Exact meaning of dB?
Typically the noise floor is the ratio of the noise with input shorted to the maximum output. 

If you have an amplifier capable of 1V RMS output and the noise floor is -120dB then the noise is 1uV RMS at the output with the input shorted. Use \$20\log(\frac{V_N}{V_{MAX}})\$ since it is voltage, not power. 

Divide by the gain to get the input-referred noise voltage.