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Break feedback loop for PSRR (Read 4141 times)

supermoment Senior Member Offline Posts: 167	Break feedback loop for PSRR Oct 25^th, 2012, 1:48am hi When measuring PSRR for a opamp with feedback loop, do I need to break the feedback loop as mandated in gain plot measurement.. the ac source is in the vcc when measuring psrr. i did a two cases, it gives me huge differences.
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buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: Break feedback loop for PSRR Reply #1 - Oct 25^th, 2012, 2:27am One of the benefits of negative feedback is - among others - the improvement of the PSSR. Thus, of course the loop should remain closed during PSSR determination.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Break feedback loop for PSRR Reply #2 - Oct 25^th, 2012, 6:19am hello Casual, as you said both will be differed by exactly by loopgain. If you break the loop then simply noise will come and appear at output but if you have loop then it will start correcting, hence less PSRR. Thanks, Raj.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Break feedback loop for PSRR Reply #3 - Oct 28^th, 2012, 12:03pm One other important thing here - mismatched elements. Your PSRR will be very good with everything nicely matched. Progressively worse when you put in expected mismatches. Consider some Monte Carlo type simulations.
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supermoment Senior Member Offline Posts: 167	Re: Break feedback loop for PSRR Reply #4 - Oct 28^th, 2012, 6:48pm yes, i did not break the loop when sim the PSRR. BUT i look at the Jacob Baker book, page 791 figure 24.27, the PSRR test setup with big RC to break the loop. It confused me. is the setup diagram wrong?
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buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: Break feedback loop for PSRR Reply #5 - Oct 29^th, 2012, 1:00am I don`t know the Baker book but perhaps he wants to demonstrate the improvement in PSSR due to feedback (comparison with/without loop closed)?
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Break feedback loop for PSRR Reply #6 - Oct 29^th, 2012, 1:15am hello casual, what buddypoor is saying correct. According to baker book he is trying to show loop gain and open loop PSRR in the same simulation and by dividing both you get closed loop PSRR. Thanks, Raj.
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