Siemens Simatic S7-300 Technical Data Manual page 145

Measurement principle
31xC CPUs use the measurement principle of actual value encoding. Here, they operate
with a sampling rate of 1 kHz. That is, a new value is available at the peripheral input word
register once every millisecond. This value can then be read via user program (e.g. L PEW).
The "previous" value is read again if access times are shorter than 1 ms.
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.
Internal
signal level
Figure 6-3
Note
The maximum frequency of the input signal is 400 Hz.
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
Attenuation
< 1 %
Attenuation
< 1 0 %
100 %
63 %
50 Hz
Low-pass characteristics of the integrated filter
6.6 Technical data of the integrated I/O
strong
attenuation
200 Hz
Technical data of CPU 31xC
Inadmissible
input frequency
Input frequency
400 Hz
6-35