Patent ID: 7683814

Claim:
An A/D converter comprising: a ramp voltage generation circuit, comprising: a capacitive load to be charged and discharged based on a constant current generated from a constant current source, comprising: a constant voltage source; and a resistive load converting a voltage generated from the constant voltage source to a constant current contributing to charge and discharge of a capacitive load, wherein the resistive load comprises a switched capacitor, wherein a ramp voltage is generated based on an output at one end of the capacitive load; a voltage comparison circuit sampling and holding one or more analog voltages to be converted, and comparing the sampled and held one or more analog voltages to be converted with a reference voltage given by a voltage change value of the ramp voltage generated from the ramp voltage generation circuit or a voltage value proportional to the voltage change value by an arithmetic unit, and changing its output when the reference voltage equals the analog voltage to be converted; a counter counting a digital value according to the reference voltage and outputting the digital value; and a latch circuit latching the digital value outputted from the counter and outputting the latched digital value when the output of the voltage comparison circuit is changed, wherein a ratio between a frequency of a sampling clock signal used in the switched capacitor in the ramp voltage generation circuit and a frequency of a counter clock signal used in the counter is kept constant.