Thermal Noise Measurement

Application

This circuit models the input-referred thermal noise of a 100 kOhm source observed across a high-impedance measurement load. It is useful for estimating the noise floor seen by sensor, amplifier, and data-acquisition inputs.

Design Context

VNOISE represents a 100 kOhm source at 127 degC over a 10 MHz noise bandwidth. RLOAD is 1 MOhm, representing a practical high-impedance measurement input, and the Noise voltage probe monitors the loaded output relative to ground. The schematic uses the standard circular source symbol and one compact grounded return loop.

Expected Behavior

The 2 ms transient plot shows a bipolar, zero-centered waveform. With the shown resistance, temperature, bandwidth, and noise ratio, the modeled output is approximately -220 uV to +190 uV, with a mean close to 0 V.

Suggested Experiments

• Reduce Bandwidth from 10 MHz to 1 MHz to lower the noise amplitude.
• Change Resistance from 100 kOhm to 10 kOhm to compare source impedances.
• Return Temperature to 27 degC to observe the lower room-temperature noise level.

Featured Component

The Thermal Noise Source converts resistance, absolute temperature, bandwidth, and noise ratio into a time-varying voltage suitable for transient noise-floor studies.