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Design >> Analog Design >> Resistor not used in parallel to f/b Cap of RC integrator in DeltaSigma Modulato
https://designers-guide.org/forum/YaBB.pl?num=1458832129

Message started by jdp on Mar 24^th, 2016, 8:08am

Title: Resistor not used in parallel to f/b Cap of RC integrator in DeltaSigma Modulato
Post by jdp on Mar 24^th, 2016, 8:08am

Hi.
We know that in an RC active integrator (using op amp), to avoid very large gain for dc-input (for which the feedback capacitor behaves like an open-circuit) resulting in the saturation of the output voltage due to small dc-offset at input, a resistor, RF is connected in parallel with the feedback capacitor C as shown in attached Figure.

But, in continuous time Delta Sigma Modulator that uses RC integrator as loop filter, no such resistor is used in parallel to the feedback capacitor (as seen in all such literature/papers). Isn't the same fear of dc-offset present here as well?

Title: Re: Resistor not used in parallel to f/b Cap of RC integrator in DeltaSigma Modulato
Post by Ken Kundert on Mar 26^th, 2016, 7:49pm

In the ΔΣ converter, the resistor is not needed because the output of the quantizer will be fed back into the input of the integrator. This negative feedback will keep the integrator in range.

-Ken

Title: Re: Resistor not used in parallel to f/b Cap of RC integrator in DeltaSigma Modulato
Post by jdp on Mar 29^th, 2016, 8:08am

Hi Ken,

Many thanks for your reply :)

Then, can it be said that input offset of constituent op-amps are not of much concern in Delta Sigma Modulator due to the presence of negative-feedback arrangement in the modulator?

Title: Re: Resistor not used in parallel to f/b Cap of RC integrator in DeltaSigma Modulato
Post by Ken Kundert on Mar 29^th, 2016, 1:30pm

It is no more of a concern than in any other feedback system. The measured result will be in error by the amount of the offset, but that offset will not be amplified.

-Ken

Title: Re: Resistor not used in parallel to f/b Cap of RC integrator in DeltaSigma Modulato
Post by jdp on Apr 1^st, 2016, 7:45pm

Thank you Ken for your help :)

With regards.
JDP