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 = 1.380649e-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.
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 |
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