode A/D converter.