pmos diode in series with a nmos diode, why?

[tommydidi]

Guys,

I sometimes saw a nmos-connected diode in series with a pmos-connected diode. The circuit is somewhat like the one I put in the attachment. The Vg of M1 is regulated through other circuit, so the current through it goes up and down. I don't quite understand the purpose of having two mosfet diodes there. Can anyone explain this to me? Thanks.

 

[dick_freebird]

Many process variations move VTN and VTP together and
since they're opposite, the stack nulls some variations
where (say) 2 stacked NMOS would have double VTN
variation.

 

[tommydidi]

Thank you for your answer, dick_freebird!