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Design >> Analog Design >> chopper stabilization noise folding
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Message started by aaron_do on Sep 17^th, 2009, 8:12pm

Title: chopper stabilization noise folding
Post by aaron_do on Sep 17^th, 2009, 8:12pm

Hi all,

for chopper stabilization, I read that ideally there is no noise folding. I can see how noise at the input of the system will have no noise folding, but does this still apply to noise in the internal amplifier? My thinking is that since this noise is generated after the first modulation, it will still fold.

thanks,
Aaron

Title: Re: chopper stabilization noise folding
Post by RobG on Sep 24^th, 2009, 8:13pm

I did this analysis several years ago, but was hoping someone else would reply because I can't remember the result precisely.

I believe the noise at the output is the thermal noise of the opamp modulated by a +/-1 square wave with frequency Fchop. This will have "carriers" at 3*Fchop, 5*Fchop, etc, so you have aliasing. However, when you add up and integrate all the noise that is aliased back it turns out to be the same as if you did nothing. At least that is how I remember it. The Enz/Temes paper has an analysis, but I found it easier to do the math myself.

rg

Title: Re: chopper stabilization noise folding
Post by thechopper on Sep 27^th, 2009, 9:03am

Hi Aaron,

As RG pointed out acutal aliasing occurs during chopping operation. The key is that the weigth of each harmonic component after modulating the noise is less than 1. When you add them up and consider the components left in the base band (in other words aliasing them) you will find the total power associated to the aliased noise is ideally equal to the one before chopping it.

In other words, you do alias noise components; but only attenutated versions of it that after summing them up totalize the same input noise power.

Hope this helps
Tosei