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Design >> Mixed-Signal Design >> Less Number of 1s in Delta Sigma ADC: Negative ENOB!!
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Message started by ksnf3000 on Sep 2^nd, 2013, 10:06am

Title: Less Number of 1s in Delta Sigma ADC: Negative ENOB!!
Post by ksnf3000 on Sep 2^nd, 2013, 10:06am

Hi All,

I have a question related to Delta Sigma ADC. I am using an OSR of 64 for a 1st order modulator. However,the input signal (sine wave of amplitude 0.5 to 4V @ 200kHz) is being sampled with a very less number of 1s, leading to a very low SNR value. (Negative!!)
Please let me know why I have such a less number of 1s in my output signal. I am using a switched capacitor design for the modulator.
Please see the attachment.
The red signal is the input sine wave.
The blue is the output of the 1-bit DAC which is fed back to the switch.

Thanks!

Regards,
ksnf3000

Title: Re: Less Number of 1s in Delta Sigma ADC: Negative ENOB!!
Post by avlsi on Sep 6^th, 2013, 9:53am

It looks you have used real circuits and not some ideal elements schematic. can you try to use an ideal circuit to see whether you had this issue??

Title: Re: Less Number of 1s in Delta Sigma ADC: Negative ENOB!!
Post by ksnf3000 on Sep 6^th, 2013, 11:26am

Hi avlsi,

Yes your right. I have used real circuits -transistor level design for 0.35 µm AMS technology. I have used ideal in MATLAB/Simulink and it works fine. I have checked my integrator and comparator also. Seems some issue with my integrator getting saturated. However,do you think this may be the reason for limited number of samples?

Thanks!!

Title: Re: Less Number of 1s in Delta Sigma ADC: Negative ENOB!!
Post by Raphael Viera on Oct 20^th, 2013, 8:46am

Did you solve your problem? I had a negative SNR, but my problem was the input frequency that I changed an forgot to change in the calculation. Is your spectrum according to the NTF or it differs?

Title: Re: Less Number of 1s in Delta Sigma ADC: Negative ENOB!!
Post by ksnf3000 on Oct 21^st, 2013, 3:13am

Hi Raphael,

No.it still persists. My input frequency is around 1 kHz. In which calculation have you changed?

My spectrum is according to the NTF. The problem is my opamp (integrator) is getting saturated quickly, thats why it is not able to resolve further values,as you can see in the constant and large number of 1's in the figure when the sine curve is about to change.

Any ideas on this?

Regards,
Kashif