Hi,

I am trying to calculate SNR of my sampling circuit by doing the following:
1. Transient sim using spectre.  fin/fsample = M/N, where 2M<N, N and M are prime to each other. N is the total number of samples (eg. 256)  
2. In ADE, apply calculator function dft to my transient output waveform --- dft(waveform, 2n, 514n, 256, 'rectangular')
3. Export the dft data to a file.
4. In matlab, I loaded the exported data, then
VFund=max(mag) //find the fundamental
VNoise=sqrt(sum(mag.^2)-VFund.^2) // Noise
snr = 20*log10(VFund/VNoise
NumberOfBits = (snr-1.76)/6.02

Am I correct?  Did I calculate SNDR instead?
When tuning the switches  of S/H circuit, what is the right way to calculate the number of bits the circuit has already achieved?  

Thanks for the help.

LRC