LENS CURRENT SETTING CIRCUIT

摘要

PURPOSE:To enable an exciting current to change extremely little by little by adding the output of a D/A converter having resolution of M bits to the output of the D/A converter having resolution of L bits and supplying a lens with a current according thereto. CONSTITUTION:An object glass current setting circuit for an electron microscope is formed with D/A converters 2 and 3 having resolution of M bits and L bits respectively as well as with an adder 4 and a constant-current circuit 5. And the data corresponding to signals from a focal length regulating knob are given from a control circuit 1 to the D/A converters 2 and 3, the D/A converter 10 of N bits is formed by piling up the lower A bits of the converter 2 and the higher A bits of the converter 3 to add the output of the converters 2 and 3 together by weighting according to a bit number of a non-piled part of the converter 3 and supply an exciting coil LD with the current according thereto. Consequently, it is possible to change the exciting current extremely finely and obtain a required focussing step.