Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.10 Common mode interference (UCM)
Analog value processing
30
Function Manual, 06/2014, A5E03461439-AC
Example
The following figure of an analog module with two inputs (Ch
1
/Ch
2
) shows the coupled
interference voltages at the inputs (U
CM1
/U
CM2
) as well as an interference voltage coupled
between these inputs (U
CM3
). Common mode rejection indicates the extent to which these
interference signals can be suppressed. It is calculated using the following equation:
CMR [dB] = 20 · log (U
CM
/ U
a
)
Figure 2-15 Common mode voltage coupled between two inputs
If there is common-mode voltage (U
CM
), the equation is solved for U
a
to calculate the
measuring error (U
a
):
Note
The common mode rejection (CMR) is detailed in "Common mode interference" in the
technical data of the device manual for your analog module.