Siemens SIMATIC ET 200AL, System Manual

Page: 2058 / 2255

What you should know about analog technology
2.9 Interference frequency suppression
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
27
2.9 Interference frequency suppression
Definition
The interference frequency suppression function is applied to analog input modules to
suppress the noise caused by the frequency of the AC voltage supply used.
The frequency of the AC voltage supply is likely to have a negative effect particularly on the
values returned by measurements in the low voltage range and on thermocouples.
Parameter assignment
The line frequency at which the system is operated can be set by means of module
parameters, for example, in STEP 7.
Figure 2-14 Interference frequency suppression
The conversion time changes depending on the set interference frequency suppression. The
data sheets of the respective module define this dependency.
Please note the following when selecting interference frequency suppression:
The higher the frequency setting, the shorter the conversion time.
Note
Line frequency
Always select the interference frequency based on the line frequency used. Mismatch errors
must be expected if a frequency that deviates from this line frequency is set, for example, to
reduce the conversion time. In this case, the specified technical data and, in particular,
precision are possibly outside the specified range.
Smoothing
By smoothing analog values, you achieve an additional improvement for the suppression of
interference frequencies. For more information on the smoothing of analog values, refer to
the section Smoothing (Page 54).