An analog to digital converter Ivhich is applicable to mass production of digital multimeter is developed. The solid state digital instrument has accuracy $pm$0.1% of feading $pm$1 digit over 1 mV to 1000 volts with high input impedance and automatic function. All possibility to affect the distortion of A/D converter is tudied. As a result, useful linearity with high temperature stability of integrating waveforms is achieved by the very simplified circuit configuration to assure the proposed accuracy under various ambient condition.