back when I was characterizing some ultra low noise things, we found that using a battery as the power source, and fixed resistor dividers with filter capacitance got it done.
Resistors/Temperature low enough in value that thermal noise did not rise up, and the capacitor such that the BW was like 0.01Hz or something similar.
We were looking at flicker noise at the time and we could actually see the transitions associated with single electrons from a CMOS amplifier.
Neat stuff, electricity is discrete units of charge if you look close enough. 8-)