inputstøjen

Inputstøjen, or input noise, is the unwanted random variation present at the input of an electronic system. It represents the intrinsic noise that a device presents to the signal before amplification or processing, and it sets a fundamental limit on the smallest detectable signal.

The main sources of input noise are thermal (Johnson–Nyquist) noise from resistive elements, shot noise from discrete charge carriers in diodes and transistors, and flicker (1/f) noise that dominates at low frequencies. External factors such as electromagnetic interference, grounding issues, and inadequate shielding can also contribute. The combination of these sources determines the overall noise that an input stage contributes to the signal chain.

Measured performance is often described using input-referred quantities. Noise can be expressed as a noise density, such as nV/√Hz for voltage noise or pA/√Hz for current noise, which describes the noise level per unit bandwidth. The equivalent input noise (EIN) or input-referred noise is the total noise that would be present at the input to produce the observed output noise, allowing comparison between devices independent of gain. The signal-to-noise ratio (SNR) is linked to input noise: higher input noise lowers SNR for a given signal amplitude and bandwidth.

Design and usage considerations include selecting low-noise components, minimizing source impedance where appropriate, ensuring sound grounding and shielding, using short and well-shielded cables, and employing careful layout and filtering. Inputstøjen is a critical factor in audio equipment, sensors, and instrumentation where detecting weak signals is essential.