Filter 1 (low-pass)

A digital filter with predefined properties is available in the analog channel. Depending on the setting, it can take on the characteristics of a filter with finite impulse response (FIR filter) or with infinite impulse response (IIR filter).

The filter properties of all input channels are set via the 1st channel, the filter settings of the other input channels have no function.

This filter still works on the integer values and is therefore independent of the interface.

In CoE 0x90n0:1B “Sampling Rate” the current conversion rate is displayed in [Hz], depending on the filter setting

Fig.8: Index 0x9000:1B, Sampling Rate

Parameter:

- Filter activation: CoE Index 0x80n0:06

The filter is enabled by default in this analog channel, see below for properties. Channel properties with disabled filter:

Fig.9: Example square wave signal 0->4.52 V on channel 1, filter disabled, EtherCAT cycle time 100 µs

- Filter type: CoE Index 0x80n0:15

The available options are:

• FIR filter
  
  The filter performs a notch filter function and determines the conversion time of the terminal. The higher the filter frequency, the faster the conversion time. A 50 Hz and a 60 Hz filter are available. Notch filter means that the filter has zeros (notches) in the frequency response at the filter frequency and multiples thereof, i.e. it attenuates the amplitude at these frequencies. The FIR filter operates as a non‑recursive filter.

Fig.10: FIR 50 Hz filter

Filter data FIR:

• IIR filter
  
  The filter with IIR characteristic is a time-discrete, linear, time-invariant 1st order low-pass filter (-20 dB/decade), which can be set in 8 levels, i.e. cut-off frequencies (level 1 = weak recursive filter, up to level 8 = strong recursive filter) The IIR can be understood to be a sliding average value calculation after a low-pass filter.

Fig.11: Comparison of square wave signal 0.2 Hz/4.5 V, EtherCAT cycle time 100 µs, left filter disabled, right IIR8