Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.11 Series mode interference (USM)
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
31
2.11 Series mode interference (USM)
Definition
Series mode interference is interference voltages and currents affecting the connection lines
in opposite directions. They have opposite polarity in the positive and negative directions.
Series mode interference currents cause a voltage drop at the input impedance that has the
effect of an interference voltage.
Causes
Series mode interference is caused by capacitive or inductive coupling. Inductive coupling
generates a magnetic flow that is radiated between adjacent current-carrying conductors.
This induces an interference voltage in the conductor. Galvanic coupling is given where
different circuits influence each other due to common conductor sections, e.g. common
ground connection.
The diagram below shows series mode interference as a voltage source (U
SM
) connected in
series to the actual measuring signal (U
M
). The index "SM" stands for "Series Mode". Series
Mode rejection indicates the extent to which these interference signals are suppressed. It is
calculated using the following equation: SMR [dB] = 20 · log (U
SM
/ U
a
)
Figure 2-16 Capacitive or inductive interference coupling to the signal cable
The voltage (U
a
) measurement error is calculated by solving the equation for U
a
.
The following equation shows the calculation of current (I
a
) measuring errors using a current
input.
Note
The series mode rejection (SMR) is detailed in "Series mode interference" in the technical
data of the device manual for your analog module. The level of input resistance (R
ein
) is also
specified in the technical data.