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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
 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #23 - Oct 10th, 2017, 12:35pm   DanielLam wrote on 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 I was preparing your answer. Back to top IP Logged
 DanielLam Community Member Offline Posts: 76 Re: SAR ADC SNR variations for different FFT numbers Reply #24 - Oct 10th, 2017, 12:52pm   Just to double check, your Fin has as many digit precision as possible right? For example, for Fin = 7/64 * Fs. That is the Fin input, and you are not using 0.11 * Fs as your Fin.Otherwise, I am a little stumped. Best advice I can give is to make an ideal ADC in matlab, and make sure that works. If that works fine, I'd suspect a rounding issue somewhere. I'm assuming these measurements were from a Cadence sim? Back to top IP Logged
 RobG Community Fellow Offline Posts: 569 Bozeman, MT Re: SAR ADC SNR variations for different FFT numbers Reply #25 - Oct 10th, 2017, 6:30pm   Yeah, something's messed with the 64 pt sim. You shouldn't need windowing to get the noise floor down. My best guess is that the first few samples are corrupted (either the ADC or the way you are processing the data). Sometimes this can happen if your reference is settling, but that usually produces a skirt. It is really hard to debug from here. Let us know what you find out. Back to top IP Logged
 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #26 - Oct 11th, 2017, 12:45am   DanielLam wrote on Oct 10th, 2017, 12:52pm:Just to double check, your Fin has as many digit precision as possible right? For example, for Fin = 7/64 * Fs. That is the Fin input, and you are not using 0.11 * Fs as your Fin. Yes exactly, for instance when using N=64 and cycles=7 for Fs=200K, I choose Fin to be => 200k*7/64=21.875KDanielLam wrote on Oct 10th, 2017, 12:52pm:Otherwise, I am a little stumped. Best advice I can give is to make an ideal ADC in matlab, and make sure that works. If that works fine, I'd suspect a rounding issue somewhere. I have simulated an ideal sine (only including quantization noise) and this SNR decline didn't happen at all(with and without a window). DanielLam wrote on Oct 10th, 2017, 12:52pm:I'm assuming these measurements were from a Cadence sim? Yes, I have imported data from Cadence to Matlab.Sounds that RobG is totally right, the first samples are corrupted, I will post the results below. Thanks so much for your help. Back to top IP Logged
 niloun Community Member Offline Posts: 47 Re: SAR ADC SNR variations for different FFT numbers Reply #27 - Oct 11th, 2017, 1:09am   RobG wrote on Oct 10th, 2017, 6:30pm:Yeah, something's messed with the 64 pt sim. You shouldn't need windowing to get the noise floor down. My best guess is that the first few samples are corrupted (either the ADC or the way you are processing the data). Thank you so much, your suggestion worked, I simulated the ADC for N=64 and cycles=7, I ran it for a longer time (640u instead of 320u) then I used output data from 320u to 640u :N=64, cycles=7, Fin=20K, Fs=200K RobG wrote on Oct 10th, 2017, 6:30pm:Sometimes this can happen if your reference is settling, but that usually produces a skirt. It is really hard to debug from here. Let us know what you find out. One of the professors in our department said that the circuit is trying to wake up in the first conversion cycles that's why the first few samples are corrupted. I don't understand "Circuit's trying to wake up"!I am very thankful. Back to top IP Logged
 RobG Community Fellow Offline Posts: 569 Bozeman, MT Re: SAR ADC SNR variations for different FFT numbers Reply #28 - Oct 11th, 2017, 7:23am   Glad that worked. Waking up just means initial transients in the reference and/or bias currents are settling out. Back to top IP Logged
 DanielLam Community Member Offline Posts: 76 Re: SAR ADC SNR variations for different FFT numbers Reply #29 - Oct 11th, 2017, 9:49am   Nice one Rob. The good old transient startup times. Back to top IP Logged
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