Calculating RMS Noise from Sensor Spectral Noise Density Datasheet Specifications
Root-mean-square noise equals spectral density multiplied by the square root of equivalent noise bandwidth, adjusted for flicker corner frequency integration.
Input Referred Noise
Equivalent Parameter
Signal-to-noise ratio calculations evaluate system sensitivity by referring all downstream noise sources back to the primary input terminal. The performance parameter known as input referred noise represents the equivalent noise voltage or current source placed at ideal noiseless amplifier inputs. Dividing total measured output noise by system voltage gain isolates amplifier input noise performance from gain factors.
This metric enables direct comparison of preamplifier components regardless of circuit gain configurations.
Back Calculation
Analog signal chains combine noise contributions from input resistors, amplifier input stages, and subsequent digitizer circuits. Calculating input referred noise requires dividing total output noise voltage by the overall closed-loop gain of the circuit. Low-gain amplifier stages expose downstream ADC noise, increasing equivalent input noise levels.
High-gain front ends suppress downstream noise contributions to negligible levels relative to input stage noise.
Front-End Optimization
Sensor interface design for low-output transducers requires minimizing noise sources at the front-end amplifier stage. Optimizing input referred noise allows detection of microvolt-level sensor signals above background electronic noise floors. Thermocouples and strain gauge bridges benefit directly from ultra-low input noise preamplifiers.
Matching source resistance to amplifier optimum noise impedance minimizes total input-referred noise power. Printed circuit board layout minimizes stray capacitance and trace resistance that degrade front-end noise performance. Temperature drift alters internal semiconductor noise currents, elevating equivalent input noise at high ambient temperatures.
Sensitivity Limit
True-RMS voltmeters measure total output noise across specified measurement bandwidths with inputs shorted or terminated in reference impedances. Input noise density expressed in nanovolts per root hertz provides bandwidth-independent comparison values. High source resistance increases thermal Johnson noise beyond intrinsic amplifier input noise limits.
Exceeding recommended operating temperature ranges increases carrier recombination noise in front-end transistors.