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I have designed a slightly modified second order (3-bit) multi-bit incremental sigma delta modulator (IDSM) in simulink (system level). The SNR that I get is 45dB, which is not as expected. But ideally with 3-bit IDSM, we should get 78 dB. So, I wanted to troubleshoot why my designed second order system is not behaving as it should.

For copyright reasons, I could not share the exact designed architecture. But the architecture is similar to the model shown below:

enter image description here

(I use the fine ADC as 2nd order sigma delta). The reason that I compared with (3-bit) multi-bit incremental sigma delta modulator is that I have 3-bit coarse ADC.

I would like to know any other way (other than SNR) to verify/troubleshoot why we would not get a second order behavior?

1-noise shaped output spectrum (which i could not for Incremental architecture, but I have posted a question in DSP forum for this - https://dsp.stackexchange.com/questions/54548/how-to-plot-noise-shaped-spectrum-of-first-order-incremental-sigma-delta-adcs-o)

I guess that this is a control system loop and is there any other way to troubleshoot like - Impulse response or any other way?

Thank you.

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    \$\begingroup\$ The impulse response needs to decay, down to the -78dB level in a predictable time. So yes, impulse response should be useful. Show what you learn, please. And predict what coefficients will cause the loops to be unstable, then evaluate at those values. Also, given a 3-bit ADC supports 8 levels, thus a ZERO value is not included (-4 -3 -2 -1 +1 +2 +3 +4)???, does the imbalance cause the SNR problem? \$\endgroup\$ – analogsystemsrf Jan 2 at 0:47
  • \$\begingroup\$ What you depicted is not a sigma-delta. There isn’t even a feedback loop involved. It’s just two ADCs being added together, the precision of which will be given by the matching between the two ADCs. \$\endgroup\$ – Edgar Brown Jan 2 at 14:12
  • \$\begingroup\$ @analogsystemsrf I am investigating about the impulse response and stability. After performing some experiments, I will edit this post. Thanks. \$\endgroup\$ – sundar Jan 5 at 20:13

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