An analog interface has been developed in order to perform very accurate noise spectral analysis with currently available instrumentation. It employs the device under test as a front‐end element, whose input‐referred noise voltage is amplified by a feedback loop which adds an extremely small noise contribution, followed by two cascaded low‐noise amplifiers. The interface is suitable for spectral analysis in the 100 mHz–100 kHz frequency range. Its intrinsic noise contribution is estimated around 1 nV/&sqrt;Hz at 100 mHz and 50 pV/&sqrt;Hz at frequencies beyond 1 kHz. ©1994 American Institute of Physics.