Thermal noise of passive components makes big influence on overall noise performance. Values of passive components should be carefully evaluated, especially in case of low noise design. Otherwise, you will not be able to take advantage of low noise performance of active components.

RMS value of thermal noise generated by resistor within Δf frequency range could be calculated by formula:

Vn=√(4*Kb*T*R* Δf) (V)

Where Kb is Boltzmann constant: Kb = 1.380?6504e-23 (joules/kelvin),

T is absolute temperature in kelvin,

R is resistance in Ohm and Δf is frequency range in Hz

This calculator will help you to evaluate noise performance of resistors.

** 100.0** Ohm resistor at **25.0** °C within **20.0** Hz to **20000.0** Hz frequency band will have :

Noise Spectral Density = **1.283185e-9** V/√Hz or **1.2832** nV/√Hz

Noise within desired bandwidth = **1.813790e-7** V or **0.1814** uV

Dynamic range re 1V RMS = **134.8** dBV

Dynamic range re 3V RMS = **144.4** dB

Dynamic range re 10V RMS = **154.8** dB

As a comparison, here is noise data for some commonly used low noise operational amplifiers:

Part Number |
Input Noise Density |
Vendor |

LME49713 | 1.9nV/√Hz | National Semiconductor |

LM4562 | 2.7nV/√Hz | National Semiconductor |

LME49990 | 0.9nV/√Hz | National Semiconductor |

OPA1611 | 1.1nV/√Hz | Texas Instruments |

OPA211 | 1.1nV/√Hz | Texas Instruments |

OPA827 | 4nV/√Hz | Texas Instruments |

AD797 | 0.9nV/√Hz | Analog Devices |

AD8597 | 1.1nV/√Hz | Analog Devices |

LT1028 | 0.85nV/√Hz | Linear Technology |

LT1115 | 0.9nV/√Hz | Linear Technology |

The recent samples of SSM2019 from Analog Devices measure just under 1.0nV/RtHz.

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any change to bandwidth high limit still gives 20khz result ie 1.28nv/hz

Yes, and this is how it should be. Because it is Noise Spectral Density. Bandwidth changes “Noise within desired bandwidth”.