Filter 2 (high-pass)

CoE Index 0x80nD:1A "Filter 2 Settings" provides another digital filter with predefined properties for processing the signal. A digital high-pass filter is available here, for example, to eliminate the DC component of the input signal so that only the AC component of the signal is processed. However, it should be noted that the absolute signal remains within the technical measuring range, i.e. any positive DC component (offset) reduces the remaining measurable positive range by the same amount.

Parameter: "Filter 2 Settings" (Index 0x80nD:1A) [ENUM]

The high-pass filter is of type IIR 1st order and therefore has a slope of +20 dB/dec.
Depending on the set cut-off frequency, the following actions lead to a settling time

• Change of the DC component (rapid change of the DC bias voltage).
• Change the setting in Filter 2 from “Off” to a filter cut-off frequency.

Example: A 10 Hz, ±1 V sine wave is applied simultaneously to Ch1 + Ch2 of an EL4374 using a signal generator.
Setting: Ch1 without filter treatment, Ch2 with filter 2 Settings = "HP 1 Hz". With (A) an electrical offset of +1 V is added, the filter eliminates this within approx. 3 s. With (B) the electrical offset is removed again.

Fig.154: Example signal generator, sine Ch.1 + 2

Effect of rapid temperature changes on the filter Both firmware and hardware are involved in this high-pass filter. The controller compensates the DC component in the output signal. Since hardware is involved, the filter has a slight temperature coefficient, i.e. rapid temperature changes can lead to offset shifts in the output signal. In this case, the high-pass filter has to settle again, which takes a relatively long time, especially at the lowest cut-off frequencies. Continuous operation at a constant ambient temperature is therefore advantageous.