How to caculate the PSRR of Bandgap circuit

[she_long]

Schematic a is the basic bandgap circuit, in small signal circuit Va equals to vdd approximately, so I redraw schematic a to schematic b.
In schematic b V2 and V1 have the same ac parameters acm=1 acp=0, For M21, the small signal vsg=0 and vref=Rout/(Rout+rds)*vdd, but the simulation result sames different from it.
How do I find out the detail small signal funtion of vref?
Thanks!

 

[kumard35b]

you can give a ac signal of magnitude 1v and do the ac simulation ,you can plot the waveform of the vref and see how much db is your PSSR.

 

[mitgrace]

YOu can change the power variation with bandgap output. delta V / Delta power variation

 

[she_long]

Thanks