Abstract
Industrial instrumentation signals to analog input unit of process control computer are usually low level differential d.c. voltage between the lines of the signal wire pair. For this type of signals, the common mode voltage appearing between each line of the signal wire pair and the system ground is considered a major source of input error voltage. Increasing the common mode rejection ratio (CMR) is important to improve the accuracy of analog input signals. The CMR is defined as the ability of the input system to distinguish the signal voltage from the common mode voltage. It has been extremely defficult to cope quantitatively with these problems because of the multiplicity of the common mode voltage source, problems encountered in the input scanner of a computer, necessity to take the distributed constants of signal lines in consideration and the like. Described in this paper is the derivation of equivalent circuits expressible by the lumped constants and equations of the CMR for each common mode voltage source in a typical analog input system of process control computer. By using these circuits and equations, proper handling of the common mode voltage will be realized. Difficulties in quantitative analysis of errors of analog input signals have been overcome.
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