The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Break feedback loop for PSRR
https://designers-guide.org/forum/YaBB.pl?num=1351154931

Message started by casual on Oct 25^th, 2012, 1:48am

Title: Break feedback loop for PSRR
Post by casual on 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.

Title: Re: Break feedback loop for PSRR
Post by buddypoor on 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.

Title: Re: Break feedback loop for PSRR
Post by raja.cedt on 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.

Title: Re: Break feedback loop for PSRR
Post by loose-electron on 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.

Title: Re: Break feedback loop for PSRR
Post by casual on 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?

Title: Re: Break feedback loop for PSRR
Post by buddypoor on 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)?

Title: Re: Break feedback loop for PSRR
Post by raja.cedt on 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.