Siemens CPU 31 Series Technical Data Manual page 147

Integrated hardware low-pass filter
An integrated low-pass filter attenuates analog input signals of channel 0 to 3. They are
attenuated according to the trend in the figure below.
Figure 6-3
Note
The maximum frequency of the input signal is 400 Hz.
Input filters (software filter)
The current / voltage inputs have a software filter for the input signals which can be
programmed with STEP 7. It filters the configured interference frequency (50/60 Hz) and
multiples thereof.
The selected interference suppression also determines the integration time.
At an interference suppression of 50 Hz the software filter forms the average based on the
last 20 measurements and saves the result as a measurement value.
You can suppress interference frequencies (50 Hz or 60 Hz) according to the parameters set
in STEP 7. A setting of 400 Hz will not suppress interference.
An integrated low-pass filter attenuates analog input signals of channel 0 to 3.
CPU 31xC and CPU 31x, Technical Data
Manual, 01/2006 Edition, A5E00105475-06
Low-pass characteristics of the integrated filter
Technical data of CPU 31xC
6.6 Technical data of the integrated I/O
6-35