Abstract: Taking the fundamental principle of log-antilog true rms-responding ac-to-dc converters as the starting point, the paper points out that the bandwidth of the operational amplifier in input log converter is frequently the primary source determining the upper limit on the ac-to-dc conversion bandwidth, and from the theory, drawing the formula of the bandwidth of the converter.The work has important significarce for the research of high precision DMM.

Key words: AC-DC converter, Operational amplifier, Frequency domain analysis.