The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Mixed-Signal Design >> Adder stage in feedforward SD modulators
https://designers-guide.org/forum/YaBB.pl?num=1186712800

Message started by tosei on Aug 9^th, 2007, 7:26pm

Title: Adder stage in feedforward SD modulators
Post by tosei on Aug 9^th, 2007, 7:26pm

Hi All,

What is the criteria for selecting between a passive adder network vs an active one (with an opamp) in a SD feed-forward topology? I could guess it mainly depends on the coefficients for such stage, it there is one coefficient larger than one the only choice is active. Is this assumption correct? Am I missing something more important to consider?

Many thanks
Tosei

Title: Re: Adder stage in feedforward SD modulators
Post by Berti on Aug 28^th, 2007, 1:21am

Hi Tosei,

I think whether you implement a passive or an active adder depends on how much signal attenuation
you can accept. With reduced supply voltage, the signal swing in SD modulators has become smaller and
the attenuation of a passive adder cannot be tolerated anymore. Furthermore, the coefficients of a passive
adder are sensitive to parasitic capacitance. An active adder isn't and therefore often results in a more
roboust design.

Regards,

Thomas

Title: Re: Adder stage in feedforward SD modulators
Post by tosei on Aug 28^th, 2007, 9:57am

Thomas,

Thanks for your reply.
Hernan