| 摘要 |
PURPOSE:To provide a digital-analog converter capable of converting the data of large number bits with high accuracy by providing a DAC of apparent 20 bits, means for precisely measuring a gain of the DAC and means for calibrating the gain of the DAC based on the measured result. CONSTITUTION:Lower 18-bit data among 20-bit data in an output data line 10 from a CPU1 is applied to a first DAC3 via a correction arithmetic unit 23, while upper 2-bit data among the 20-bit data in the output data line 10 is applied to a second DAC5. Outputs from both DAC's 3 and 5 are added by an adder 4 in an analog fashion to drive a deflector 6, so that an electron beam 7 irradiating a reference sample 8 is deflected accordingly. In an offset correction arithmetic unit 23, the data in the data line 10 is corrected base on an offset amount given from a memory 21 and then inputted to the first DAC3. In a gain correction arithmetic unit 24, output of the first DAC3 is corrected base on a gain value given from a memory 22 and then inputted an adder 4. Every time when the electron beam 7 irradiates a mark on the reference sample 8, this is detected by a mark detector 9 to be informed to the CPU1. |
