Thermal Noise
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Any component that has resistance also has some amount of thermal noise (also called Johnson noise) that is generated by the random motion of electrons inside it. The noise is less as the temperature goes down and also as the resistance decreases. Larger resistance will have more noise as will higher temperature.
This calculator finds the RMS noise Voltage for a resistance at a given temperature, resistance and bandwidth.
Vn=√4*Kb*T*R*F
Kb is the Boltzmann constant, T is the temperature in Kelvin, R is the resistance in Ohms and F is the frequency bandwidth in Hz
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If you use the default values in the calculator, you will find that a 1k resistor at 25°C (77°F) will have 0.287 microvolts of noise across a bandwith of 5000 Hz, and the noise floor is at -130.8 dBv.
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