Take a 'NAP' and enjoy a moment of silence

Description

When measuring ultra small signals with accuracy, an ultra-low noise amplifier is needed.
In AN-159, engineers from Linear Technologies have made such a device, which referred input voltage noise is down to about 500pV / √Hz, and being able to measure wideband signals from 10Hz to 2MHz.

Based on the works from Linear, an ultra-low noise amplifier (named EmoeNAP) capable to measure low-noise LDO/OPA's wide band output noise, and many other interesting measurements was made. Using four opa2211/opa1612 from texas instruments, building an wideband ultra-low noise pre-amplifier stage with 80dB voltage gain is achivable. But caution still has to be made during the building process.

To prevent nearby EM fields interference, the circuits and battery are built into a 2-cascade shielding structure(A cookie box+copper clad board with Mu-Metal film attached inside), to minimize both the low frequency and high frequency interferences.

More details can be found on external lin

Details

With input shorted, 10Hz-100kHz output is about 1.76mVrms, which indicates the referred input voltage noise density of the EmoeNAP is about 450pV/√Hz (at 1kHz).

And connect a 50-ohms load to input, the 10Hz-100kHz output is about 3.654mVrms.

The theoreticle Johnson-Nyquist noise (thermal noise) of a 50-ohms resistor in 10Hz-100kHz bandwidth is 316nVrms, combined with EmoeNAP's noise floor (1.76mVrms/10000)，the total input voltage noise should be about 361nVrms, and after 80dB voltage gain, 3.61mVrms is very close to 3.654mVrms. This test verified the EmoeNAP's extraordinary ability to measure a 50 ohms resistor's thermal noise precisely.

And I alse tested the LM399 voltage reference's output voltage noise.

According to LM399's datasheet, we can estimate it's output voltage noise in 100Hz-100kHz should about 30uVrms (the best). And the datasheet also gives the voltage noise in 10Hz-10kHz, we can see it varies.