Based on three demodulations, this smart voltmeter is capable of the automatic measurement of peak value, effective value and average value of direct voltage and any periodic AC signals. By applying excluding extreme value averaging filter algorithm, the voltmeter provides better resistance to burrs and low amplitude random noise, which as a result, improved the systematic accuracy. The voltmeter input frequency is ranging from 1 Hz to 1 MHz, with an amplitude range from 0.1 V to 10 V and a relative error less than ±1%. With its tiny size and light weight, this voltmeter drastically promoted the function upgrade of digital multimeter products on the market.