opamp psrr measurement

[pianomania]

about the OPamp psrr measurement , should we test it in
1 openloop like connect the two input , then measure the output variation

or

2 close loop : like connect the op as the Opamp buffer then measure the output variation?

the two kind of testing have very different result.
which one is reasonale , why?

 

[fcfusion]

I believe the closed-loop is the correct one because Open-loop opamps have practically 0dB PSRR.

 

[cesczhang]

I think the first open loop is like a dc psrr,the second is ac psrr

 

[dedalus]

Both open-loop and closed-loop approaches are possible ("open loop" means closed-loop with large RC in the feedback, not "true open loop" configuration).

Open loop simulation is described in Backer/Li/Boyce "CMOS Circuit Design, Layout, and Simulation" (1st ed., p. 633).

Closed-loop simulation is described in Allens/Holberg "CMOS Analog Circuit Design" (2nd ed., p. 432).

In the attached figure results of both simulations are shown for comparison. A considerable difference is observed after cut-off frequency, so in most cases either method can be used.

the two kind of testing have very different result.

As you can see from attached figure results are very close, so likely you made some mistake.

... because Open-loop opamps have practically 0dB PSRR.

Didn't you mix up PSRR (Adm/Add) with (1/Add)?

 

[love_analog]

I have typically used closed loop in past life. The open loop is too susceptible to "instability" etc. Maybe fine for sims but for testing ...