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Design >> Analog Design >> Input signal level in a delta sigma modulator
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Message started by naderi on Mar 12^th, 2011, 7:14am

Title: Input signal level in a delta sigma modulator
Post by naderi on Mar 12^th, 2011, 7:14am

Hello all,

What can limit the input signal level for a delta sigma modulator?
and How can the maximum stable input be increased?

I used an example given in the delta-sigma toolbox from Matlab and optimized a 3rd-order NTF for:
-Continuous-time with RZ DAC
-all poles at DC.
-out-of-band noise: Hinf=1.7
-BW = 20kHz
-OSR = 128
-cascaded-integrator-feedback (CIFB) structure.
(fs=5.12MHz, U(1:3)=[0.1727, 0.7209, 1.4812])

After optimization and conversion to continuous-time, I tested in time-domain by the simulink. It seems that the maximum input signal level cannot be above 1/4 of U1 (coefficient of the outer feedback loop shown in the figure), which is 0.043. Otherwise the modulator becomes unstable. Such a low input makes problem for a resolution above 13 bits as it can be limited by thermal noise floor of the modulator.

I appreciate your comments.

Thanks,

Title: Re: Input signal level in a delta sigma modulator
Post by Rakesh on Mar 12^th, 2011, 7:31am

The maximum stable amplitude of the sigma delta modulator will limit the amplitude of the input signal. MSA can be increased by either increasing the resolution of the quantizer of by reducing the out of band gain.

Title: Re: Input signal level in a delta sigma modulator
Post by naderi on Mar 12^th, 2011, 7:42am

Thanks for your reply.
Could you please elaborate why resolution of the quantizer effects on MSA?

Best,
Ali

Title: Re: Input signal level in a delta sigma modulator
Post by Rakesh on Mar 12^th, 2011, 8:40am

If u increase the resolution of the quantizer the quantisation noise of the quantizer (delta^2/12) will become low. since delta reduces with increase in the resolution. So since the total noise becomes low for the same out of band gain we get lower noise which doesnot saturate the quantizer. So the MSA increases..
Is it fine

Title: Re: Input signal level in a delta sigma modulator
Post by naderi on Mar 12^th, 2011, 9:31am

Yes, Thanks a lot.

Ali