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Design >> Mixed-Signal Design >> 2.0*a in Digital Delta Sigma Modulator
https://designers-guide.org/forum/YaBB.pl?num=1412268685

Message started by cheap_salary on Oct 2^nd, 2014, 9:51am

Title: 2.0*a in Digital Delta Sigma Modulator
Post by cheap_salary on Oct 2^nd, 2014, 9:51am

Assume multiplying 20bits data a[19:0] with 2.0.
Generally, we realize it as {a[18:0], 1'b0}.

However I find {a[18:0], a[0]} in digital single loop Delta-Sigma Modulator for Fractional-N Synthesizer.
http://m.eet.com/media/1055785/mingliang-fig6.gif

I tried simulation.
But I can't find out any remarkable difference of noise shaping characteristics between them.

What effect can we expect by {a[18:0], a[0]} instead of {a[18:0], 1'b0} ?

Title: Re: 2.0*a in Digital Delta Sigma Modulator
Post by carlgrace on Oct 3^rd, 2014, 3:29pm

Well {a[18:0], a[0]} is equivalent to rounding after the multiplication. {a[18:0], 1'b0} is truncation. I've only used truncation but I guess rounding might be slightly more accurate.

Title: Re: 2.0*a in Digital Delta Sigma Modulator
Post by cheap_salary on Oct 3^rd, 2014, 8:13pm

carlgrace wrote on Oct 3^rd, 2014, 3:29pm:

{a[18:0], a[0]} is equivalent to rounding after the multiplication.
{a[18:0], 1'b0} is truncation.

Thanks.