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扫描电镜的物焦镜距与加速电压和物镜电流有关。扫描电镜要求物镜电流具有较大的变化范围。T_300扫描电镜物镜电路如图所示。IC8为D/A转换器。波段开关S_3、二极管矩阵电路(D_10～D_27)、R_52～R_60、R_B_(23)组成了二进制码编码电路,为D/A转换器提供八位二进制码。D_8、R_(50)、IC9_1组成的电路为D/A转换器提供参考模拟电压V_REF。D_8(IS2192)的稳压值为8.2V,即A点的电压为8.2V。电压跟随器IC9_1输出的电压V_B=V_REF=V_A=8.2V。D/A转换器的输出电压Vout=-(V_REF/R_49)·R_51(A_8/2~1+A_7/2~2+A_6/2~3+A_5/2~4+A_4/2~5+A_3/2~6+A_2/2~7+A_1/2~8)。若当S_3的刀与第一掷相接时,D_10导通,D_11～D_27均不导通,D/A转换器的数字输入端A_8=0,A_7～A_1,均为1。因此D/A转换器的输出电压Vout=4V。D/A转换器的输入状态和输出电压与S_3对应关系如表1所示。从中可看到D/A转换器的输出电压范围为-4～-8.1V。D/A转换
The focal length of the SEM is related to the acceleration voltage and the objective current. Scanning electron microscopy requires a larger objective lens current range. T_300 scanning electron microscope lens circuit as shown. IC8 is a D / A converter. The band switch S_3, the diode matrix circuit (D_10 ~ D_27), R_52 ~ R_60, R_B_ (23) form the binary code circuit, providing the binary code for the D / A converter. D_8, R_ (50), IC9_1 circuit for the D / A converter provides a reference analog voltage V_REF. The regulation value of D_8 (IS2192) is 8.2V, that is, the voltage of point A is 8.2V. Voltage follower IC9_1 output voltage V_B = V_REF = V_A = 8.2V. The output voltage of the D / A converter Vout = - (V_REF / R_49) · R_51 (A_8 / 2~1 + A_7 / 2~2 + A_6 / 2~3 + A_5 / 2~4 + A_4 / 2~5 + A_3 / 2 ~ 6 + A_2 / 2 ~ 7 + A_1 / 2 ~ 8). If S_3 knife and the first throw phase, D_10 turn on, D_11 ~ D_27 are not turned on, D / A converter digital input A_8 = 0, A_7 ~ A_1, are 1. Therefore, the D / A converter output voltage Vout = 4V. D / A converter input status and output voltage corresponding relationship with S_3 shown in Table 1. It can be seen from the D / A converter output voltage range of -4 ~ -8.1V. D / A conversion