An A/D conversion system comprising:
- plural loop filters including at least a first-stage loop filter to which an input signal is inputted and a last-stage loop filter to which an output signal of a preceding-stage loop filter is inputted as a subject signal to be processed;
- a quantizer connected to the last-stage loop filter;
- and
- an A/D converter connected to the last-stage loop filter through a last converting switch, so that higher bits are formed based on an output of the quantizer and lower bits are formed of an output of the A/D converter,
- wherein the last-stage loop filter includes:
- a last sampling capacitor for sampling the subject signal;
- a last integrating capacitor for integrating the subject signal sampled by the last sampling capacitor;
- a last amplifier circuit for amplifying the subject signal with a gain, which is determined by a ratio between a capacitance value of the last sampling capacitor and a capacitance value of the last integrating capacitor;
- last plural path switchover switches arranged to switch over signal paths for a last discharging operation, a last sampling operation, a last integrating operation and an error cancelling operation;
- and
- a control circuit for controlling on/off states of the last converting switch and the last plural path switchover switches,
- the control circuit connecting, for the last discharging operation, both ends of each of the last sampling capacitor and the last integrating capacitor to an analog ground,
- the control circuit connecting, for the last sampling operation, one end of the last sampling capacitor to the output of the preceding-stage loop filter and connecting an other end of the last sampling capacitor to the analog ground thereby to sample the subject signal in the last sampling capacitor,
- the control circuit connecting, for the last integrating operation, the one end of the last sampling capacitor to the analog ground, connecting the other end of the last sampling capacitor to an input terminal of the last amplifier circuit, connecting one end of the last integrating capacitor to the input terminal of the last amplifier circuit and connecting an other end of the last sampling capacitor to an output terminal of the last amplifier circuit thereby to maintain an integration result of the subject signal inputted to the last sampling capacitor,
- the control circuit connecting, for the error cancelling operation, the one end of the last sampling capacitor to the output terminal of the last amplifier circuit, connecting the other end of the last sampling capacitor to the input terminal of the last amplifier circuit, connecting the one end of the last integrating capacitor to the input terminal of the last amplifier circuit and connecting the other end of the last integrating capacitor to the analog ground thereby to cancel errors of the capacitance values of the last sampling capacitor and the last integrating capacitor and output an output signal of the last-stage loop filter from the output terminal of the last amplifier circuit, and
- the control circuit turning on the last converting switch after controlling the last plural path switchover switches to perform the last discharging operation, then repeat the last sampling operation and the last integrating operation and finally perform the error canceling operation.