Analog-to-digital conversion apparatus and method

1. An A/D conversion apparatus comprising:

an A/D converter that converts a potential difference between a reference voltage input and a voltage input to be measured to a digital signal and outputs the digital signal;

a first switch connected between a voltage source to be measured and the voltage input to be measured;

a first sampling capacitor having a first end connected to the voltage input to be measured and to a first end of said first switch, and having a second end connected to a reference power source;

a second switch connected between a reference voltage source and the reference voltage input;

a second sampling capacitor having a first end connected to the reference voltage input and to a first end of said second switch, and having a second end connected to the reference power source; and

an impedance adjusting circuit which is connected between the reference voltage source and a second end of said second switch and changes, in stepwise fashion, impedance between the reference voltage source and the second end of said second switch.

2. The apparatus according to claim 1 , further comprising a control unit that fixes the voltage of the voltage source to be measured, varies the impedance of said impedance adjusting circuit, causes said A/D converter to perform an A/D conversion a plurality of times for each impedance value, selects the impedance value for which A/D conversion error is smallest between identical impedance values, sets the selected impedance value as the impedance value of said impedance adjusting circuit, and causes measurement of the voltage value of the voltage source to be measured.

3. The apparatus according to claim 2 , wherein maximum and minimum values of the A/D conversion values for each impedance value set by said impedance adjusting circuit are obtained, and the impedance value for which the difference between the maximum and minimum values is smallest is selected as the impedance value for which the A/D conversion error is smallest.

4. The apparatus according to claim 2 , further comprising a memory circuit;

wherein said control unit stores the impedance value, which has been selected as the impedance value for which the A/D conversion error is smallest, in said memory circuit beforehand.

5. The apparatus according to claim 2 , further comprising a noise generating circuit connected between said first and second sampling capacitors and said reference power source;

wherein said A/D converter is caused to perform an A/D conversion in a state in which said noise generating circuit is caused to generate noise, the impedance value for which the A/D conversion error is smallest is selected, the selected impedance value is set as the impedance value of said impedance adjusting circuit, said the A/D conversion is performed upon halting operation of said noise generating circuit.

6. The apparatus according to claim 5 , wherein said noise generating circuit is one that generates an operating clock as noise.

7. An A/D conversion method comprising:

providing an A/D conversion circuit that includes an A/D converter for converting a potential difference between a reference voltage input and a voltage input to be measured to a digital signal and outputting the digital signal; a first switch connected between a voltage source to be measured and the voltage input to be measured; a first sampling capacitor having a first end connected to the voltage input to be measured and to a first end of the first switch, and having a second end connected to a reference power source; a second switch connected between a reference voltage source and the reference voltage input; a second sampling capacitor having a first end connected to the reference voltage input and to a first end of the second switch, and having a second end connected to the reference power source; and an impedance adjusting circuit, which is connected between the reference voltage source and a second end of the second switch, for changing, in stepwise fashion, an impedance value between the reference voltage source and the second end of the second switch;

changing the impedance value of the impedance adjusting circuit with the voltage of the voltage source to be measured held fixed, performing an A/D conversion a plurality of times for each impedance value, and obtaining a variation in A/D conversion measurement for each impedance value;

obtaining an impedance value for which the measurement variation found in said obtaining the variation is smallest; and

setting the impedance value obtained in said obtaining the impedance value in the impedance adjusting circuit and performing an A/D conversion with regard to the voltage source to be measured.

8. The method according to claim 7 , wherein the A/D conversion circuit has a noise generating circuit connected between said first and second sampling capacitors and said reference power source;

wherein in said obtaining the variation, the measurement variation is obtained upon actuating the noise generating circuit; and

at said performing the A/D conversion with regard to the voltage source to be measured, the A/D conversion is performed upon halting operation of the noise generating circuit.

9. The method according to claim 8 , wherein the noise generating circuit is one that generates an operating clock as noise.

10. The method according to claim 8 , wherein the noise generating circuit generates a noise based on an operating clock of a digital circuit, the digital circuit being integrated in a semiconductor substrate with said A/D conversion circuit.