Berti
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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
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