Hi.
Because the S/H is the first stage, so it should have 14 bits accuracy. But usually we don't calculate this accuracy for each part of the stage such as switches. Instead we can calculate the ess (steady-state error) for the whole stage and it should be at least 1/2^14. But if you want to desig na really good ADC, that's better always to design a more accurate stage in order to have the appropriate accuracy in all process and temperature corners. You can simply calculate ess by applying a step input (pulse input with large period) and watching the output transient response.
For FFT, you can peak up output voltage samples (by using .tran statement in HSpice) and use this data in MATLAB® to find SNDR, SFDR and other dynamic specifications. But be careful in using FFT function. e.g. you want to do a 31/64 FFT (Nyquist rate) on your output signal. You MUST set the input freq in simulator to (31/64 * fs), where fs is the sampling freq. and you MUST let the transient response at least pass 64*Ts, where Ts is sampling period (i.e. 1/fs).
Regards,
EZT