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Design >> Mixed-Signal Design >> Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
https://designers-guide.org/forum/YaBB.pl?num=1486297269

Message started by polyam on Feb 5^th, 2017, 4:21am

Title: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Post by polyam on Feb 5^th, 2017, 4:21am

Hello,

Let's assume that I have synthesized a CT delta sigma modulator (DSM) while having NRZ DACs for the main path and fast loop (DAC_A and DAC_B).
Now, I want to replace the DAC_A, which has an NRZ shape, with RZ one. What changes I have do on the loop filter to restore the loop-response?

Many thanks

Title: Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Post by cheap_salary on Feb 6^th, 2017, 11:28pm

Time width for 1bit is a Tbit.
DAC Amplitude for NRZ is Ampl.

Time width for RZ DAC is a Trz.
If RZ DAC Amplitude is equal to Ampl*(Tbit/Trz), NTF will be unchanged roughly.

If you want to design strictly, use followings.
https://jp.mathworks.com/matlabcentral/fileexchange/19-delta-sigma-toolbox/content/delsig/pulse.m
https://jp.mathworks.com/matlabcentral/fileexchange/19-delta-sigma-toolbox/content/delsig/impL1.m

Title: Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Post by polyam on Feb 25^th, 2017, 12:49pm

Thank you cheap_salary.
This method works.

Title: Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Post by cheap_salary on Feb 26^th, 2017, 2:18am

RZ system require more large slew-rate for integrator and DAC than NRZ system.
And jitter sensitivity is worse than NRZ.

Title: Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Post by polyam on Feb 26^th, 2017, 3:48am

Yes! that's completely right. Very useful info. Thanks.
But when it comes to excess loop delay it works better. Anyway, it's a compromise. In a case that the architecture has more than one clock cycle delay, RZ DAC seems to be a good choice.
That's why I am using that.

Title: Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Post by cheap_salary on Feb 26^th, 2017, 4:13am

One merit of RZ system is less interference feedback DAC data between sequential two bits.

Achievable SNR is limitted by this issue in NRZ system.

So if you would like to aim high SNR, RZ system is good candidate.