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Design >> Mixed-Signal Design >> Issues in SNR calculation for Delta Sigma ADC
https://designers-guide.org/forum/YaBB.pl?num=1346330825

Message started by sourav on Aug 30^th, 2012, 5:47am

Title: Issues in SNR calculation for Delta Sigma ADC
Post by sourav on Aug 30^th, 2012, 5:47am

Hi Guys,
I am finding issues in SNR calculation for my Simulink model and the model given in Screier toolbox.A pdf file is attached with the SNR figures and the model i have used

Specifications for my simulink model:

Amplitude= 0.8
Samples per period= 1170
Sample time= 10^-6
Frequency=2*pi*854.49 Hz

a,g,b,c have been calculated for CIFB model as:

H = synthesizeNTF(2,256,0,2);
[a,g,b,c] = realizeNTF(H,'CIFB')
a =
0.4721 1.2361
g =
0
b =
0.4721 1.2361 1.0000
c =
1 1

Matlab code for SNR calculation:
OSR = 256;
H = synthesizeNTF(2,OSR,0,2);
N = 8192; fB = ceil(N/(2*OSR));
f=7;
u = 0.8*sin(2*pi*f/N*[0:N-1]);
v = simulateDSM(u,H);
%%
t = 0:1000;
stairs(t, u(t+1),'g');
hold on;
stairs(t,v(t+1),'b');
axis([0 200 -1.2 1.2]);
ylabel('u, v');
%%
spec=fft(v.*ds_hann(N))/(N/4);
%figure
plot(linspace(0,0.5,N/2+1), ...
dbv(spec(1:N/2+1)));
axis([0 0.5 -120 0]);
grid on;
ylabel('dBFS/NBW')
snr=calculateSNR(spec(1:fB),f);
s=sprintf('SNR = %4.1fdB\n',snr)
text(0.25,-90,s);
s=sprintf('NBW=%7.5f',1.5/N);
text(0.25, -110, s);

Problem is that SNR for my model does'nt scale with OSR and also significantly smaller than theoretical one....

All sorts of suggestions are solicited....

Thanks and Regards,
Sourav

Title: Re: Issues in SNR calculation for Delta Sigma ADC
Post by nrk1 on Sep 1^st, 2012, 9:53am

You need to look at the signal band in more detail. From the plots, it appears that your model has more junk in the signal band, but impossible to say for sure. You should also do it for lot more than 8192 points(2^16 or more), take the PSD with averaging so that you can discern small differences.

Title: Re: Issues in SNR calculation for Delta Sigma ADC
Post by ywguo on Oct 6^th, 2012, 5:52am

Hi Sourav,

Would you please show the plots in log scale for x-axis? And it is good to attach it as picture?

Yawei

Title: Re: Issues in SNR calculation for Delta Sigma ADC
Post by sourav on Mar 16^th, 2013, 12:53pm

Hi Guys,
sorry for late response. I have found out the actual problem. Notice the sine wave block in pdf. In matlab simulink there are two options to define a sine wave; one is time based and another is sample based. In early simulations i have tried with the sample based sine wave. But when i select the time based sine wave option in sine block the problem get resolved. I dont know the actual reason but its working. :-?

sourav