FFT results back to ADC voltage?
Obviously you can just do the IFFT.

If your question is instead "what waveform does the i'th bin in the FFT output represent", that depends on the particular version of FFT you used.  The safe answer is to look up the documentation for your FFT.  For instance, a normal FFT vs. a real FFT are two different things.  There is also the issue of the ordering (zero frequency at the beginning or at zero?) and overall scale factor.  There are standard ways to do this, but some special purpose library might make non-standard choices -- for instance if their only purpose is to plot a spectral density.

The most common "regular" FFT will actually never produce the result you suggested, as the basis waveforms are complex, and your input is real valued.  For instance, the waveform: (1/8) * cos(2*pi*x*4/16) for x=0..15 (a cosine with exactly 4 cycles in 16 samples), will have an FFT of [0,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0].  On the other hand the real FFT will have the result: [0,0,0,0,1,0,0,0,0] (only 9 elements).

The general form is that an N point RFFT with a '1' in index F represents an ADC waveform:  (2/N) * cos(2*pi*F*x/N).

Again, you really have to check the documentation for the function you are using.  What I have described is the most standard definition of the FFT.  For instance here is the documentation for numpy and scipy:

http://docs.scipy.org/doc/numpy/reference/routines.fft.html#module-numpy.fft