> Is this value useful in calculating signal to noise ratio? Or what fun calculations can I do with this number?

To convert the spectral density \$\tilde v\$ (in nV/√Hz) to a voltage (in V<sub>RMS</sub>), you need to multiply it by the square root of the bandwidth:
$$
v_\mathrm{RMS}=\tilde v \cdot \sqrt{\Delta f}
$$
For example, if the op-amp is a [TLC071][1], with equivalent input noise voltage density of 7 nV/√Hz, and audio bandwidth, the total equivalent input noise is:

- 7 nV/√Hz ⋅ √(20000 Hz - 20 Hz) = [0.99 μVrms][2]

Assuming this is the dominant noise source, if the noise gain of your amp is 10× (= +20 dB) the output noise is then:

- 0.99 μVrms ⋅ 10 = 9.9 μVrms

Note that the actual noise curve is not always 7 nV/√Hz, it [slopes up at low frequencies][3]:

[![TLC071 equivalent input noise voltage vs frequency][4]][4]

Turns out that's ok because the X axis is logarithmic and the units of noise are not, so it has very little effect on the total (the non-flat part below 1 kHz is only 5% of our total bandwidth, measured linearly).  If you need a more accurate value you can (numerically) integrate and get the area under the (squared) curve:
$$
v_\mathrm{RMS}=\sqrt{\int^{f_2}_{f_1} \! \tilde v(f)^2\,df}
$$
Or simulate it in SPICE (I get 0.82 μVrms EIN).

Also, real circuits do not have ideal brickwall HPF and LPF filters, so you can compensate for this using "[brickwall correction factors][5]" to calculate the "[equivalent noise bandwidth][6]".

If your circuit has 1-pole filters, for instance, the total noise would then be

- 7 nV/√Hz ⋅ √(1.57 ⋅ (20000 Hz - 20 Hz)) = 1.24 μVrms

(Sanity check: SPICE with noiseless filters measures at 1.22 μVrms.)


  [1]: http://www.ti.com/product/TL072
  [2]: http://www.wolframalpha.com/input/?i=7nV%2Fsqrt(Hz)*sqrt(20%20kHz)%20to%20microvolt
  [3]: https://en.wikipedia.org/wiki/Flicker_noise
  [4]: https://i.sstatic.net/nSEmc.png
  [5]: https://electronics.stackexchange.com/q/281155/142
  [6]: http://analog.intgckts.com/equivalent-noise-bandwidth/