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SAR ADC SNR variations for different FFT numbers (Read 4397 times)
niloun
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Re: SAR ADC SNR variations for different FFT numbers
Reply #30 - Oct 11th, 2017, 11:35am
 
RobG wrote on Oct 11th, 2017, 7:23am:
Glad that worked. Waking up just means initial transients in the reference and/or bias currents are settling out.

Thanks, since I haven't used voltage reference generators ( just a DC voltage as supply) and the implemented comparator is a dynamic StrongArm one without preamp and the rest are digital circuits it's a bit confusing me.
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Frank_Heart
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Re: SAR ADC SNR variations for different FFT numbers
Reply #31 - Nov 16th, 2017, 3:23pm
 
sheldon wrote on Oct 3rd, 2017, 9:02am:
Niloun,

 Comments:
1) Carl's approach includes an integer number of clock and signal
   periods, for example: 13 periods of the signal and 256 periods
   of the clock. So, the signals will be periodic in the data record
   used for the FFT
2) The reason for using the approach Carl suggest is to assure that
   the clock and signal are not harmonically related. For example,
   if the clock is 100MHz and the signal is 10MHz, there are only
   10 lsb levels tested in the simulation. So the SNDR/SINAD will
   be correct, but the SNR and THD will be incorrect. Since all the
   energy will be concentrated in 10 bins of the FFT
3) If you follow the guidelines that Carl suggests, you can use the
   Rectangular window which means that there is no signal spreading
4) For Delta Sigma, #1 is still applied but you will need to use a
   window function

                                                                    Sheldon


Hi, Sheldon,

   It sounds correct to me,

   if the clock is 100MHz and the signal is 10MHz, there are only
   10 lsb levels tested in the simulation.      So the SNDR/SINAD will
   be correct, but the SNR and THD will be incorrect. Since all the
   energy will be concentrated in 10 bins of the FFT

   
   But why SNDR/SINAN is correct, while SNR/THD not?

Thanks
Feng



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