inngangsfilter

An inngangsfilter, or input filter, is a circuit placed at the input of an electronic device to attenuate unwanted signals or disturbances before they reach the internal circuitry. The primary purpose is to improve signal integrity, reduce electromagnetic interference, protect components, and help meet EMC standards. Input filters can be passive or active. Passive implementations use resistors, capacitors, and inductors (for example RC, LC, or pi-filter configurations). Active filters employ operational amplifiers to achieve higher-order responses or better input impedance characteristics but require power.

Common filter types include low-pass filters to remove high-frequency noise, high-pass filters to block DC offsets, band-pass filters to pass a specific frequency range, and notch filters to suppress narrowband interference such as mains hum. In digital systems, input filters also serve as anti-aliasing filters before analog-to-digital conversion.

Placement is important: the filter is usually located as close as possible to the device input to minimize the influence of external noise on internal traces. The design is influenced by the input impedance of the subsequent stage and the source impedance; mismatches can alter the intended response and cause reflections.

Common practical implementations include EMI filters on mains inputs, which may combine X and Y capacitors with a common-mode choke, and ferrite beads or feed-through capacitors for high-frequency suppression on signal lines. Designers balance attenuation with potential effects on timing, phase, and amplitude, and must consider applicable standards and regulatory requirements.