‘How do I convert it into Acceleration vs time data using ifft function.’
Thet depends entirely on what the data are. If you have only magnitude data, then inverting it is impossible.
If you have complex acceleration data, you can transform it using ifft (although it would be necessary to know what the data are and what the frequency vector is). If you have magnitude and phase data, you can convert them to complex frequency domain data, and then use ifft.
If you have ‘one-sided’ complex Fourier transform data (the frequency vector going from 0 to the Nyquist frequenncy), it would be necessary to flip a complex-conjugate version of that and concatenate it to the end of the data you have, then do the ifft. The time vector would then go from 0 the length of the reconstructed vector, with the sampling interval being 1/(2*Nyquist_frequency).
