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Dec 4th, 2020, 2:14am
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 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #15 - Oct 04th, 2017, 11:59pm   sheldon wrote on Oct 4th, 2017, 12:58pm:Niloun,   The noise should not increase with the number of bins, just guess, but there must be something in the way you do the calculation. Are you adding noise voltage or noise power? I use power as I said above:%noise = [s(1:sigbin-1);s(sigbin+1:end)];%snr = 10*log10( s(sigbin)^2/sum(noise.^2) )When I use a Hann Window (which reduces signal discontinuity) there is negligible SNR variations, I guess it is something to do with that, If the calculations were wrong, ideal sine (only containing quantization noise) would also give wrong results. Back to top IP Logged
 RobG Community Fellow Offline Posts: 569 Bozeman, MT Re: SAR ADC SNR variations for different FFT numbers Reply #16 - Oct 8th, 2017, 4:43pm   With coherent (prime) sampling and rectangular window I see only a dB or so of SNDR variation as I increase the number of samples. Perhaps it is your "integer sampling" instead of prime sampling that is throwing it off. I suppose the first few samples of your ADC could also be bad. Back to top IP Logged
 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #17 - Oct 8th, 2017, 9:57pm   RobG wrote on Oct 8th, 2017, 4:43pm:With coherent (prime) sampling and rectangular window I see only a dB or so of SNDR variation as I increase the number of samples. Perhaps it is your "integer sampling" instead of prime sampling that is throwing it off. Thanks RobG, I stated above that by "integer cycles" I mean integer/prime number of cycles. For example I use 7 cycles for N=64.RobG wrote on Oct 8th, 2017, 4:43pm:I suppose the first few samples of your ADC could also be bad. I have checked the digital output codes both manually and by Matlab, I don't see much difference between the first samples and the rest but maybe you are right I must try. Back to top « Last Edit: Oct 9th, 2017, 4:44am by niloun »     IP Logged
 RobG Community Fellow Offline Posts: 569 Bozeman, MT Re: SAR ADC SNR variations for different FFT numbers Reply #18 - Oct 9th, 2017, 9:21am   Sorry I missed that you were doing what I call prime sampling. I did not see where you told us how much it is varying - what is the change you are seeing? You seem to be doing everything correct as far as I can tell. Back to top IP Logged
 DanielLam Community Member Offline Posts: 76 Re: SAR ADC SNR variations for different FFT numbers Reply #19 - Oct 10th, 2017, 11:37am   Hi Niloun,Can you post pictures of what your FFT's look like before and after increasing the number of bins? (say number of points > 1024). The examples where you say increasing the number of bins helps the ENOB.I am suspecting you might be getting spectral leakage. That might be why changing bins with a Hann Window does not change much. Back to top IP Logged
 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #20 - Oct 10th, 2017, 12:14pm   RobG wrote on Oct 9th, 2017, 9:21am:Sorry I missed that you were doing what I call prime sampling. I did not see where you told us how much it is varying - what is the change you are seeing? You seem to be doing everything correct as far as I can tell. For N=64 cycles=7without windowing: SNR=22.7184 ENOB=3.4815with windowing:     SNR=41.6099 ENOB=6.6196For N=4096 cycles=409 without windowing SNR=38.1335 ENOB=6.0421with windowing:     SNR=40.0398 ENOB=6.3588 Back to top IP Logged
 DanielLam Community Member Offline Posts: 76 Re: SAR ADC SNR variations for different FFT numbers Reply #21 - Oct 10th, 2017, 12:21pm   Hey Niloun,I asked for the pictures because it will tell us if you have spectral leakage or not. I can't just tell from the numbers.Basically, I am guessing your 64 pt FFT with rectangular window (what you are calling no window) looks like the 2nd picture on this page (triangular towards the bottom of the fundamental tone). https://www.maximintegrated.com/en/app-notes/index.mvp/id/1040 Back to top IP Logged
 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #22 - Oct 10th, 2017, 12:34pm   DanielLam wrote on Oct 10th, 2017, 11:37am:Hi Niloun,Can you post pictures of what your FFT's look like before and after increasing the number of bins? (say number of points > 1024). The examples where you say increasing the number of bins helps the ENOB.I am suspecting you might be getting spectral leakage. That might be why changing bins with a Hann Window does not change much. For N=64 cycles=7 without windowing For N=64 cycles=7 with a Hann window For N=4096 cycles=409 without windowing For N=4096 cycles=409 with a Hann window Back to top IP Logged