Instrumentation Problems Caused by Common Mode Interference Conversion

In measuring the level of conducted interference on active signal lines, instruments are connected across the lines. This paper describes how the connection of certain measuring instruments across balanced circuits can produce both system errors and data errors. The error considered here is caused by the conversion of a common mode interference voltage to a differential voltage by the input network of the measuring device. Equations are developed to provide a mathematical description of the problem. Some techniques for reducing the effects of the interference are inferred from the equations. The behavior predicted by the analysis is compared with the results of a laboratory investigation.