This paper presents the study of common-mode (CM) voltage measurements for IEC62684 regulatory. Due to the digitization error caused by the limited vertical bit number of the analog-to-digital converter (ADC) in oscilloscopes, the CM voltage spectrum measurement of the flyback converter with AC source occurs significant inaccuracy. In order to avoid the digitization error, the CM voltage measurement of the flyback converter with DC source is proposed in this paper. The simplified equivalent circuits of the flyback converter with AC and DC input sources are derived to calculate the ratio of their CM voltages. With 64-bit circuit simulation software SIMPLIS®, the digitization error can be significantly reduced to have high-resolution measurements for the verification of the calculated CM voltage ratio. The ratio of the calculated CM voltages can be used to modify the measurement result with DC source to obtain the equivalent result with AC source.