LTSpice PSRR

[Neutrality]

How do you measure PSRR in LTSpice?

Been looking all over for any info on eaxctly how to do it, but have had no luck sofar.

I just want a simple explanation on how to do it on a amplifier simulation and maybe also an example.

Thanks.

 

[Mooly]

Good question. I would imagine the calculation is the usual 20log (v1/v2) where one of the voltages is the ripple on the rails.

Here's an LTspice file showing noise on the rails and the corresponding amplifier output. You can set your supplies up in a similar way.

This is what you should see on the amp output.

 

[jcx]

the .TRAN is always best - you can see nonlinear effects of bias cutting off, clipping

.AC analysis, adding the AC value to the existing DC Vsource lets you do a frequency plot - but like all .AC is only a linearized small signal evaluation around the operating point found at the start of the sim - no nonlinear effects will be seen

 

[Neutrality]

Then how do you make LTSpice calculate 20Log(v1/v2).

Tried to add a new trace and enter the equation, no luck. Maybe I am doing it wrong.

Hmm.

 

[Mooly]

I wouldn't know how you would set LTspice up to calculate something like that... I'd just use a calculator

 

[jcx]

.AC does display the Bode plot

for a .TRAN you can look at the waveforms in the FFT plot window, ratio the any signal's frequency component's magnitude, phase to the PS waveform's components with the cursors

 

[jazbo8]

PSRR Example

You can also try right-click on the trace and enter the plot expression manually, e.g., 20*log(V(n001)/V(n002)).