Equivalent input voltage noise density can be calculated with the following equation:
Figure 1. Equation
For example, 50uV noise at 400kHz bandwidth would have an 80nV/root Hz equivalent voltage noise density:
Figure 2. Equation example
Additional Information
Other mathematical equation and explanation related to frequency domain, you can find in
The Fundamentals of FFT-Based Signal Analysis and Measurement in LabVIEW and LabWindows/CVI.