SNRin

SNRin refers to the input signal-to-noise ratio of a device or system, measuring how much the desired input signal stands out from the noise at the input over a defined bandwidth. It is commonly used for sensors, front‑end amplifiers, analog-to-digital converter inputs, and radio receivers to indicate how clean the input signal is before processing. SNRin is expressed in decibels (SNRin_dB = 10 log10(P_signal_in / P_noise_in)) and, for a system where signal and noise are represented as voltages into the same source impedance, approximated as SNRin_dB ≈ 20 log10(V_signal_rms / V_noise_rms). The value depends on the source, the input filtering, and the intrinsic noise added by the device, sometimes described as input-referred noise.

SNRin is used to predict dynamic range and to assess how subsequent gain stages will affect overall performance. It is distinct from SNRout, which measures signal-to-noise after amplification or processing. The relationship to the device’s noise figure (NF) is NF = SNRin / SNRout in linear terms, or NF_dB = SNRin_dB − SNRout_dB. Measurement of SNRin typically involves applying a known reference signal at the input and measuring the resulting signal and noise within the bandwidth of interest, or using calibrated input suppression to isolate noise contributions.

See also: signal-to-noise ratio, input-referred noise, noise figure, dynamic range.