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An analog-to-digital conversion method using an RC time constant calibrator is provided. The method includes the operations of comparing a crossing time point at which a first reference signal and a second reference signal cross each other with a target time point and calibrating an RC time constant

An analog-to-digital conversion method using an RC time constant calibrator is provided. The method includes the operations of comparing a crossing time point at which a first reference signal and a second reference signal cross each other with a target time point and calibrating an RC time constant according to a result of the comparison. A length of time until the crossing time point at which a first analog signal and a second analog signal cross each other is counted based on a calibrated RC time constant. The counted value is output.

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1. A method for converting an analog signal to a digital signal using an RC time constant calibrator, the method comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point and calibrating an R

1. A method for converting an analog signal to a digital signal using an RC time constant calibrator, the method comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point and calibrating an RC time constant according to a result of the comparison;counting a length of time until an analog signal crossing time point at which a first analog signal and a second analog signal cross each other based on the calibrated RC time constant and outputting counted length of time as an analog signal crossing time point count value; andinverting the analog signal crossing time point count value. 2. The method of claim 1, wherein the step of counting and outputting the analog signal crossing time point count value comprises: detecting a time delay value representing a length of time until the analog signal crossing time point of the first and second analog signals based on the calibrated RC time constant; andcounting time corresponding to the time delay value and outputting the analog signal crossing time point count value. 3. The method of claim 1, wherein the first and second analog signals are each differential output voltages. 4. The method of claim 1, wherein the step of comparing the reference crossing time point with the target time point and calibrating the RC time constant comprises: comparing a reference signal crossing time point count value obtained by counting a length of time until the crossing time point of the first and second reference signals with a externally input target count value corresponding to the target time point;generating an RC time constant calibration code for controlling the reference signal crossing time point such that the reference signal crossing time point becomes close to the target time point when a length of time between the reference signal crossing time point count value and the crossing time point is not the same as the target count value; andcalibrating the RC time constant based on the RC time constant calibration code such that the count value counting the length of time until the reference signal crossing time point is the same as the target count value and adjusting the reference signal crossing time point based on the calibrated RC time constant. 5. A method for converting an analog signal to a digital signal using an RC time constant calibrator, the method comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point and calibrating an RC time constant according to a result of the comparison; andcounting a length of time until an analog signal crossing time point at which a first analog signal and a second analog signal cross each other based on the calibrated RC time constant and outputting counted length of time as an analog signal crossing time point count value,wherein the step of counting and outputting the analog signal crossing time point count value comprises:detecting a time delay value representing a length of time until the analog signal crossing time point of the first and second analog signals based on the calibrated RC time constant; andcounting time corresponding to the time delay value and outputting the analog signal crossing time point count value, andwherein the time delay value is inversely proportional to a difference between the first and second analog signals. 6. A method for converting an analog signal to a digital signal using an RC time constant calibrator, the method comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point and calibrating an RC time constant according to a result of the comparison; andcounting a length of time until an analog signal crossing time point at which a first analog signal and a second analog signal cross each other based on the calibrated RC time constant and outputting counted length of time as an analog signal crossing time point count value,wherein the step of comparing the reference crossing time point with the target time point and calibrating the RC time constant comprises:comparing a reference signal crossing time point count value obtained by counting a length of time until the crossing time point of the first and second reference signals with a externally input target count value corresponding to the target time point;generating an RC time constant calibration code for controlling the reference signal crossing time point such that the reference signal crossing time point becomes close to the target time point when a length of time between the reference signal crossing time point count value and the crossing time point is not the same as the target count value; andcalibrating the RC time constant based on the RC time constant calibration code such that the count value counting the length of time until the reference signal crossing time point is the same as the target count value and adjusting the reference signal crossing time point based on the calibrated RC time constant, andwherein the RC time constant calibration code is set in order from a most significant bit to a least significant bit, and bits set in the RC time constant calibration code are maintained when the target count value is greater than the count value counting the length of time until the reference signal crossing time point and are the RC time constant calibration code is reset when the target count value is less than the count value counting the length of time until the reference signal crossing time point. 7. An analog-to-digital converter comprising: a switching unit configured to perform a switching operation according to a control signal and to output a pair of analog signals forming differential outputs;an RC time constant calibrator configured to output an analog signal crossing time point count value obtained by counting a length of time until an analog signal crossing time point at which the pair of analog signals output from the switching unit cross each other based on a calibrated RC time constant; andan inversion unit configured to invert the analog signal crossing time point count value. 8. The analog-to-digital converter of claim 7, wherein the RC time constant calibrator comprises: an RC timer configured to output a hold signal according to a time delay value as a result of the crossing time point of the pair of signals based on the RC time constant; anda counter unit configured to count the time delay value until the hold signal is output and to output the count value. 9. A method for converting an analog signal to a digital signal, comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point using a comparison device;calibrating an RC time constant based on the results of the comparison using an RC time constant calibration device;counting a length of time elapsed prior to the occurrence of the crossing time point based on the calibrated RC time constant using a counting device;inverting the counted length time; andoutputting the inverted counted length of time using an output device. 10. The method of claim 9, wherein the step of counting comprises: detecting a time delay value representing a length of time elapsed prior to the occurrence of the crossing time point based on the calibrated RC constant; andcounting a length of time corresponding to the detection of the time delay value. 11. The method of claim 9, wherein the step of comparing includes comparing a count value obtained by counting a length of time until the crossing time point of the first and second reference signals with a externally input target count value corresponding to the target time point. 12. The method of claim 9, wherein the step of calibrating an RC time constant includes: generating an RC time constant calibration code for controlling the crossing time point such that the crossing time point is close to the target time point when the count value counting the length of time until the reference signal crossing time point is not the same as the target count value; andcalibrating the generated RC time constant based on the RC time constant calibration code such that the count value counting the length of time until the crossing time point is the same as the target count value and adjusting the crossing time point based on the calibrated RC time constant. 13. A method for converting an analog signal to a digital signal, comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point using a comparison device;calibrating an RC time constant based on the results of the comparison using an RC time constant calibration device;counting a length of time elapsed prior to the occurrence of the crossing time point based on the calibrated RC time constant using a counting device; andoutputting the counted length of time using an output device,wherein the step of counting comprises:detecting a time delay value representing a length of time elapsed prior to the occurrence of the crossing time point based on the calibrated RC constant; andcounting a length of time corresponding to the detection of the time delay value, andwherein the time delay value is inversely proportional to a difference between the first and second reference signals. 14. A method for converting an analog signal to a digital signal, comprising: comparing a reference signal crossing time point at which a first reference signal and a second reference signal cross each other with a target time point using a comparison device;calibrating an RC time constant based on the results of the comparison using an RC time constant calibration device;counting a length of time elapsed prior to the occurrence of the crossing time point based on the calibrated RC time constant using a counting device; andoutputting the counted length of time using an output device,wherein the step of calibrating an RC time constant includes:generating an RC time constant calibration code for controlling the crossing time point such that the crossing time point is close to the target time point when the count value counting the length of time until the reference signal crossing time point is not the same as the target count value; andcalibrating the generated RC time constant based on the RC time constant calibration code such that the count value counting the length of time until the crossing time point is the same as the target count value and adjusting the crossing time point based on the calibrated RC time constant, andwherein the RC time constant calibration code is set in order from a most significant bit to a least significant bit, and bits set in the RC time constant calibration code are maintained when the target count value is greater than the count value counting the length of time until the crossing time point and are the RC time constant calibration code is reset when the target count value is less than the count value counting the length of time until the crossing time point.