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Break feedback loop for PSRR (Read 4140 times)
supermoment
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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
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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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LvW (buddypoor: In memory of the great late Buddy Rich)
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raja.cedt
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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
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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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Jerry Twomey
www.effectiveelectrons.com
Read My Electronic Design Column Here
Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical
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supermoment
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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
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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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LvW (buddypoor: In memory of the great late Buddy Rich)
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raja.cedt
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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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