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Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator (Read 338 times)
polyam
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Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Feb 05th, 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
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DSM.png
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cheap_salary
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Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Reply #1 - Feb 6th, 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/conte...
https://jp.mathworks.com/matlabcentral/fileexchange/19-delta-sigma-toolbox/conte...
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polyam
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Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Reply #2 - Feb 25th, 2017, 12:49pm
 
Thank you cheap_salary.
This method works.
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cheap_salary
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Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Reply #3 - Feb 26th, 2017, 2:18am
 
RZ system require more large slew-rate for integrator and DAC than NRZ system.
And jitter sensitivity is worse than NRZ.
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« Last Edit: Feb 26th, 2017, 3:30am by cheap_salary »  
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polyam
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Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Reply #4 - Feb 26th, 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.
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cheap_salary
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Re: Replacing an NRZ DAC with an RZ DAC in a CT delta sigma modulator
Reply #5 - Feb 26th, 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.
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