Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.12 Interference voltage suppression
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
33
Common mode rejection
The table below sets out the possible common mode rejection specifications in the technical
data of an analog input module:
Interference voltage suppression for f = n x (f1 ± 1 %), (f1 = interference frequency), n = 1, 2, ...
Common mode interference
> 100 dB
Common mode voltage, max. 10 V
If the interference frequency is configured as 50 Hz, the specified interference voltage
suppression for common mode interference will only apply for the frequencies 50 Hz ±1 %,
100 Hz ±1 %, 150 Hz ±1 %, etc. Immunity is lower for all other frequencies. Interferences
caused by the line voltage distort the measured utility signal in the ratio of 100,000 : 1.
An interference voltage amplitude of 1 V, for example, will affect the utility signal by 10 μV.
The Common Mode voltage (U
CM
) defines the maximum permitted deviation of the potential
between two channels and between one channel and analog ground. The interference
voltage suppression defined in the technical specifications must not be exceeded. If the
maximum common mode voltage specified, for example, 10 V, is exceeded, the analog
signal is processed incorrectly. Depending on the module used, this error may be diagnosed
and an error message output. The maximum common mode voltage may be exceeded as a
result of varying input or overly long lines.
Series mode rejection
The table below sets out the possible series mode rejection specifications in the technical
data of an analog module:
Interference voltage suppression for f = n x (f1 ± 1 %), (f1 = interference frequency), n = 1, 2, ...
Series mode interference
> 60 dB
If the interference frequency is configured as 50 Hz, the specified interference voltage
suppression for series mode interference will only apply for the frequencies 50 Hz ±1 %,
100 Hz ±1 %, 150 Hz ±1 %, etc. Immunity is lower for all other frequencies. Overall,
interference caused by the line voltage distorts the utility signal measured by 1000 : 1.
For example, an interference voltage amplitude of 1 V will affect the utility signal by 1 mV.